A Passive Wood-Based Building in Slovakia: Exploring the Life Cycle Impact

Abstract

The aim of the study is to point out the burden of passive wood-based buildings throughout the life cycle from the environmental point of view to better understand the consequences and importance of building design in Slovakia. The analysis was carried out according to the Life Cycle Assessment methodology. The results were calculated by the CML-IA baseline method. The impacts of the product stage and operational energy use were the highest throughout the considered life cycle. Substances contributing to eleven impact categories were identified. Foundations, especially foam glass, were found to bear the majority of the impact of the overall construction materials. The normalization category showed considerable impact on marine aquatic ecotoxicity mainly due to building energy consumption over the course of 50 years. Loads connected to the replacement stage were the third highest. The study also proved high demand on elements of photovoltaics.

1. Introduction

Currently, one of the biggest issues worldwide is to mitigate climate change. In recent decades, global society has faced constant environmental problems and challenges, such as increasing greenhouse gases emissions (especially CO₂), higher energy demand and consumption, and ultimately, global warming and climate change. The building construction industry is one of the major contributors to global carbon emissions. Greenhouse gases produced by buildings and the construction industry represent about 39% of global CO₂ emissions [1,2,3]. Therefore, it is necessary to reduce the energy consumption of buildings. Passive buildings constitute high energy saving systems by decreasing the thermal transmittance of the building envelope together with controlling ventilation and airtightness [4]. It is generally accepted that the energy demand for heating decreases with improved thermal-technical properties of external structures. On the other side, the scientific literature provides evidence that mitigating the effects of a building’s operation does not in itself ensure an overall improvement in its environmental performance [5].

The passive building requirements are among the most stringent energy efficiency standards for buildings [6] and can meet the range of high energy performance requirements set by the European Commission [7]. A Passive House is defined as a building, for which thermal comfort (ISO 7730) can be achieved solely by post-heating or post-cooling of the fresh air mass, which is required to achieve sufficient indoor air quality conditions—without the need for additional recirculation of air. Furthermore, the Commission also suggests passive housing to be a key strategy to shift towards a low-carbon economy by 2050 [8].

Each human activity affects the environment. The Life Cycle Assessment (LCA) methodology expresses the impacts of human activities in units corresponding to a specific impact category. The Centre of Environmental Science of Leiden University [9] proposed a set of 11 impact categories that include abiotic depletion of elements and fossil fuels, the potential for global warming, ozone depletion, photochemical oxidation acidification and eutrophication, toxicity to humans, and freshwater, marine, and terrestrial ecotoxicity.

The building sector is responsible for 19% of the global energy-related greenhouse gases (GHGs) emissions [10] and 36% of the total CO₂ emissions in Europe [11]. Sartori, I., Hestnes, A.G., and Karimpour, M. et al. [12,13] show that the share of production energy can increase up to 60% of the total life cycle energy use, as the operational energy use is reduced through thermal envelope improvement. Case studies on structures built according to different design criteria, and at parity of all other conditions, showed that the design of low-energy buildings induces both a net benefit in the total life cycle energy demand and an increase in the embodied energy. Retrofitting existing buildings to high energy performance levels reduces the operational energy use and carbon dioxide equivalent (CO₂eq) emission, but usually entails the use of additional materials.

The choice of construction materials affects the building’s environmental impact within the whole life cycle [14]. Studies from several countries show that wood materials used in building frames usually release less CO₂ than other materials throughout the life cycle [15,16,17]. This is due to the relatively small amount of energy needed to manufacture wood products compared to other materials and the opportunity to replace fossil fuels with wood byproducts during the manufacturing process. Dodoo et al. [18] also show that the post-use energy recovery of wood has a higher carbon benefit compared to both the recycling of reinforcing steel and the carbonation of post-use concrete from reinforced concrete. It was demonstrated that embodied energy can represent 35% of the future emissions target of a building in a mild climate [13]. The findings of another work [19] indicated that concrete is the greatest contributor across all environmental impact categories, except in the abiotic depletion of elements category, and that employing insulating materials in the building’s wall systems can have a beneficial effect in the energy efficiency of a building, without substantially burdening its total embodied energy.

Buildings are difficult to compare with each other in terms of energy efficiency notwithstanding the other parameters [20]. Energy-efficient buildings, sustainable buildings, smart buildings, nearly zero-energy buildings, and passive and active buildings are construction concepts widely recognized as the latest trends. The purpose of their design is to create an optimal thermal microclimate by means of heat flows that are either formed within the building or enter it [21]. High-performance building envelopes are designed to achieve target reductions in operational energy. However, these wall assemblies often require initial investments in material cost and manufacturing energy. Wood-framed wall systems were the most cost-effective and exhibited lower LCE (life cycle energy) and LCC (life cycle cost) compared with the other wall assemblies [22].

Life Cycle Assessment (LCA) is one of the environmental management tools that evaluate the impact of a product on the environment. Its principles and guidelines are described in respective legislation [23,24]. In overall consumption, all energy from the production and distribution of materials, construction, and use to the eventual disposal of the building and generated waste must be included [12]. LCA methods can be applied in various scopes, particularly for building materials and their production [25,26,27] or to buildings as a whole [28,29]. Standards of ISO 14040 and 14044 [30,31] define the life cycle stages of buildings. LCA is a detailed methodology, and its application is internationally harmonized, standardized, and used [32].

