Evaluation of Untreated and Surface-Treated Wooden Facades of Buildings in Sweden

: In this research, untreated and surface-treated wooden facades of buildings are investigated and compared with regard to durability, environmental impact, and cost using the document and literature studies supplemented with quantitative data. The investigation is based on the inﬂuence on the wood by external factors, either in its natural form or with a protective layer of a surface treatment. It resulted from the study that from a durability viewpoint, it is not always advantageous to paint a wooden facade. Yakisugi, a thermal modiﬁcation for wooden facades that involves burning the wood to create a protective surface layer, has shown better properties, which, in combination with a lower need for maintenance, can be considered the leading treatment for facades. From an environmental perspective, Yakisugi, like untreated wood, has demonstrated advantages compared with conventional paints. Paints often come with maintenance requirements that should be carried out correctly to avoid extensive costs. Cost is an important factor that can be decisive when users choose a surface treatment. What determines the cost is the relationship between durability and maintenance intervals. The need for the maintenance of paints leads to large ﬁnancial costs during the working life of a wooden facade, which differs from Yakisugi or an untreated wooden facade that can be considered maintenance-free. In order to meet the growing environmental demands from a national level to a global scale, unconventional surface treatments for wooden facades should be promoted in the future.


Introduction
The use of wood in buildings is growing in popularity because it is thought to be an environmentally friendly, lightweight, and attractive building material; thus, it is attracting many designers and engineers throughout the world.Wooden buildings allow architects and engineers a flexible design process thanks to the suitability and feasibility of the wood material [1,2].Several hybrid wood-based buildings have been built during the recent years [3][4][5].
Using wood as a facade material in Sweden contributes to sustainable and environmentally friendly constructions.As the Swedish building and real estate sector accounts for a significant part of the society's environmental impact, the use of wood should be promoted.To guide users in the choice of a wooden facade and its surface treatment, there are several aspects to take into account.A wooden facade must be durable, have a low environmental impact, and not be too expensive.Many new houses are built in Sweden yearly.A lot of choices appear with new constructions, including the houses' facades, which should act as a protective layer against the outdoor exposure.Wood is the most common facade material in Sweden, with a rich tradition in the country as a sustainable and renewable building material [6,7].
Wood has been the most important building material in Sweden's history.Today, wood is still Sweden's most significant and only renewable building material.With the Coatings 2023, 13, 746 2 of 23 increased environmental awareness and pursuit of a sustainable society, the demand for wood-based products has increased.An advantage of wood is that it never needs to be landfilled because the material can be reused or recycled.Wood that cannot be reused or recycled can be disposed of by incineration [8].
Wood can be modified in different ways, including chemically and thermally.Chemical modification involves treating the wood with chemicals or changing the chemical structure.Thermal modification is a type of heat treatment where the wood is exposed to high temperatures, which creates a protective surface layer [9].
When choosing a facade, the user is often guided by aesthetic wishes and personal preferences.For this reason, it is common to treat the surface of the facade with paints.However, mandatory maintenance that should be performed regularly comes with paint.Paints are often manufactured with additives of chemical substances, for example, biocides that reduce the appearance of living organisms on the surface treatment.Another trend that has gained increased popularity is to leave the wooden facade in its natural state due to the rustic feel, increased environmental awareness, and reduced maintenance.
The builder should take into account that the facade must form the protective part of the exterior wall structure and withstand moisture, wind, temperature differences, and UV radiation from the sun.In addition to the fact that wood has environmental advantages, it also has good durability and a characteristic appearance [10].
A wooden facade that is surface treated has two basic goals: to protect the wood against degradation and to create an aesthetic appearance [11].The surface treatment improves the aesthetics of the building, while complying with architectural and environmental trends and legal requirements [10].
When wood is used as a facade material, the interaction between the wood and the surface treatment is important to achieve a good result [12].A wooden facade can be surface treated with, for instance, paint or thermal modification.In Sweden, Falu Rödfärg, alkyd oil paint, and acrylate paint or a mixture of them, the so-called hybrid paints, are commonly utilized to paint wooden facades.The thermal modification of the wood can be implemented in several different ways.One way is a traditional Japanese method, Yakisugi, that has attracted attention in Sweden in recent years and involves burning the wood to create a protective surface layer [13].
In the case of new construction, legal requirements are imposed on, for example, design and environmental aspects based on a sustainability perspective.A building that is erected must have a good form, color, and material impact.In addition, requirements for the technical properties regarding durability and protection with respect to the environment should be met with normal maintenance during an economically reasonable working life [14].
There are several aspects that should be considered by a user when choosing a wooden facade.To avoid problems with a wooden facade, it can be beneficial for the user to have knowledge about wooden facades and their properties.
The users' preconceived notions about an increased rot attack and their aesthetic wishes could be reasons why untreated wood is not used as a facade material to the same extent as surface-treated wood.Furthermore, a lack of knowledge among users is a decisive factor for why untreated wood is not a frequent choice as a facade material, despite the possibilities for lower maintenance.
Most surface treatments require maintenance, which users may perceive as problematic and time-consuming.Moreover, the maintenance of a surface treatment can be costly.In addition to the cost of paint, one must decide who will do the work and count on costs for materials and fixtures.To maintain a painted wooden facade, the right conditions are required, which can be problematic because weather cannot be influenced by humans [15].
According to [16], there is no evidence that painting is necessarily needed for facades, because several old buildings with untreated wooden facades in Sweden have survived with the outdoor exposure for hundreds of years without durability deterioration.Leaving a wooden facade untreated compared with painting it or using another surface treatment Coatings 2023, 13, 746 3 of 23 differs in terms of several various factors.This study examines untreated wooden facades and different surface-treatment methods for wooden facades with the aim of facilitating the users' choice of wooden facades of buildings.

Untreated Wood
Untreated wooden panels, in contrast to surface-treated wooden panels, take up a large amount of water by capillary action [17].Untreated wooden panels can, therefore, be exposed to attack, for example, by mold, which can lead to discoloration of the surface [18].Discoloration gives a rustic impression with a raw surface that exposes the wood's characterizing elements and which, over the years, gives the wood a monochrome gray tone [19].Facades made of untreated wood can have a long working life with minimal maintenance; however, the surface erodes when exposed to the weather and wind [20,21].