Wood-based buildings cause relatively small environmental damage as they use environmentally friendly materials [33]. Wood-based construction materials are generally considered sustainable as they bind carbon until they become waste [34]. A careful selection of materials can reduce the net CO₂eq by up to 68%, especially when using wood materials. Results show that the operation CO₂eq emission decreases by between 50% and 82% in the retrofitted buildings depending on the passive house requirement. The non-operation CO₂eq emissions correspond to 4% to 25% of the operation CO₂-eq savings depending on the passive house standard and material option [14]. The research results of Mitterpach et al. [35] also showed that the material assembly of the external wall significantly influences the life cycle impact assessment result. The selection of structural and especially insulating materials plays an important role in the case of wood constructions, as the insulation is more than 80% of the volume of the wood-based structures on average. Results of the life cycle assessment for single-family housing show that the majority of the life cycle environmental impacts (95%) occur at the use and operation stage. Only 4% is added by the pre-use stage and 1% by the post-use stage [36].

Due to population growth and consequent expansion of industry and services, the need for energy is continuously increasing [37]. Energy consumption plays a substantial role in global warming as most of this energy comes from fossil fuels. Therefore, the transition to renewable energy sources could partially reduce the negative effects of energy needs. The share of renewable energy in the global energy mix should increase substantially by 2030 [38].

The current practice of energy-environmental assessments of buildings, which focuses only on the operational stage, reveals a gap in knowledge about the overall impact on the environment [36,37,38]. Many scientific studies examine or analyze the calculated, simulated, or actual measured energy consumption, emissions, and other impacts, only during the operational stage of a building. However, relevant knowledge and data on the overall impact throughout the life cycle are lacking. This article presents a well-known and widely applied method of evaluating buildings, construction systems, and materials for individual quintessential solutions (including the project in question) in terms of the complex impact on the environment and energy consumption taking into account the impacts on the use of natural resources and occupant health, as well as the impacts on the environment in terms of climate change, ozone layer depletion, ecotoxicity, acidification, and eutrophication. It analyzes the characteristic reference family house, which reflects the current typical low-energy and green building.

The aim of this study was to identify the selected environmental impacts of a two-story wood-based passive building throughout the selected life cycle based on the CML-IA baseline calculation method [9] in the national conditions of Slovakia. The study aims at life cycle stages and substance contribution analysis from raw material extraction, the production of construction materials, the construction process, and the use of the building. The results could provide valuable information for LCA experts and help stakeholders in making decisions.

2. Materials and Methods

The reference structure was a two-story residential building with a simple compact shape and rectangular floor plan (Figure 1, Figure 2 and Figure 3) representing a typical family house for a family of four characterized by a gross floor area (GFA) of 210.0 m² and a gross internal area (GIA) of 159.6 m², which covers 80–90% of similar residential buildings. The choice of construction system corresponded to the economically most widespread timber frame construction system based on ecological and renewable wood raw material and regular filling materials available on the market. Moreover, the construction and technologies were designed to meet the requirements for nearly-zero-energy buildings—energy efficiency class A0—in accordance with the Directive on the energy performance of buildings [7], which is a requirement for all new residential buildings since 2021.

A reinforced concrete foundation slab was thermally insulated with 600 mm foam glass granules. The roof was designed as flat with extensive vegetation and photovoltaic panels. The load-bearing construction of the roof and the exterior walls was made from wood-stud frames. The mineral wool was placed between the studs to ensure the required insulation. The exterior walls were designed as a diffusely open construction. A brick placed in the ceiling and in the inner partition walls served as an energy accumulation layer. The high standard of thermal protection was complemented by high-quality, low-energy wood-frame windows with triple glazing presented by an extremely low heat transfer coefficient U = 0.8 W/(m²·K). However, the transmittance of solar radiation (g = 0.63) was high. The supply of thermal energy came from a heat pump with a heating output of 3 kW. Heating was provided by the wall heating system and supported by controlled ventilation with recuperation. The heating system was also used for summer cooling. The system ensured thermal comfort with optimal heat distribution in the winter heating season and low-energy cooling in the case of extreme summer heat. All devices were controlled and harmonized by a central intelligent control system. The calculated specific heat demand for heating of the construction was 13.3 kWh/(m²·a).

The functional unit was represented by the whole reference structure with a 50-year lifespan. The construction consisted of a timber structure and other components listed in

Table 1. Specification of construction materials.

Structure	Specification
Foundations	Gravel
	Foam glass
	Concrete
	Reinforcement
External walls	KVH Structural timber
	Oriented strand board (OSB)
	High density fiberboard (HDF)
	Rock wool
	Gypsum plasterboard
	Steel connections
	Mineral plaster
Partitions I	KVH Structural timber
	Oriented strand board (OSB)
	Brick
	Gypsum plasterboard
	Steel connections
Partitions II	KVH Structural timber
	Rock wool
	Gypsum plasterboard
	Steel connections
Flooring	KVH Structural timber
	Steel connections
Roof	KVH Structural timber
	Oriented strand board (OSB)
	High density fiberboard (HDF)
	Rock wool
	Gypsum plasterboard
	Steel connections
	Extensive vegetation
Openings	Wood-glass entrance door
	Wood inner door
	Wood-frame windows with triple glazing

. Data on the components were taken from the building area statement for the reference building. Geotextiles, waterproof foil, and a vapor barrier were excluded from the assessment. Life cycle inventory details of construction materials are given in

Table A1. Life cycle inventory of input construction materials.