Painted Wood
When a wooden facade is painted, moisture absorption is reduced by slowing down diffusion and preventing the wood from absorbing water by capillary action [17].The moisture ratio differs depending on the paint used [22].A wooden facade that is covered with paint can be expected to have less rot and, thus, an increased durability [23].
The experience of a painted surface is characterized by the material, color, surface texture, gloss, dullness, and luster.Wooden facades age in different ways; usually it is the paint that ages when it is chalked, bleached, or cracked due to the fact that the paint does not interact with the wood in the correct way [20].In accordance with [23], paints in Sweden are usually solvent-or water-based and consist of pigments and binders of organic or inorganic origin.In addition, paints often contain additives that extend the working life, simplify production, and improve the stability and usability of paints.The performance of paints depends on their chemical compositions and physical conditions.
Paints consist of different components that are mixed in varying forms.Pigment forms the color of paints, resists abrasion, and is not soluble.Binder is a polymeric substance and binds the pigment particles with the substrate.Solvents make paints thinner so that they can easily be applied [23].
Biocides are a common additive in paints and are used to reduce the mold growth.Previously, biocides contained, among other things, arsenic and chromium, but in the 21st century these were replaced with organic substances.If biocides are added to the paint, the product should be an active component for at least 10 years, which is also an approximate maintenance interval for paints.When replacing a wooden facade, it must be possible to recycle the wood without affecting the environment.It should, therefore, be ensured that biocides do not have an active impact when changing wooden facades [24].

Falu Rödfärg
Falu Rödfärg plays a societal role as Sweden's oldest and most used type of paints.The color is characterized by tradition and a sense of freedom and can be noticed on everything from old mansions to modern villas.Adhesive paints have water as a solvent and starch paste as a binder.After painting, water evaporates, and the paint dries matte.The pigment in the paint consists of iron oxide that comes from the Falun mine's mineralization and is unique in its composition of 20 different minerals, including silicic acid, iron ore, copper, and zinc [25].

Alkyd Oil Paint
Alkyd oil paint is a predecessor of linseed oil paint, which consists of a mixture of linseed oil and other synthetic binders.Alkyd oil paint is a solvent-based paint that was recently developed and is now also offered as water-based.Alkyd oil paint has a short drying time and creates a durable and strong surface that protects against degrading factors [22].

Acrylate Paint
In connection with an increased awareness of paints' environmental impact, acrylate paint became popular as a water-based paint in the second half of the 20th century in Sweden [22].Based on [26], the binder of acrylate paint is produced from natural gas or crude oil in the form of small sticky plastic balls that stick together when the added water evaporates.In general, acrylate paint adheres well to clean and solid surfaces and can contain a lot of pigments.Acrylate paint resists acids and alkalis better compared with alkyd oil paint and also has a high elasticity that gives a long working life.Under the microscope, however, acrylate paint shows a pore structure in the form of craters, which increases the risk of dirt sticking to the surface.

Hybrid Paint
With the help of new technology, paints and paint systems can be developed in step with increased environmental requirements.Usually, white spirit is used as a thinner in alkyd oil paint while acrylate paint is water based [27].For environmental reasons, white spirit is being phased out.Very fatty alkyds (>70%) are used as an additive in acrylate paint as the high fatty acid content gives increased durability [26].A mixture of acrylate and alkyd oil results in hybrid paint [27].Hybrid paint has improved properties in terms of moisture resistance and adhesion [26].The ability to adhere, also called adhesion, consists of the force and interaction between two bodies.

Yakisugi
Yakisugi is a centuries-old method of heat-treating wood.The method has a long tradition in Japan, but its popularity declined in the 1950s when new and cheaper alternatives were introduced to the market.In Japan, cedar wood was initially used for burning, but as the method gained increased popularity in Europe, with several distributors and retailers, other types of wood such as spruce and pine were also used [13,28].The popularity of the method is rapidly increasing as it is seen as organic, natural, and aesthetic.It is also marketed as maintenance-free and durable with a long working life [29].
It is possible to burn the wood traditionally, manually, or by machine.In the traditional burning, three facade planks are tied together in a triangle and paper is stuffed at one end.When the paper is burned, a chimney effect is created.The wood should have a moisture content between 10%-15% before burning to obtain a good result and durable material.To obtain a burnt surface of 3-4 mm, approximately 5 min of firing is required where the temperature needs to be at least 250 • C; however, it can reach a little over 400 • C.After burning is complete, the planks are untied, and the fire is extinguished with water [13].
However, burning with a manual method makes it difficult to achieve high quantities of the material, which is the reason why it is usually private persons who burn manually with LPG.In order to burn larger quantities of the wood with the least possible environmental impact throughout the process, KOLSVART has developed a machine for burning.When burning in a machine, it is expected that 1 m of the wood takes about 1 min to burn.Burning an entire facade can, therefore, be time-consuming.Even though spruce is the most common facade material, KOLSVART recommends burning pine.Pine is favored because spruce is softer and harder to burn evenly.
Soot occurs regardless of how the wood is burned.Depending on the type of wood, soot is brushed away with a suitable brush in the same direction as the wood fibers, which highlights the structure of the wood.After brushing, the wood is cleaned, either by blowing away soot with an air compressor or wiping with a wet cloth.Often the process is finished by treating the wood with a wood or linseed oil [30].

Maintenance
The maintenance of a wooden facade is carried out to maintain the technical function of the surface and, at the same time, to preserve the desired color [21].There is periodic and need-based maintenance.Periodic maintenance means performing maintenance at intervals determined based on experience or recommendations.Need-based maintenance means carrying out actions when there is a specific need for them, for example, if damage is detected in the surface treatment.However, both maintenance and facade replacement can take place before there is a need.
According to [15], a survey done by Colorama, which is Sweden's largest specialist paint store, has demonstrated that maintenance of facades is ignored for far too long due to the fact that it is costly and time-consuming.Lack of maintenance can lead to moisture entering the construction.Moisture problems can result in extensive renovation costs of up to several hundreds of thousands of SEK.
In the case of new construction, future maintenance intervals should be considered with regard to durability, environment, and cost.The facade's design, color choice, and climate zone determine upcoming maintenance intervals where the user should have sufficient knowledge to be able to make an appropriate choice [19].
The periodic maintenance intervals depend on the degradation process of the different surface treatments [19].Untreated wood erodes at a rate of approximately 1 mm/10 years, which, over time, can lead to a need to replace the facade; however, it can last for several hundred years with minimal maintenance [20].Falu Rödfärg has a maintenance requirement of 7-10 years, alkyd oil paint of 9-12 years, and acrylate paint of 12-15 years [31].Yakisugi has a working life of at least 80 years and is almost maintenance-free [28,32].
Falu Rödfärg is easy to maintain; only brushing off the paint and repainting the facade are needed [26].Alkyd oil and acrylate paints can be scraped off and then repainted.In [27], we are informed which paints are compatible with each other, as listed in Table 1.Falu Rödfärg and acrylate paint are only compatible with the same type of paint.An advantage of alkyd oil paint is that it can be painted over previously painted acrylate paint.

Replacement of Facade
Untreated wooden facades can have a working life of up to 100 years, but in the Nordic climate, it is estimated to be 60 years [33].Painted wooden facades have an estimated working life of 60 years; however, it can vary depending on how the maintenance is managed [34].
However, replacing the facade involves extensive work as the existing facade panels must be taken down and the layers inside inspected.However, if the existing facade has protected the inner layers, there should be no need to replace the studs or the layers inside.

Durability
The durability of a wooden facade is believed to depend on its ability to resist several different factors, mainly weather exposure and rot attack.In addition, the choice of surface treatment and the properties of wood can be decisive factors [19,25].
The durability of the wood depends on its resistance to biological, electromagnetic, and chemical attacks.Wood is one of Sweden's most vulnerable materials when talking about biological attack, and in the production stage, the material is already at risk of attack by, for example, molds.Molds are a collective name that include discoloring fungi and rotting fungi.Molds consist of a mycelial network of hyphal threads that form the structure of the fungus in which the fruiting bodies grow.Discoloring fungi are airborne and grow only on the surface of the wood without breaking it down to any great extent.It mainly affects the aesthetic character of the wood because the mycelial network is pigmented and Coatings 2023, 13, 746 6 of 23 has a slow decomposition process.When attacked by rotting fungi, the wood darkens, which reduces its durability [25].
Mold growth and moisture absorption differ depending on whether heartwood or sapwood is used.It is established that the heartwood in both pine and spruce has better resistance to fungi and moisture compared with the sapwood [35].The sapwood has a larger moisture ratio variation compared with the heartwood [22].
Electromagnetic attacks by UV radiation create a change in the molecular structure of the wood that, over time, produces a darker surface.The change in the photodegradation of the UV radiation can lead to the chemical bonds between the lignin and the wood structure being broken, causing the material to become more brittle and the color to change [25,29].
Wood exposed to chemical attacks by precipitation, which may contain acidic salts and acids, affects the structure of the wood, and breaks down its cellulose.Wood is also broken down by alkaline solutions that attack the lignin and hemicellulose.Despite this, wood is considered to have a good resistance to chemical attack [25].

Untreated Wood
A facade made of untreated wood can be stable despite its large moisture ratio variations if it is given the opportunity to dry between the times it is exposed to water [19,22].However, the researchers in [19] believe that the moisture ratio variations can lead to cracks, which was also confirmed by [36] where they stated that weather conditions are the most important factor when structural differences arise in wood.Wood types with a large proportion of heartwood and vertical annual rings have less moisture movement and a low and stable moisture content, which improve durability [19,37].The risk of cracks decreases with vertical annual rings [37].
Molds need factors such as heat, moisture, biodegradable material, mold spores, and oxygen to grow [23].
Untreated wood darkens early due to the combination of mold growth and photochemical degradation [36,38].Mold growth can be a reason why untreated wood discolors over time [39].Mold growth is caused by a group of discoloring fungi that attack the outer part of the wood and affect moisture permeability, making the wood susceptible to rot fungi.Mold growth is promoted by a humidity between 85% and 95%.If the wood has large moisture ratio variations, it takes longer for molds to grow, and the rate of decomposition is, thus, slower.
The degradation of the wood due to climatic impacts is defined by the erosion rate on its surface and changes in terms of color and loss of fibers [39].Untreated wood exposed to the UV radiation undergoes a photodegradation of lignin whereupon the wood fibers detach and create the gray tone [19,39].The degradation of lignin creates a surface with loosely bound cellulose that, with erosion, gives an uneven and rough surface, which also increases the risk of cracking [29].The UV radiation only penetrates the outermost layer of wood, which leads to a superficial degradation [39].Untreated wood exposed to natural weathering showed visible color changes after one month, attributed to photodegradation [36,38].
Despite the degradation that untreated wood is exposed to during the outdoor exposure, the durability and working life are usually not impaired to the point of serious damage [39].However, an untreated wooden facade can have a very long working life.

Painted Wood
Paint can peel, crack, mold, erode, or fade.The degradation process by molds is one of the most important parameters when concerning the durability of paint in the Swedish climate.Attacks by decaying molds are reduced by surface treating the wood with paint.Other degradation processes that can affect the paint's durability and working life are radiation, moisture, and pollution.The durability of the wood is directly related to water in various forms.The water mechanism through paint can either take place as absorption of liquid water, water vapor into the wood through absorption, or water vapor from the wood through desorption.It is very important that there should be a balance between absorption and desorption in the paint, as a higher absorption than desorption over time leads to blisters, cracks, and mold [23].
The paint layer risks cracking as a result of the moisture-related movements in the wood.If moisture penetrates the cracks of the paint layer, the risk of cracking in the wood also increases.Today, paints are manufactured with a low resistance to vapor passage, but despite this, the wood can suffer from the rot damage [25].

Falu Rödfärg
Wooden facades painted with Falu Rödfärg have a higher moisture ratio and a tendency to flake more compared with alkyd oil and acrylate paints.Falu Rödfärg's diffusion properties, with the ability to allow the wood to absorb water and then dry out, lead to a large moisture ratio variation in comparison with alkyd oil and acrylate paints.Falu Rödfärg has good properties for longer periods apart from its ability to crack and crackle [22,40].Furthermore, Falu Rödfärg has the same risk of cracking as untreated wood [22].Cracks that occur in the wood painted with Falu Rödfärg are larger compared with alkyd oil and acrylate paints [40].

Alkyd Oil Paint
Alkyd oil paint applied on dry and non-basic substrates is weather-resistant and quick-drying and has good adhesion and stretchability.Alkyd oil paint for outdoor use contains air-drying fatty alkyds and should, therefore, be avoided when painted in thick layers [26].Excessively thick layers of alkyd oil paint can cause deep cracks.Alkyd oil paint painted in thick layers has a higher tendency to crack compared with acrylate paint [41].However, alkyd oil paint applied with the recommended thickness can result in a smaller proportion of cracks in the wood than Falu Rödfärg and acrylate paint [22].
Solvent-based paint was compared with water-based paint in [42] without the addition of biocides to see which could resist mold growth the best.A high porosity in both alkyd oil and acrylate paints created moisture penetration and diffusion, which increased the risk of mold growth.However, the comparison demonstrated that solvent-based alkyd oil paint resists mold growth better compared with the water-based ones.The water balance turned out to be completely decisive for the growth of mold.Both paints revealed, despite the lack of biocides, surprisingly good resistance to discoloring fungi.
Solvent and water-based alkyd oil paints with additives of biocides were investigated in [43].The solvent-based alkyd oil paint illustrated a better adhesion and penetrated further into the wood compared with the water-based ones.Water-based alkyd oil paint instead had a greater resistance to the outdoor exposure.

Acrylate Paint
Acrylate paint is considered to have good resistance to weather exposure, both in terms of color and gloss [19,43].Dirt sticks more easily to the porous surface of acrylate paint compared with other types of paint [19].Acrylate paint has a high elasticity, which means that it does not crack but can follow the moisture movements of the wood [22].
Acrylate paint was found to flake more than alkyd oil paint over a shorter period of time [41].However, acrylate paint reached the same degree of flaking as alkyd oil paint after a few years [22].Acrylate paint has good chemical resistance and resists acids and alkalis better than alkyd oil paint [19].