Construction Material	Database *	Share on Overall Construction Weight (%) **
Gravel	Gravel, crushed {RoW}|market for gravel, crushed|APOS, U	15.82
Foam glass	Foam glass {GLO}|market for|APOS, U	8.05
Concrete	Concrete, 25 MPa {RoW}|market for concrete, 25 MPa|APOS, U	30.15
Reinforcement	Reinforcing steel {GLO}|market for|APOS, U	1.00
KVH Structural timber	Own assumption 1	10.70
Oriented strand board (OSB)	Oriented strand board {GLO}|market for|APOS, U	5.99
High density fiberboard (HDF)	Fiberboard, hard {GLO}|market for|APOS, U	1.87
Rock wool	Stone wool, packed {GLO}|market for stone wool, packed|APOS, U	1.36
Gypsum plasterboard	Gypsum plasterboard {GLO}|market for|APOS, U	5.12
Steel connections	Steel, chromium steel 18/8 {GLO}|market for|APOS, U	1.55
Mineral plaster	Base plaster {GLO}|market for|APOS, U	1.25
Brick	Clay brick {GLO}|market for|APOS, U	3.84
Extensive vegetation	Own assumption 2	11.21
Windows and doors	Door, outer, wood-glass {GLO}|market for|APOS, U Door, inner, wood {GLO}|market for|APOS, U Window frame, wood, U = 1.5 W/(m2·K) {GLO}|market for|APOS, U Glazing, triple, U < 0.5 W/m2K {GLO}|market for|APOS, U	2.08
Recuperation system	Ventilation system, central, 1 × 720 m3/h, polyethylene ducts, with earth tube heat exchanger {GLO}|market for|APOS, U	-

* Data refer to ecoinvent database [40]. ** total weight of construction was 149.935 t. ¹ Database is based on the following assumption: Density of KVH is 440 kg·m⁻³. 1 m³ of Sawnwood, beam, softwood, raw, dried (u = 10%) {GLO}|market for|APOS, U and 0.3 kg of Urea formaldehyde resin {RER}|market for urea formaldehyde resin|APOS, U is used to manufacture 1 m³ of KVH and 141.67 kWh of Electricity, medium voltage {Europe without Switzerland}|market group for|APOS, U is consumed. ² Database is based on the following assumption: Inputs to manufacture 1 kg of the substrate for extensive vegetation consist of: 0.05 kg of Compost (industrial) market for; 0.45 kg of Cleft timber, measured as dry mass {Europe without Switzerland}|market for|APOS, U; 0.40 kg of Shale {GLO}|market for|APOS, U; 0.10 kg of Peat {GLO}|market for|APOS, U; 0.01 kg of Grass seed, organic, for sowing {GLO}|market for|APOS, U and 0.028 kg of Packaging film, low density polyethylene {GLO}|market for|APOS, U. Transportation accounts for 0.03052 tkm of Transport, freight, lorry > 32 metric ton, euro5 {RER}|market for transport, freight, lorry > 32 metric ton, EURO5|APOS, U. Energy requirements are 0.00185 kWh of Electricity, low voltage {SK}|market for|APOS, U and 0.00227 MJ of Heat, central or small-scale, natural gas {Europe without Switzerland}|market for heat, central or small-scale, natural gas|APOS, U.

.

According to EN 15978 [30], the system boundaries (Figure 4) were set from stage A1 to B7. The analysis was carried out using the CML-IA baseline v3.06 [8] calculation method and SimaPro 9.1.1.1 Analyst software [39] developed by PRé Consultants in the Netherlands. LCI data were taken from ecoinvent v3.6 [40]. Slovak datasets were only used for energy consumption, and the remaining datasets covered the average global data. Attributional modeling, especially Allocation at the Point of Substitution (APOS), was chosen to reflect the average impact of products within the supply chain.

The selected calculation method transforms input and output materials and energy flows into a specific impact on the environment through default characterization factors. The CML-IA baseline method covers the impact of the depletion of abiotic elements and fossil fuels, global warming potential, ozone depletion potential, toxicity on humans, freshwater, and marine aquatic environments, terrestrial ecotoxicity, and the potential for photochemical oxidation, acidification, and eutrophication.

Transportation distances from the distributor to the construction site were assumed based on qualified estimates taking into account actual Slovak conditions described in

Table 2. Modelling transportation distances from distributor to the construction site.

Distance (km)	Product
100	Timber
	Steel
	Gravel
	Foam glass
	Windows and doors
50	OSB
	HDF
	Gypsum plasterboard
	Rock wool
	Brick
30	Concrete
20	Extensive vegetation
	Base plaster
	Transport of excavator
200	Recuperation system

. Vehicles meeting the Euro 5 European emission standards were chosen for transportation.

The construction activities of the A5 stage requiring heavy machinery consisted mainly of groundworks, landscaping, and foundation building, estimated to last 11 h. The rest of the construction was assumed to be built manually. The electricity consumption for electrical appliances used during construction was set to 2.6 kWh. Construction wastes were assumed to be 10% used KVH timber, OSB, HDF, gypsum fiberboard, rock wool, and brick. All construction wastes were considered to be transported 20 km and landfilled.

Emissions from the B1 stage included leakages of cooling agents from the heat pump system, assumed to reach 10% of the overall coolants consumption over 50 years.

Maintenance (B2), replacement (B4), and refurbishment (B5) of the building were scheduled according to the manufacturer’s instructions and assumptions listed in

Table 3. Specification of building maintenance, replacement, and refurbishment.

Stage	Activity	Frequency
B2 Maintenance	Varnishing	Interior—every 20 years
		Exterior—every 10 years
	Cleaning of recuperation and photovoltaic panels Cleaning of windows	Two times a year
	Cleaning of floor Vacuum cleaning	Every week
B4 Replacement	Filters for recuperation system	Once a year
	Blower and heat exchange unit Heat pump	Every 20 years
	Photovoltaic panel	Every 30 years
	Coolants	Every 16 years
B5 Refurbishment	Inner door Repainting	Once in a lifetime

.

The B6 stage was modelled as follows: The energy consumption of the building was calculated according to the Slovak standard [41]. The annual electricity consumption was 4141.79 kWh and heat production from the heat pump was 5806.51 kWh. Furthermore, 95.51% of the consumed electricity was produced by photovoltaics, while the rest of the electricity was covered by public power distribution. The entire photovoltaic energy produced was considered as used in the building. Seasonal weather changes were not calculated. Controlled ventilation with recuperation together with the brine water heat pump circuit was used to support wall heating with distribution under plasterboard cladding. Since the recuperation system did not produce energy, it was assigned to the product stage. In the B7 stage, the water necessary for heating and humidification of vegetation roof over the course of 50 years was included, accounting for 13,451.5 liters. The handling of waste through life cycle stages is presented in

Table A2. Waste management of produced wastes.

Stage	Waste Type	Amount	Waste Scenario
A5 Construction process	Timber	1604.37 kg	L
	OSB	898.35 kg	FD
	HDF	280.28 kg	FD
	Gypsum plasterboard	767.22 kg	FD
	Rock wool	204.17 kg	L
	Brick	576.01 kg	FD
	LDPE waste foil	0.45 kg	M
	Reinforcement steel	72.50 kg	M
B2 Maintenance	Wastewater	32 m3	T
	Electronic waste	40 kg	M
	Waste paint	4 kg	I
B4 Replacement	Used air filter in central unit	50 u	T
	Used air filter in exhaust air	50 u	T
	Electronic scrap	54.67 kg	M
	Steel and iron	241.67 kg	R
	Waste glass sheet	3.33 kg	M
	Used blower and heat exchange unit	2.5 u	T
	Used refrigerant	8.23 kg	T
	Spent antifreezer liquid	178.75 kg	I
B5 Refurbishment	Used inner door	16.2 m2	FD
	Waste paint	2 kg	I
	Waste plastic	2 kg	M

Note: L—landfill; FD—final disposal; M—market; T—treatment of; I—incineration; R—recycling; u—unit. Market activities comprises various waste scenarios covering average global treatment of specific waste.

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3. Results

3.1. Impact Assessment of Individual Life Cycle Stages

Table 4. Characterization results of the building life cycle.

Stage	AD-E	AD-F	GWP	ODP	HT	FAE	MAE	TE	POP	AP	EP
A1–A3	7.85	865.08	78.92	5.65	267.34	86.86	142.73	763.29	24.52	393.34	189.53
A4	0.03	18.01	1.20	0.22	0.55	0.19	0.38	1.84	0.16	3.87	0.89
A5	0.03	8.20	1.03	0.05	11.60	2.28	4.39	17.94	0.80	11.28	14.75
B1	0.00	0.00	0.09	0.00	0.00	0.00	0.00	0.00	8.17	0.00	0.00
B2	0.03	18.65	1.72	0.21	2.08	2.65	4.75	11.45	0.57	12.50	7.50
B4	4.79	103.16	20.98	9.36	69.51	47.86	59.70	57.43	5.17	91.72	37.23
B5	0.13	13.81	1.26	0.09	0.95	0.73	2.01	10.19	0.62	7.96	5.55
B6	4.06	754.61	79.09	13.68	130.27	142.88	229.95	507.36	20.24	481.80	323.66
B7	0.00	0.05	0.00	0.00	0.00	0.00	0.01	0.06	0.00	0.02	0.01

Note: AD-E—abiotic depletion of elements (kg Sb eq); AD-F—abiotic depletion of fossil fuels (GJ); GWP—global warming potential (t CO₂ eq); ODP—ozone depletion potential (g CFC-11 eq); HT—human toxicity (t 1.4-DB eq); FAE—freshwater aquatic ecotoxicity (t 1.4-DB eq); MAE—marine aquatic ecotoxicity (kt 1.4-DB eq); TE—terrestrial ecotoxicity (kg 1.4-DB eq); POP—photochemical oxidation potential (kg 1.4-DB eq); AP—acidification potential (kg 1.4-DB eq); EP—eutrophication potential (kg 1.4-DB eq).

and Figure 5 reflect the impact of building life cycle stages. Product stages (A1–A3) had the highest impact across five categories—Abiotic Depletion either for elements (AD-E; 46.40%) or fossil resources (AD-F; 48.56%); Human Toxicity (HT; 55.43%); Terrestrial Ecotoxicity (TE; 55.73%); and Photochemical Oxidation Potential (POP; 40.70%).

The impact of stages A4, A5, B2, B5, and B7 was negligible. The emissions from Building Use (B1) had the third highest impact in POP accounting for 13.56%. The replacement stage (B4) had the second highest impact in AD-E (28.35%) and ODP (31.98%). The B6 stage corresponded to the highest negative environmental impact in six categories—Global Warming Potential (GWP; 42.91%); Ozone Depletion Potential (ODP; 46.75%); Fresh Water (FAE; 50.41%) and Marine Aquatic Ecotoxicity (MAE; 51.80%); Acidification (AP; 48.06%); and Eutrophication Potential (EP; 55.89%).

The product stage and stage B6 had nearly the same impact on GWP indicating the energy consumption over 50 years approximately matches the embodied energy of the construction materials.

Normalization assigns the magnitude of impact categories within a selected region in a year, thus providing the relative significance of the impact category indicator results. EU25 normalization factors were for, representing the impact of 25 European countries in 2006. MAE was determined to be the most-hit category. The share of particular life cycle stages on the overall MAE impact was as followed: 51.80% of the B6 stage, 32.15% of the A1–A3 stages, and 13.45% of the B4 stage.

3.2. Impact Assessment on Individual Impact Categories

3.2.1. Depletion of Abiotic Resources

Abiotic depletion is divided into element and fossil parts, expressed as the rate of extraction of minerals and the deaccumulation rate (kg Sb eq) and the lower heating value (MJ per kg of 1 m³ fossil fuel), respectively.