Yakisugi
Since wood undergoes a degradation process when exposed to the outdoors, several different techniques, including heat treatment, have been developed with regard to improving the material's properties [44].Five-hundred-year-old buildings in Japan were treated with Yakisugi and only required need-based maintenance [45].The traditional Japanese method can advantageously be used as a chemical-free surface treatment in Europe because it gives the wood good durability.It is estimated that the burnt wood has a long life of 80 years or more [46].
Yakisugi gives the surface of the wood a protective structure against radiation and decay [47].The burnt carbon layer is hydrophobic and chemically stable with an ability to repel water and resist the UV radiation [29].In addition, the cellulose in the wood is neutralized by the heat, and the burnt wood becomes resistant to mold [48].
The chemical changes that occur during the heat treatment process break down the building blocks of the wood: cellulose, hemicellulose, and lignin.The decomposition results in new properties depending on the temperature with which the wood was treated [45].Wood must be heated to a temperature of 300 • C to achieve changes to the building blocks [32].One should not burn the wood in excessively high temperatures, as a slower process with lower temperatures produces a more stable and thicker carbon layer, which is crucial in terms of resistance to the outdoor exposure.The surface of the carbon layer is porous, which means that it absorbs the moisture capillary [29].
Yakisugi provides a natural protection against moisture.Furthermore, a higher pH value gives the wood an alkaline surface that protects against mold.Burning of the wood is indicated to raise the pH value, which gives increased durability [45].
Wood burned with a low temperature had a higher sorption, while burning with high temperature had a higher absorption [49].The higher absorption caused cracks in the wood, and the absorption in the burnt wood was less when compared with the untreated wood [32].The temperature of firing is decisive not only for the durability but also for the moisture absorption [29,49].
Temperatures above 1000 • C can be reached when the wood is burned with LPG.Temperatures above 700 • C create a carbon layer with lignin components that have a lower resistance to the UV radiation [29].The lignin helps the wood resist photodegradation by the UV radiation [29,45].The burnt surface of the wood has a long working life.However, the modified lignin is not completely stable, and over time, it undergoes a degrading process [29].

Environmental Impact
The Swedish building and real estate sector today accounts for a significant part of society's environmental impact, and it expects major challenges as there is an extensive need for new housing.Increased reuse and recycling in built environments contribute to a circular economy.Built environments include surface treatments that also have a circular economy.
As the national environmental goal, a good built environment must make use of and develop natural and cultural values, be located and designed in an environmentally friendly way, and constitute a good and healthy living environment.A good built environment links to sustainable development goal (SDG) 11, sustainable cities and communities, as well as SDG 12, responsible consumption and production, in Agenda 2030 (the United Nations' global action plan with 17 SDGs).Sweden also has a national climate goal, which includes the building and real estate sector, to have no net emissions of greenhouse gases (GHGs) that then turn into negative emissions [50].
In connection with Agenda 2030, the government has decided on new milestones that focus on a society free from harmful substances and increased protection for health and the environment.The new milestones include the reduced use of biocide products with particularly dangerous properties and work for sustainable stormwater management.
The Swedish Chemicals Agency is tasked with controlling, informing, and classifying chemicals.With regulation at the EU level, the Swedish Chemicals Agency works to reduce the use of dangerous chemicals.Biocidal products contain environmentally hazardous chemicals and require the Swedish Chemicals Agency's review and approval to be sold or used in Sweden [51].
When choosing a surface treatment, there are often aesthetic wishes that need to be met, in combination with maintaining current environmental requirements.It is reported in [52] Coatings 2023, 13, 746 9 of 23 how the government submitted the proposal "policy for designed living environment" to the Parliament.The "policy for designed living environment" is a national architecture policy whose goal is, among other things, that architecture, design, and form should have a vital importance when it comes to spatial planning.The policy focuses on well-designed environments but also pays attention to long-term sustainability where architecture can be a tool to achieve change and promote sustainable development in both existing buildings and new constructions.The goal can be obtained by ensuring that factors such as quality and sustainability are not chosen for economic reasons and that knowledge about architecture, design, and form is disseminated and continues to develop.

Untreated Wood
The building and real estate sector in 2015 accounted for 18% of Sweden's total emissions of GHGs, of which 26% came from new construction and demolition, which included transport and work machinery.GHG emissions from the building and real estate sector have decreased sharply in the last 30 years as a result of actions such as stricter energy requirements and the development of the Swedish energy system.The progress in the construction and real estate sector has led to an increased focus on continuing the trend of climate-neutral products throughout the life cycle [50].
The factors that affect the environment during the production of wooden facades must be documented in the form of an Environmental Product Declaration (EPD).EPD contains a life cycle analysis of the material in a compressed format [50].The increased use of wood material leads to reduced environmental impact because the material binds carbon and, thus, does not contribute to increased GHGs.However, the climate is affected by the need for maintenance, the working life of the facade, and how forestry is managed.If one practices sustainable and ethical forestry, wood production does not contribute to GHG emissions.Forests must be managed responsibly, and wood should be sustainably produced [19].
Wood production requires less energy in the form of fossil fuels compared with, for example, steel and concrete.There is also a connection in reduced carbon dioxide emissions when using wood [50,53].
Wooden facades constitute a significant volume of the total amount of material used in the construction of a building [33].Furthermore, wood presents environmental benefits from production to management [54].

Painted Wood
Treating a wooden facade with paint contributes to the emission of GHGs.In accordance with [29], paint has a relatively high climate impact, and it is insisted that a longer maintenance interval would reduce the climate impact.However, the working life of the facade is reduced if periodic maintenance is extended.
When pesticides are added to paints, a careful balancing of their effectiveness and environmental impact is required.Since regulations govern how large an environmental impact the pesticides in paint can have, it sets limits for the composition of the paint [55].Biocides can contaminate the runoff water from buildings.Since biocides have a short degradation time, they are always on the ground around buildings due to the fact that the time period between rainfall in Sweden is shorter than the half-life of biocides [56].The biocides used today have a lower environmental impact and are more degradable without reducing their effectiveness [55].

Falu Rödfärg
Falu Rödfärg contains binders from renewable raw materials.Falu Rödfärg comprises a pigment mixture from the Falu copper mine that contains residues of copper, lead, and zinc compounds.Regarding the production of the pigment, the contaminated land around the copper mine is cleaned.The lead content in the pigment has nowadays been reduced to less than 0.5%.Falu Rödfärg often has biocides.Despite the environmental disadvantages, Falu Rödfärg is considered to have a low overall impact on the environment [26].
The most common outdoor paints in Folksam's large paint test were mapped and ranked in [57] according to their environmental impact and function after two years of the outdoor exposure.Falu Rödfärg was rated "Questionable Choice" because it was considered a "Good Choice" in terms of function but "Not Recommended" due to its environmental impact.Falu Rödfärg could not be assessed due to incomplete documentation; however, the paint was considered to contain environmentally hazardous substances both during manufacture and as a finished product.

Alkyd Oil Paint
The raw materials for the production of alkyd oil paint come from renewable raw materials such as pine oil or non-renewable raw materials such as crude oil [26].The production and use of crude oil affect the environment at different levels.The oil industry's biggest negative environmental impact is considered to be various forms of pollution.The oil industry also risks eradicating unique ecosystems and biological diversity [58].
The environment can be affected by the components of alkyd oil paint.The pigment titanium oxide is produced through manufacturing processes that cause a negative environmental impact.The environmental impact of alkyd oil paint also depends on whether it is solvent-based or water-based.Volatile organic compounds (VOCs) affect photosynthesis, which leads to increased aging and major damage to plants.Solvent-based alkyd oil paint emits, immediately after painting, high levels of VOCs that decrease over time, while water-based alkyd oil paint emits a more even level of VOCs over a longer period.Solvent-based alkyd oil paint contains 30%-50% hydrocarbons, which can, for example, increase the level of ground-level ozone.Water-based alkyd oil paint may contain organic solvents with an undefined effect on the environment.They may also contain metal salts and cobalt siccative, which are highly toxic to aquatic organisms [26].
With regard to the environmental impact, during the latter half of the 20th century, alkyd oil paint changed from solvent-based to water-based [19].Solvent-based paint has properties similar to linseed oil paint, while water-based paint resembles acrylate paint.

Acrylate Paint
The binder in acrylate paint is produced from non-renewable raw materials in the form of crude oil or natural gas [26].The production of natural gas does not contribute to particularly high emissions of carbon dioxide, but the gas consists of a large proportion of methane, which has a high climate impact [59].Titanium oxide has been used in the production of acrylate paint.An environmental advantage of acrylate paint is that it is water-based and does not contain the plasticizer phthalates [26].

Yakisugi
Since Yakisugi can be made in several different ways, the environmental impact differs depending on which method is used.If the wood is burned following the traditional method, carbon dioxide is formed, which contributes to the greenhouse effect.Burning the wood manually with LPG burns non-renewable sources, which also contributes to the greenhouse effect.If the wood is burned industrially in a machine, the energy can come from renewable sources such as water and wind or non-renewable sources such as nuclear and coal power [60].The climate is affected by the soot particles that are spread when burning the wood.The soot particles absorb heat in the air and can accelerate, for example, melting of the sea and inland ice [61].
Wood should not be burned with a manual method.In addition to the difficulty of producing a uniform wooden facade, it is not environmentally beneficial as it requires a large consumption of LPG [45].There are functional advantages to using a machine over LPG because the temperature can be more easily controlled, which gives a better result [29].Yakisugi does not apply any chemicals during the manufacturing process, has low maintenance and a long working life, and is, therefore, considered environmentally friendly [29,44].

Cost
The Swedish National Board of Housing, Building, and Planning [14] sets requirements for form, color, and material effect when constructing new buildings.They also require that the normal maintenance of a building be pursued during an economically reasonable working life.
The cost of maintaining a wooden facade depends on whether periodic or need-based maintenance is applied.The cost of maintenance also depends on whether the wooden facade is surface-treated or not.An untreated wooden facade is considered maintenancefree and, like Yakisugi, requires no financial investment.A painted wooden facade requires mandatory maintenance and, thus, financial efforts.An issue that should also be taken into account when maintaining painted facades is the number of coats required, as this can increase costs during the facade's working life.

Materials and Methods
The current research consists of the document and literature studies with collected secondary data supplemented with quantitative data.

Research Strategy
With the help of secondary data, various areas were examined broadly to create a basic knowledge.The widespread examination of the secondary data was refined into essential knowledge within this specific area.The secondary data were selected, compressed, and compiled into a theoretical framework.The important aspects of the theoretical framework regarding sustainability and environmental impact were tied together in a summary of the document and literature study.Parallel to the work with the secondary data, quantitative data were generated in the form of calculations, 3D modeling, and contact with building companies.The collection and structuring of quantitative data helped answer the cost differences between the selected surface treatments.

Document Study
The document study is based on magazines, reports, and industry websites, which laid the foundation for the theoretical framework.The documents were reviewed and analyzed in a source-critical and systematic way, with careful consideration of the content's relevance.Valuable information was substantiated with the help of scientific articles.

Literature Study Method of the Current Research
The articles were also selected geographically with a focus on regions that have a similar climate to Sweden to give the study a verifiable result.Areas of interest included Norway, Finland, and Denmark.In order to fulfill the purpose of the research, the secondary data and quantitative data were analyzed and are compiled in text, table, and figure format in the next sections.

Quantitative Data Collection
The quantitative data were collected from actual values from building companies in the form of quotes and Wikell's construction calculations, which resulted in a cost analysis.The received offers were based on the cost of maintenance for the facade, as well as the replacement and construction of a new facade.After processing and careful evaluation of the quotes and Wikell's construction calculations, tables could be drawn up to clarify the cost differences.The selection for the quotation request was made according to the study's geographical delimitation and was based on whether the company had available contact information.

Input Data for Cost Analysis
An inquiry document with specifications was sent to many building companies within the geographical delimitation.Finally, five companies gave a positive response to the request, which were Byggdesign Gävleborg AB, Holmgren Bygg AB, KOLSVART, Kossan o Kalven Bygg AB, and Pierre Entreprenad AB.
Wikell's construction calculations contained current prices regarding different paints and could give an overview of the cost difference for the paints.Regarding the maintenance interval, an average value was calculated for how often a wooden facade painted with the respective paint should be maintained [31].To obtain the number of times a wooden facade should be maintained, the working life of a facade was divided by the average value of the maintenance interval.To generate a total cost of maintenance during the whole working life of a facade, the cost of each paint was multiplied by the number of times the maintenance should be carried out.The result of the cost analysis was based on the construction companies' quotations because they ensure a professional execution that can provide a long working life.

Prototype Model
For the quotation request, a prototype model was created in the software Revit, as illustrated in Figure 1

Results and Discussion
This section presents the results of the secondary data and the quantitative data lection reported in previous sections.It is based on differences between untreated surface-treated wooden facades and how the surface treatments differ in terms of du bility, environmental impact, and cost.

Results and Discussion
This section presents the results of the secondary data and the quantitative data collection reported in previous sections.It is based on differences between untreated and surface-treated wooden facades and how the surface treatments differ in terms of durability, environmental impact, and cost.

Comparison of Untreated and Surface-Treated Wooden Facades
The differences between untreated and surface-treated wooden facades and their properties with respect to the durability and environment are highlighted.