Elements’ depletion (Figure 6) was mainly caused by gold in nearly all life cycle stages except for the A5 and B1 stages. Altogether, gold reached 91.85% of all elements’ depletion. The impact of chromium (59.80%) dominated in the A5 stage, although it represented only 1.88% of all elements.

The depletion of fossil fuels (Figure 7) was primarily assigned to hard coal in A1–A3, A5, B4, B5, and B7 stages. The impact of brown coal prevailed in the B2 and B6 life cycle stages. The transportation of construction materials (A4) was especially bound to crude oil consumption (89.36%). The impact of natural gas varied and accounted for a maximum of one-third of the impact within the stages. Generally, the depletion of fossil fuels within the life cycle was dominated by hard coal (34.57%), followed by natural gas (25.03%), crude oil (21.22%), and brown coal (18.58%).

3.2.2. Global Warming Potential, Ozone Layer Depletion, and Photochemical Oxidation

At first, GWP was specified (Figure 8). Fossil CO₂ emissions accounted for more than 72% of the GHGs in nearly all stages, excluding B1 and B4. Emissions from the Replacement stage (B4) consisted of 57.24% HFC-134a and about 38% CO₂. Stage B1, referring to emissions during the normal use of the building, produced only HFC-134a emissions. More than 5% of the GHGs contribution were detected for fossil CH₄ (A1–A3, A5, B5, and B7) and 20.57% of those emissions were generated by biogenic methane in the A5 stage. Emissions of CO₂ from land transformation were detected to be 5% in the B5 stage.

In terms of the three most-polluting substances, fossil CO₂, HFC-134a, and fossil CH₄ emissions reached the highest share on overall GWP accounting for 83.05%, 8.71%, and 4.97%, respectively.

In the case of ODP, the most representative substance was halon 1301, which led in A1–A3, A4, A5, and B5 (Figure 9). The considerable prevalence of CFC-113 was detected in the B4 stage (76.22%). CFC-114 dominated in B2, B6, and B7. The rest of the freons formed a minority of the ozone depletion impact.

Overall assessment of the emitted ozone-depleting substances during the selected life cycle of the building showed 37.57% of that impact was caused by CFC-113, followed by CFC-114 (23.68%) and halon 1301 (15.78%).

Figure 10 explores the contribution of substances responsible for photochemical oxidation. Apart from the B1 stage, which was hit by propylene glycol, each of the rest of the life cycle stages suffered the largest impact from sulfur dioxide, from 38.46% of the impact on B5 to 78.06% of the impact on B6. The second most contributing substance was found to be fossil CO emissions responsible for 9.23% of the B6 impact and up to 39.95% of the A4 impact. Formaldehyde was detected in the A1–A3 and B5 stages, representing 7.07% and 7.70% of the impact, respectively. A high share of other emissions represented the vast amount of substances liable to photochemical oxidation.

According to the overall POP assessment, SO₂ was proved to be the most-emitted substance, referring to 29.50% of all contributing emissions (Figure 10). Next, fossil CO and propylene glycol accounted for 10.40% and 8.17% of the overall POP emissions.

3.2.3. Toxicity and Ecotoxicity

This section describes substances representing the potential toxic effect on human health (HT); freshwater (FAE) and marine aquatic environments (MAE); and the terrestrial environment (TE).

Figure 11 revealed thallium in the water as the most contributing substance to HT in B2, B4, B5, B6, and B7. Chromium emissions in the air were depicted to reach 73.32% and 90.41% of the A1–A3 and A5 impact, respectively. Antimony emissions prevailed in the A4 stage, peaking at 31.01%. The impact of selenium ranged from 7.48% (B5) to 15.48% (B7). Copper emissions were detected in the A4 stage (10.51%). Benzene emissions were responsible for 12.73% of the impact in the B5 stage and 7.24% of the impact in the B7 stage. Relatively high shares of other substances in A4 and B2-B7 account for many different pollutants with toxic natures.

Overall assessment of substances contributing to HT affirmed chromium and thallium as the two most-emitted pollutants, causing 44.75% and 33.48% of those emissions, respectively.

Figure 12 characterizes the distribution of substances responsible for FAE. The main pollutant was detected to be copper, mostly represented in the A4, B2, B4, B5, and B6 stages. Nickel emissions were dominant in A1–A3, A5, and B7. The second highest shares of FAE in the B5 (23.64%) and B7 stage (24.87%) belonged to vanadium. Cobalt was found to be the second highest pollution contributor (18.49%) in the A5 stage. The share of beryllium emissions ranged from 8.41% in the A1–A3 stages to 14.28% in the B5 stage.

Investigating the overall substance contribution to FAE proved the dominance of copper (47.31%) and nickel (23.05%) emissions followed by beryllium (10.24%). The share of cobalt and vanadium was 6.90% and 6.24%, respectively.

The impact on MAE (Figure 13) was predominantly caused by airborne hydrogen fluoride emissions, especially in the A5 (50.82%) and B5 (43.83%) stages; and beryllium leakages to water mostly burdening the rest of the life cycle stages, represented by 30.21% of the impact on A1–A3 up to 47.97% of the impact on B4. Nickel emissions were the second highest in the A5 stage (22.92%) and the third highest in the A1–A3 stages (16.68%). Other emissions created a minority of the ecotoxicity impact within the particular stages.

Altogether, beryllium, hydrogen fluoride, and nickel bore the majority of the MAE impact, accounting for 38.58%, 23.81%, and 10.24%, respectively. Copper and cobalt constituted 6.09% and 5.66% of those emissions, respectively.