Durability
The durability of a wooden facade is directly related to the proportion of heartwood and sapwood.The stem of a young tree is entirely made of sapwood.However, after a certain age, the sapwood close to the pith begins to convert into completely dead heartwood, depending on the species of the tree, climate, and geographic location of the tree stand [62,63].Heartwood development starts when extractive matter separates from the parenchyma cells, and resin is driven out from the cells of the resin canals [64].The tracheid's pit membrane moves to the border at the same time.The pit membrane seals the aperture with the torus owing to the abundance of adhesive extractives.This is called aspiration.As a result of the cessation of water and nutrient transport, all cells in the freshly created heartwood die in this way.The sapwood is covered in the heartwood layer.Even after the wood has dried, the permeability of the heartwood is lower than that of the sapwood because the heartwood consists of aspirated cells and has a higher concentration of extractives.All moisture transport in the living tree occurs in the sapwood [35].
A type of wood with a high proportion of heartwood has a natural protection against moisture and rot, which leads to a reduction in biological attacks.Regardless of whether one chooses untreated or surface-treated wood as a facade, it is important to use, for example, spruce, which has a high proportion of heartwood [19,35].
The theoretical framework has pointed out that untreated wood differs compared with surface-treated wood.The reason why untreated wood is considered durable is its ability to absorb and desorb moisture [19,22].Using untreated wood can cause cracks, followed by moisture movement, which is reduced if one uses a type of wood with a high proportion of heartwood [19,36,37].
Even solid untreated wood is affected and broken down by external factors (Table 2); the decomposition of discoloring fungi and UV radiation affects only the surface of the wood [36,38,39].Biological attacks by discoloring fungi and UV radiation do not reduce the durability of the wood but change the architectural feeling [19,35].Cracking and decomposition of the wood do not lead to any serious consequences if a need-based maintenance is followed [39].The results of this study demonstrate that paints have individual characteristics and differ in many ways.A common strength they have is their ability to protect the wood against external factors, provided that periodic maintenance is taken (Table 2).In accordance with [23], moisture can affect the durability and working life of paints.A good balance between absorption and desorption in paints is a requirement to achieve good durability.However, moisture movements in the wood can create cracks in alkyd oil and acrylate paint layers, which trap water and lead to rot damage [25].Falu Rödfärg is the paint that is most prone to cracking, but unlike alkyd oil and acrylate paints, there is no defined surface layer, which minimizes the risk of the rot damage [22,40].Falu Rödfärg's diffusion properties, which are similar to untreated wood, enable moisture movement and do not significantly impair the durability of the facade [22,40].
Falu Rödfärg has a tendency to flake more than alkyd oil paint, acrylate paint, and hybrid paint [40].However, Falu Rödfärg is considered to be permanent for a longer period.
It was reported in [43] that solvent-based alkyd oil paint, compared with water-based oil paint, with addition of biocides, penetrates further into the wood.Despite that, waterbased alkyd oil paint has a greater resistance to the outdoor exposure.However, solventbased alkyd oil paint has illustrated resistance to mold growth better than water-based alkyd oil paint, without the addition of biocides [42].
Treating the wooden facade with Yakisugi changes the properties of the wood.The changed properties depend on the heating temperature, and the changes occur at a temperature of 300 • C [32].Compared with untreated and painted wood, Yakisugi showed good properties in terms of durability (Table 3).If the wood is burned at the right temperature, the risk of cracking and degradation by the UV radiation is minimized [29,32,49].The hydrophobic carbon layer that occurs during firing neutralizes the cellulose and gives the wood a durable surface that is highly resistant to mold and chemical attack [48].Burning the wood also raises the pH value, which increases resistance to moisture and molds [45].Yakisugi provides good durability with a working life of 80 years or more [45,46].All these properties lead to the burnt wood having a significantly longer working life compared with the painted wood.The production of wood requires less energy compared with other building materials and has environmental benefits from the cradle to the grave [53,54].Wood production does not contribute to any GHGs if sustainable and ethical forestry is carried out [19].
A positive feature of untreated wood as a facade is that one can count on minimal maintenance compared with paints [15,20,21,35].The minimal maintenance leads to a lower environmental impact because no surface treatment with harmful substances is used [19,54].
Paints have several shortcomings in terms of the environmental impact, for example, added biocides contaminate runoff from buildings.However, these are necessary for paints to resist biological attacks [56].
Falu Rödfärg is the paint that has the least impact on the environment because its binder comes from renewable raw materials, and the lead content in the pigment was reduced to less than 0.5% [26].In addition, the production of the pigment in Falu Rödfärg contributes to reduced pollution in the soil around the Falu copper mine.However, Falu Rödfärg has a higher maintenance interval compared with alkyd oil and acrylate paints, which affects the environment [19].
The production of alkyd oil paint affects the environment to some extent as the raw materials come from both pine oil and crude oil, which cause air pollution and affect ecosystems and biological diversity [26,58].Because solvent-based alkyd oil paint contains hydrocarbons and emits high levels of VOCs, these were replaced by water-based alkyd oil paint for environmental reasons [26].Water-based alkyd oil paint has organic solvents but emits a more even content of VOCs [26].Alkyd oil paint requires less maintenance than Falu Rödfärg, which results in a lower environmental impact [19].
The binder in acrylate paint comes from crude oil or natural gas, which has a high environmental impact [26].Natural gas consists of methane, which affects the climate more than carbon dioxide [59].The advantages of acrylate paint are that it is water-based, contains no additives of phthalates, and has a lower maintenance interval than Falu Rödfärg and alkyd oil paint [19,26].
If Yakisugi is performed traditionally, manually, or mechanically, carbon dioxide is formed, which has a negative impact on the environment.The manual method of burning wood should be avoided as a large consumption of LPG is required [45].If the wood is burned in a machine, the energy can come from both nuclear and coal powers, which contributes to carbon dioxide emissions.If one instead uses energy from renewable sources, the environment is not affected to the same extent [60].All burning techniques create soot particles that absorb heat in the air and can accelerate ice melting north of the polar circle [61].A positive feature of Yakisugi is that no chemicals are used during the manufacturing process and the surface treatment is, therefore, considered to have environmental benefits [29,44].

Differences between Selected Surface Treatments for Wooden Facades in Terms of Durability, Environmental Impact, and Cost
Of all the considered surface treatments, Yakisugi provides the best characteristics regarding all external factors of the degradation process of the wood that were presented in Section 4.1.1.Furthermore, Figure 2 shows the overview of the surface treatments that indicate the best characteristics regarding degradation factors arising from the outdoors.All the surface treatments affect the environment to varying degrees, as can be seen from Figure 3.The environmental impact that Yakisugi causes is, compared with other paints, considerably lower both during manufacture and its working life.Paints affect the environment both during manufacture and as a finished product, and they also require periodic maintenance.Yakisugi emits carbon dioxide during the burning of wood, but no environmentally hazardous substances are added during manufacture or as a finished product.All the surface treatments affect the environment to varying degrees, as can be seen from Figure 3.The environmental impact that Yakisugi causes is, compared with other paints, considerably lower both during manufacture and its working life.Paints affect the environment both during manufacture and as a finished product, and they also require periodic maintenance.Yakisugi emits carbon dioxide during the burning of wood, but no environmentally hazardous substances are added during manufacture or as a finished product.
environment both during manufacture and as a finished product, and they also require periodic maintenance.Yakisugi emits carbon dioxide during the burning of wood, but no environmentally hazardous substances are added during manufacture or as a finished product.

Cost
The cost analysis in this research work examines the maintenance carried out by private persons.It also evaluates the maintenance and replacement of the facade conducted by a construction company.The quotes received from the construction companies based on a facade area of 162 m 2 are the basis for the calculations.
In order to make a fair assessment of the differences between the surface treatments in terms of cost, the costs of the facade maintenance and facade replacement were assessed.A wooden facade has a working life of approximately 60 years if periodic maintenance is performed [33,34], which is used for the cost analysis.

Maintenance by Private Persons
The number of coats needed for each paint is usually controlled by the supplier's instructions; however, this study follows the AMA Hus paint type, which is indicated in Wikell's construction calculations.According to AMA Hus, two coats of the respective paint are required.Table 3 lists the costs of facade maintenance, including ROT by private persons.ROT is tax-deduction for domestic service work in the reparation, renovation, and maintenance of houses.
Table 4 provides the maintenance interval and total cost of a painted wooden facade of 162 m 2 over a working life of 60 years when conducted by private persons.Materials and scaffolding are not included in the cost.Based on the table, during a facade's working life of 60 years, Falu Rödfärg is the cheapest choice of paint with a cost of SEK 105,664.The most expensive choice is alkyd oil paint at SEK 150,709.

Maintenance by Construction Companies
If a construction company is hired to maintain a facade, labor costs will be added.Four cost quotations were received from the construction companies, including value-added tax (VAT) for facade maintenance with paint, as illustrated in Figure 4. From these, an average cost of SEK 93,813 was obtained.The figure also displays the cost differences between the quotes, including VAT from the construction companies.KOLSVART's quote regarding maintenance was not received as the method was considered to be maintenance-free.Acrylate paint turned out to be the cheapest choice of paint over the working li a wooden facade at SEK 416,947, as can be observed from Table 5.The most expen choice turned out to be Falu Rödfärg with a cost of SEK 662,209.Five quotations were received from five construction companies for costs o change and construction of a new facade.Figure 5 illustrates these costs.The average for replacing the facade was SEK 230,000.
The labor costs for the construction of a wooden facade were estimated at 165,000 by Kossan o Kalven Bygg AB and SEK 140,000 by Byggdesign Gävleborg AB. average value of the labor costs, thus, became SEK 152,500.The reason why an estim labor cost was produced was to be able to make a fair comparison of the cost of repla a wooden facade surface-treated with paint and erecting a new facade surface-tre with Yakisugi.The cost, including VAT and labor costs for constructing a new facad KOLSVART was SEK 305,590.The differences in costs for changing and erecting a fa are also depicted in the figure.Acrylate paint turned out to be the cheapest choice of paint over the working life of a wooden facade at SEK 416,947, as can be observed from Table 5.The most expensive choice turned out to be Falu Rödfärg with a cost of SEK 662,209.

Replacement of Facade by Construction Companies
Five quotations were received from five construction companies for costs of the change and construction of a new facade.Figure 5 illustrates these costs.The average cost for replacing the facade was SEK 230,000.
The labor costs for the construction of a wooden facade were estimated at SEK 165,000 by Kossan o Kalven Bygg AB and SEK 140,000 by Byggdesign Gävleborg AB.The average value of the labor costs, thus, became SEK 152,500.The reason why an estimated labor cost was produced was to be able to make a fair comparison of the cost of replacing a wooden facade surface-treated with paint and erecting a new facade surface-treated with Yakisugi.The cost, including VAT and labor costs for constructing a new facade, by KOLSVART was SEK 305,590.The differences in costs for changing and erecting a facade are also depicted in the figure.

Difference in Cost between Maintenance Carried out by Private Persons and Construction Companies
As can be seen from Table 6, there are big differences in costs if one chooses to perform the maintenance by him/herself or if one hires a construction company.The maintenance of Falu Rödfärg demonstrates the biggest difference with SEK 556,545 and acrylate paint indicates the least difference with SEK 297,356.If the maintenance were conducted by a private person, costs for materials and scaffolding would not be included in the calculations.

Untreated Wood
Untreated wood has proven to be durable over a long period despite not having a paint finish.One of the reasons why untreated wood has proven to be durable is that the large moisture ratio variations do not affect the wood to the same extent as when a painted surface layer can seal in the moisture.The moisture inside the paint can cause cracks and rot fungi, which leads to increased maintenance that most users may find problematic.Users may lack knowledge about untreated wood and its positive properties.

Difference in Cost between Maintenance Carried out by Private Persons and Construction Companies
As can be seen from Table 6, there are big differences in costs if one chooses to perform the maintenance by him/herself or if one hires a construction company.The maintenance of Falu Rödfärg demonstrates the biggest difference with SEK 556,545 and acrylate paint indicates the least difference with SEK 297,356.If the maintenance were conducted by a private person, costs for materials and scaffolding would not be included in the calculations.

Untreated Wood
Untreated wood has proven to be durable over a long period despite not having a paint finish.One of the reasons why untreated wood has proven to be durable is that the large moisture ratio variations do not affect the wood to the same extent as when a painted surface layer can seal in the moisture.The moisture inside the paint can cause cracks and rot fungi, which leads to increased maintenance that most users may find problematic.Users may lack knowledge about untreated wood and its positive properties.