The terrestrial environment (Figure 14) was hit by substances from each of the three compartments—air, water, and soil. Cypermethrin, a synthetic insecticide, caused the majority of the impact in the A1–A3 stages (47.38%) and the B5 stage (63.60%). Stages B2 (65.65%) and B6 (65.72%) were mostly affected by Cr⁶⁺ emissions in the soil. The same airborne emissions were found to bear 51.63% of the impact on A5 and involved the second highest share of emissions in the A1 to A3 stages (22.68%). Airborne mercury emissions mainly hit the B7, A4, and B4 stages, reaching 84.70%, 33.81%, and 32.14% of the impact, respectively. The impact of nickel in the air reached 17.60% in the B4 stage and was the second highest in this category.

The overall contribution of each toxic substance supports the findings that cypermethrin was the largest contributor to TE representing a share of 28.85% of emissions affecting the terrestrial environment. The rest were detected to be Cr⁶⁺ in the soil and airborne mercury and chromium emissions, responsible for 27.77%, 17.29%, and 13.92% of those emissions.

3.2.4. Acidification and Eutrophication

Acidification is primarily associated with combustion processes. The majority of the AP impact in all life cycle stages came from SO₂ emissions contributing at least 50.50% of the impact (A4) and up to 84.61% of the impact in the B2 stage (Figure 15). Ammonia was responsible for 7.41% of the impact in A1–A3 and 15.27% of the impact in B4. The NO_x impact distribution within the stages ranged from 13.54% to 48.72%.

Generally, SO₂ was the biggest contributor to AP, followed by NO_x and ammonia, accounting for 73.57%, 21.45%, and 4.98% of those emissions, respectively.

Eutrophication is a consequence of the enrichment of nutrients in the water, such as phosphorus and nitrogen, leading to decomposition processes. Figure 16 depicted phosphate leakages into the water as representing the highest burden of the A1–A3, B2, B4, B6, and B7 stages. The chemical oxygen demand was the highest impact indicator for the A5 (84.66%) and B5 stages (60.80%). Airborne NOx reached 54.91% of the impact on the A4 stage.

To sum up, phosphate contributed three-quarters of all substances in the EP category. NOx was responsible for 9.65% and chemical oxygen demand represented only 7.41% of all substances supporting eutrophication.

3.3. Assessment of Construction Materials and Operational Energy

According to the previous findings, the two most-polluting life cycle stages—A1–A3 and B6—were chosen for a closer impact assessment.

3.3.1. Embodied Environmental Impact of Construction Materials

Figure 17 depicts the impact of individual construction materials within each impact category (ICs). Only contributions higher than 10% were highlighted.

The embodied impact of foam glass dominated in most ICs, namely AD-F, GWP, ODP, MAE, TE, POP, AP, and EP. Steel parts largely affected HT (69.43%) and FAE (42.89%). Windows and doors exceeded the 10% cut-off impact in AD-E (30.41%), POP (10.44%), and AP (10.12%). More than 40% of the impact on AD-E was assigned to the recuperation system. OSB reached 13.33% and 13.64% impact on ODP and POP, respectively. KVH was detected to reach 12.38% of the ODP impact. All other contributions were below 10%.

The foundations—gravel, foam glass, concrete, reinforcement—constituted 55.02% of the total weight of the building (Table A1). Only foam glass represented 8.05% of the weight and was assessed as the most environmentally harmful construction material. Wood-based products—KVH, OSB, and HDF—accounted for 18.56% of the overall building weight. However, the cumulative impact of wood-based products was higher than that of foam glass only in POP (28.13% for wood-based products and 22.11% for foam glass, respectively).

3.3.2. Evaluation of the B6 Stage Impact

Operational energy demand was previously found to be the second worst life cycle stage. Since operational energy consumption was widely bound to photovoltaics, a comparison was made to specify differences in the possible environmental impact of the B6 stage if there was only photovoltaic energy (P), Slovak energy mix (S), and average European energy mix (EU) consumption (Figure 18). The results showed the impact of photovoltaic panels was more than 50% lower than that of the Slovak energy mix, which was found to be the worst variant in nearly all categories. Only AD-E was dominated by photovoltaics, causing an impact about 58% higher due to a higher demand for chemical elements (gold, silver, copper, lead, zinc).

3.4. Sensitivity Analysis

3.4.1. Lifetime Perspective

A sensitivity analysis was performed to indicate relative differences in the impact of each life cycle stage across 40- and 60-year lifetimes in comparison with the relative impact of each stage within the selected impact categories over the course of 50 years (

Table 5. Sensitivity analysis regarding 40- and 60-year lifetimes of the building (results are given as percentage change in impact).