Painted Wood
The construction industry has a responsibility to inform users about surface treatments without taking economic aspects into account.Since untreated wooden facades have provided good properties, there is a certain suspicion that paint and construction traders advocate paints because financial motives guide their recommendations.The studies have shown that paints require periodic maintenance, which a user should be aware of since it takes both time and money to maintain a painted wooden facade.
Moving from standardized paints to untreated wooden facades or Yakisugi would probably lead to an economic debate because paint manufacturers and craftsmen would have to change and develop their businesses.However, the reduction would take place over a long period because surface-treated wooden facades that are erected today need continued periodic maintenance during the facade's working life.
This research has pointed out several problems that can arise with the surface treatment of a wooden facade with paint.Today, biocides and other dangerous substances are added to most paints to resist mold growth, which results in environmental consequences.Alkyd oil paint is the surface treatment that has caused the most disadvantages in terms of durability and environmental impact.A study by [42] presents that solvent-based alkyd oil paint, without added biocides, surprisingly resists mold growth relatively well.If so, biocide additives could be phased out and alkyd oil paint would have less impact on the environment.However, the fact remains that paint contains hydrocarbons and emits high levels of VOCs.As people increasingly switch from solvent-based alkyd oil paint with a high content of VOCs to water-based alkyd oil paint, these levels decrease.
Acrylate paint is water-based, and it is, thus, more environmentally friendly than alkyd oil paint.However, acrylate paint consists of non-renewable raw materials that affect the environment.Although Falu Rödfärg is composed of renewable raw materials, it is the surface treatment that requires the most maintenance and has a lead content, which should be phased out completely.
It can be stated that surface treatments in the form of paints lead to several consequences at the expense of the environment.The maintenance of paints can also result in high costs.The quotations illustrated that the initial cost of erecting a facade treated with Yakisugi was higher than a painted facade.The initial cost of Yakisugi was just over SEK 300,000, which can be compared with the paints, which during the working life, indicated total costs of approximately SEK 400,000-700,000 for maintenance by a construction company.
None of the investigated paints are completely compatible with each other; the only one that has provided versatility is alkyd oil paint, which can also be painted on top of acrylate paint.If one chooses Falu Rödfärg for new construction, one is locked in the choice and cannot maintain the facade with an alkyd oil or acrylate paint in the future.Should the user for some reason carry out maintenance with incompatible paints, it may lead to having to replace the facade, which is not recommended as the study demonstrates that it is a costly action.
At first glance, one might think that Falu Rödfärg is the cheapest paint.However, after considering the facade's estimated working life of 60 years and the fact that the work is performed by a company, the result is completely different.Falu Rödfärg has a tight maintenance interval and is the cheapest to maintain privately.If a construction company maintains the facade painted with Falu Rödfärg, the cost will be higher; the difference can be as large as SEK 550,000.As a private person, one should consider whether it is the economic or the functional aspect that is decisive in the facade maintenance.Incorrect execution by a private person can lead to serious consequences, for example, an alkyd oil paint that is painted with excessively thick layers can cause cracking, which can result in moisture problems.
In recent years, hybrid paints have increased their market share due to their adaptability to current environmental requirements as well as improved properties regarding moisture resistance and adhesion.As it spreads, there will be room for stricter environmental requirements, which would lead to a decrease in the market for alkyd oil and acrylate paints.Over time, hybrid paints will probably replace today's paints, but the need for maintenance remains, which leads to continued environmental impact and high costs.

Yakisugi
Of the investigated surface treatments, Yakisugi has the most positive properties.The study's results are interesting considering how conventional it is to surface treat the wood with paints.If Yakisugi is conducted correctly, the wood receives strong protection against external factors, which makes the facade largely maintenance-free.A good prerequisite is to use spruce instead of pine and perform firing at the right temperature, which is an important aspect for the supplier to take into account.Users who choose Yakisugi do not have to be burdened with mandatory maintenance or feel that they are contributing to the negative environmental impact that some paints cause.Yakisugi has a certain environmental impact as burning requires energy, generates carbon dioxide, and emits soot particles.
Burnt wood as a facade has a long life, but the carbon layer can break down over time like untreated wood, which erodes during its working life.Although untreated wood and Yakisugi have a long working life, there is more evidence that Yakisugi generally ensures protection against cracks and rot.
An increased use of Yakisugi could contribute to Sweden achieving the set national environmental and climate goals.The goals could also be realized if the use of biocidal products, as well as other dangerous substances and non-renewable raw materials in paints, are phased out.In addition to national regulations, there are also requirements regarding aesthetics and technical properties, durability, and the environment.To reach these goals, it is essential that users and suppliers are aware of environmentally friendly and climate-neutral surface treatments.
This research presents, in addition to environmental benefits, that Yakisugi results in lower costs over the working life of a wooden facade compared with a surface treatment by paints, considering all the aspects that must be taken into account when maintaining a painted surface.

Conclusions
The main goal of this study was to facilitate the users' choice of untreated or surfacetreated wooden facades for buildings.Several aspects were considered to guide users.In order to have a good choice, a wooden facade must be durable, have a low environmental impact, and not be too expensive.Hence, in this research, untreated and surface-treated wooden facades were compared with regard to durability, environmental impact, and cost.The research work was done using the document and literature studies supplemented with quantitative data.The properties of untreated and surface-treated wood were investigated, and their ability to resist external factors was analyzed.The following conclusions can be drawn from the study:

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Despite the fact that untreated wooden facade is affected by external factors, it is considered a suitable choice as the degradation does not affect the durability to a large extent, while at the same time, it has a low environmental impact and does not require any periodic maintenance.

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It was shown that surface treatments in the form of paints and Yakisugi had different properties despite having a similar function.

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The surface treatments acted as a barrier for the underlying wood, and their durability is crucial for the maintenance and longevity of the facade.

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The results revealed that Yakisugi is the most suitable choice of wooden facade in terms of durability, environmental impact, and cost.

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Yakisugi as a surface treatment provides a stable facade with a long working life.

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The hydrophobic carbon layer created by Yakisugi, which forms a permanent surface with resistance to external factors, minimizes maintenance and, thus, costs.

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An increased use of Yakisugi contributes to sustainable development and a distinctive architectural expression.

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Unconventional surface treatments of wooden facades should be promoted in the future to fulfill the increasing environmental demands.

Figure 1 .
Figure 1.Prototype model for calculation of facade area.

Figure 1 .
Figure 1.Prototype model for calculation of facade area.

Figure 2 .
Figure 2. Overview of surface treatments demonstrating best characteristics regarding degradation factors arising from outdoor exposure.

Figure 2 .
Figure 2. Overview of surface treatments demonstrating best characteristics regarding degradation factors arising from outdoor exposure.

Figure 3 .
Figure 3. Overview of environmental impact of surface treatments during manufacture and use.

Figure 3 .
Figure 3. Overview of environmental impact of surface treatments during manufacture and use.

Coatings 2023 , 18 Figure 4 .
Figure 4. Differences in total costs for maintenance of a wooden facade during its working life formed by companies.

Figure 4 .
Figure 4. Differences in total costs for maintenance of a wooden facade during its working life performed by companies.

Figure 5 .
Figure 5. Differences in total costs for change and construction of a new wooden facade performed by companies (All the total costs considered by companies are for the change of a facade except the case of KOLSVART, which is for construction of a new facade).

Figure 5 .
Figure 5. Differences in total costs for change and construction of a new wooden facade performed by companies (All the total costs considered by companies are for the change of a facade except the case of KOLSVART, which is for construction of a new facade).

Table 1 .
[27]ts compatible with previously conducted paint on new construction in terms of maintenance[27].

Table 2 .
External factors and their effect on untreated and surface-treated wood in terms of durability, provided that periodic maintenance is performed.

Table 3 .
Costs of a wooden facade maintenance conducted by private persons.

Table 4 .
Maintenance interval and total costs of a painted wooden facade over its working life done by private persons.

Table 5 .
Maintenance interval and total cost of a painted wooden facade over its working life ducted by companies.

Table 5 .
Maintenance interval and total cost of a painted wooden facade over its working life conducted by companies.

Table 6 .
Comparison of maintenance costs of a painted wooden facade over its working life done by private persons and companies.

Table 6 .
Comparison of maintenance costs of a painted wooden facade over its working life done by private persons and companies.