Lifetime	Stage	AD-E	AD-F	GWP	ODP	HT	FAE	MAE	TE	POP	AP	EP
40 years	A1–A3	11.93	11.10	12.60	18.97	9.18	15.87	15.41	9.37	13.05	13.44	14.83
	A4	11.93	11.10	12.60	18.97	9.18	15.87	15.41	9.37	13.05	13.44	14.83
	A5	11.93	11.10	12.60	18.97	9.18	15.87	15.41	9.37	13.05	13.44	14.83
	B1			−9.92						−9.56
	B2	−10.46	−11.12	−9.92	−4.82	−12.65	−7.30	−7.67	−12.48	−9.56	−9.24	−8.14
	B4	−10.46	−11.12	−9.92	−4.82	−12.65	−7.30	−7.67	−12.51	−9.56	−9.25	−8.14
	B5	−10.46	−11.12	−9.92	−4.82	−12.65	−7.30	−7.67	−12.51	−9.56	−9.25	−8.14
	B6	−10.46	−11.12	−9.92	−4.82	−12.65	−7.30	−7.67	−12.51	−9.56	−9.25	−8.14
	B7	−10.46	−11.12	−9.92	−4.82	−12.65	−7.30	−7.67	−12.51	−9.56	−9.25	−8.14
60 years	A1–A3	−9.63	−9.09	−10.07	−13.75	−7.76	−12.05	−11.78	−7.89	−10.35	−10.59	−11.44
	A4	−9.63	−9.09	−10.07	−13.75	−7.76	−12.05	−11.78	−7.89	−10.35	−10.59	−11.44
	A5	−9.63	−9.09	−10.07	−13.75	−7.76	−12.05	−11.78	−7.89	−10.35	−10.59	−11.44
	B1			7.92						7.58
	B2	8.45	9.10	7.92	3.50	10.69	5.54	5.86	10.57	7.58	7.29	6.28
	B4	8.44	9.09	7.92	3.50	10.69	5.54	5.86	10.53	7.58	7.28	6.27
	B5	8.44	9.10	7.92	3.50	10.69	5.54	5.86	10.53	7.58	7.29	6.28
	B6	8.44	9.10	7.92	3.50	10.69	5.54	5.86	10.53	7.58	7.29	6.28
	B7	8.44	9.10	7.92	3.50	10.69	5.54	5.86	10.53	7.58	7.29	6.28

). Since stages A1–A3, A4, and A5 represented the embodied impact of the building, the input data were kept. Material and energy flows in the use stage (B1–B7) were assumed to be directly proportionate to the number of years the building was used. This means the impact of stages B1–B7 increased by 20% for each impact category every 10 years.

Positive values refer to the increased impact of the specific stage within the selected impact category while negative values indicate a reduced relative burden on the environment. Obviously, the fewer years the building is used, the higher the relative impact of the construction materials, transport to the construction site, and the construction itself, which can be observed in the case of the 40-year lifetime as an 11.93% increased impact compared to the reference impact in the 50-year lifetime.

3.4.2. Masonry Construction Perspective

In order to determine the effect of different types of construction, masonry construction was designed to meet the calculated specific heat demand for heating of the construction (13.3 kWh/(m²·a)). All characteristics belonging to the use stage of the building (stages B1 to B7) were preserved. Changes occurred in the product (stages A1 to A3) and the construction stage (A4 and A5). The timber construction of walls was replaced by brick, and the ceiling was transformed into reinforced concrete. A list of the considered construction materials is given in

Table A3. Life cycle inventory of input construction materials for masonry passive building.

Construction Material	Database *	Share on Overall Construction Weight (%) **
Gravel	Gravel, crushed {RoW}|market for gravel, crushed|APOS, U	9.68
Foam glass	Foam glass {GLO}|market for|APOS, U	4.92
Concrete	Concrete, 25 MPa {RoW}|market for concrete, 25 MPa|APOS, U	18.46
Reinforcement	Reinforcing steel {GLO}|market for|APOS, U	0.61
Reinforced concrete ceiling	fiber-reinforced concrete {BR}|market for fiber-reinforced concrete, steel|APOS, U	29.30
Rock wool	Stone wool, packed {GLO}|market for stone wool, packed|APOS, U	2.10
Mineral plaster	Base plaster {GLO}|market for|APOS, U	3.13
Brick	Clay brick {GLO}|market for|APOS, U	23.66
Extensive vegetation	Own assumption 1	6.86
Windows and doors	Door, outer, wood-glass {GLO}|market for|APOS, U Door, inner, wood {GLO}|market for|APOS, U Window frame, wood, U = 1.5 W/m2K {GLO}|market for|APOS, U Glazing, triple, U < 0.5 W/m2K {GLO}|market for|APOS, U	1.27
Recuperation system	Ventilation system, central, 1 × 720 m3/h, polyethylene ducts, with earth tube heat exchanger {GLO}|market for|APOS, U	-

* Data refer to ecoinvent database [40]. ** total weight of construction was 244.927 t. ¹ Database is based on the following assumption: Inputs to manufacture 1 kg of the substrate for extensive vegetation consist of: 0.05 kg of Compost (industrial) market for; 0.45 kg of Cleft timber, measured as dry mass {Europe without Switzerland}|market for|APOS, U; 0.40 kg of Shale {GLO}|market for|APOS, U; 0.10 kg of Peat {GLO}|market for|APOS, U; 0.01 kg of Grass seed, organic, for sowing {GLO}|market for|APOS, U and 0.028 kg of Packaging film, low density polyethylene {GLO}|market for|APOS, U. Transportation accounts for 0.03052 tkm of Transport, freight, lorry > 32 metric ton, euro5 {RER}|market for transport, freight, lorry > 32 metric ton, EURO5|APOS, U. Energy requirements are 0.00185 kWh of Electricity, low voltage {SK}|market for|APOS, U and 0.00227 MJ of Heat, central or small-scale, natural gas {Europe without Switzerland}|market for heat, central or small-scale, natural gas|APOS, U.

. Transportation distances relating to the construction stage are given in

Table A4. Transportation distances from distributor to the construction site—masonry construction.

Distance (km)	Product
100	Steel
	Gravel
	Foam glass
	Windows and doors
50	Rock wool
	Reinforced concrete ceiling
30	Concrete
20	Extensive vegetation
	Base plaster
	Transport of excavator
200	Recuperation system
	Brick

.

According to the results (

Table 6. Effect of masonry construction on the environmental impact (results are given as percentage change in impact).

Stage	AD-E	AD-F	GWP	ODP	HT	FAE	MAE	TE	POP	AP	EP
A1–A3	−31.88	−1.00	11.40	13.06	−73.80	−43.45	−13.32	−32.03	−9.14	−9.43	−14.87
A4	185.10	261.21	255.41	261.61	277.22	230.54	245.99	265.40	253.34	261.27	257.00
A5	0.32	16.17	−18.60	33.88	−3.49	−6.24	−45.09	−28.96	−74.56	−60.98	−91.27

), the masonry construction contribution was higher in two categories, namely GWP and ODP (11.40% and 13.06%, respectively). The rest of the impact categories reported lower values than the timber construction, whereas the highest difference was noted for HT (−73.80%). An increase in the impact of transportation was indicated, of at least 185.10% in AD-E and up to 277.22% in HT. The construction process of masonry construction was higher in AD-E, AD-F, and ODP, accounting for 0.32%, 16.17%, and 33.88%, respectively. The impact on the other categories was lower and ranged between −3.49% in HT and −91.27% in EP.

4. Discussions

The study identified the life cycle impacts of a balloon-frame timber structure designed as a passive building with integrated photovoltaics, a heat pump, and controlled ventilation. The proposed building represented a higher standard of a single-family building. Data on the type and mass of construction materials, operational energy, and water use are verified based on actual measurements. Other stages contain data partly based on qualified estimates and manufacturer recommendations. Transportation distances are representative for Slovak conditions. For input and output flows, a 1% cut-off rule was applied. The life cycle was modelled for a geographical region of Slovakia.

During the impact assessment of individual life cycle stages, only contributions more than 5% were accounted for in the evaluation. The product stage and operational energy reported similar impacts in several categories. In particular, the GWP impact of these stages was nearly the same and supporting passive buildings not only reduced the energy performance of the building but also the negative environmental impacts. The research also proved findings of previous studies [42,43,44] whereby the impact of construction materials and operational energy use in passive buildings can be equal over the course of 50 years. In addition, attention should be focused on the burden associated with replacement as a consequence of HVAC system choice. A similar conclusion was reached by [45], who determined HVAC retrofitting to burden the environment more than the exterior insulation.

Several methods of specifying the environmental impact exist. Generally, GWP is the main impact category assessed by many authors [2,14,15,16,17,18,19,20,25,27,28,29,33,35,36,42,44,45,46,47,48,49,50]. Other environmental impacts are rarely discussed [16,27,33,35,46]. The present study showed the alarming impact of heavy metals, according to normalization results. Ecotoxicity indicators, especially for aquatic ecosystems, should be paid more attention.

Raw material extraction, transportation to the manufacturer, construction materials manufacturing, transportation to site, and the construction process represent the embodied impacts of a building. Sensitivity analysis proved buildings with longer lifetimes reported lower embodied impact. Focusing on construction materials determined the impact of the proposed balloon-frame timber construction’s labelled foundations to bear 43.04% of the GWP impact. Compared to a previous study [45] where foundations of a light steelframed building were responsible for 29% of the embodied GWP, the wood-based constructions represent an environmentally friendlier type of construction.

Passive buildings use renewable energy sources to lower energy consumption. The energy consumption of buildings was found to have a crucial impact on the environment by several studies [47,48,51]. In the present study, the choice of electricity production mix considerably affected the B6 stage. In the case of the photovoltaic panels, 60–70% of the energy consumption relates to the manufacturing stage [47]. Thus, the positive impact they cause in GWP compensates for the negative impacts they cause in Abiotic Depletion. The results agree with the findings of Tawalbeh et al. [52] who thoroughly investigated the environmental impacts of the photovoltaic system. As photovoltaic panels require large amounts of abiotic raw materials, it is necessary to pay attention to the development of more advanced technology with lower environmental impact.

Certain authors [36,49] claim wood-based buildings achieve better environmental characteristics compared to conventional masonry buildings. Based on our findings, masonry constructions could be regarded as environmentally worse buildings in terms of the impact on GWP and ODP. However, other categories proved the contrary. The high impact on the other categories might be due to distorted data on specific types of brick filled with mineral insulation used in the masonry construction. Since the ecoinvent database only covers standard clay brick, the real impact of brick manufacturing might be higher. The relatively high impact of timber construction in the construction stage could be due to the larger amount of waste produced.

The results of this study are bound to specific types of construction, transportation distances, geographic sites, and electricity mix. The specific situation is bound to transportation distances as well as the specific type of HVAC system. Results of this study could be compared to buildings with similar passive standard systems. Thus, comparisons with other buildings should be carried out with care [50,53]. Further research should be conducted to evaluate the impact of other construction materials as well as to compare the impact of different material compositions and prefabrication on the building’s life cycle.

5. Conclusions

Building design influences its environmental impact in each life cycle stage. Therefore, it is crucial to look at the building as a whole system where even a slight modification in the design stage can lead to a substantial change within the life cycle.

The study focused on evaluating the life cycle environmental impact of a passive wood-based residential building including controlled ventilation with recuperation, photovoltaics, and a heat pump over the course of 50 years. The product stage, the replacement stage, and operational energy use were the most burdensome stages in nearly all categories throughout the life cycle. The global warming potential impact of the product stage and B6 stage was nearly the same. Normalization results showed a major negative impact on marine aquatic ecotoxicity mostly caused by the B6 stage.

The proposed wood construction was found to be environmentally friendlier than the masonry variant in terms of GWP and ODP. The environmental impact of construction materials indicated foam glass as the worst material. Wood-based materials had a lower cumulative impact in almost all categories compared to foam glass.

Moreover, renewable energy sources used in passive buildings to lower energy consumption should not be considered burden-free for the environment. In addition to many positive environmental impacts of renewable energy sources, the study showed the impact of high demand on chemical elements in the case of photovoltaics. This raises a concerns of possible depletion of non-renewable raw materials due to the probable expansion of photovoltaic power plants in the future.