U-Values for Building Envelopes of Different Materials: A Review

: In recent decades, the issue of building energy usage has become increasingly significant, and U-values for building envelopes have been key parameters in predicting building energy consumption. This study comprehensively reviews the U-values (thermal transmittances) of building envelopes made from conventional and bio-based materials. First, it introduces existing studies related to the theoretical and measured U-values for four types of building envelopes: concrete, brick, timber, and straw bale envelopes. Compared with concrete and brick envelopes, timber and straw bale envelopes have lower U-values. The differences between the measured and theoretical U-values of timber and straw bale envelopes are minor. The theoretical U-values of concrete and brick envelopes ranged from 0.12 to 2.09 W/m 2 K, and the measured U-values of concrete and brick envelopes ranged from 0.14 to 5.45 W/m 2 K. The theoretical U-values of timber and straw bale envelopes ranged from 0.092 to 1.10 W/m 2 K, and the measured U-values of timber and straw bale envelopes ranged from 0.04 to 1.30 W/m 2 K. Second, this paper analyses the environmental factors influencing U-values, including temperature, relative humidity, and solar radiation. Third, the relationship between U-values and building energy consumption is also analysed. Finally, the theoretical and measured U-values of different envelopes are compared. Three research findings in U-values for building envelopes are summarised: (1) the relationship between environmental factors and U-values needs to be studied in detail; (2) the gaps between theoretical and measured U-values are significant, especially for concrete and brick envelopes; (3) the accuracy of both theoretical and the measured U-values needs to be verified.


Introduction
Building energy use continues to increase significantly around the world.The building sector accounts for around 30% of final energy use [1,2].This exacerbates fossil fuel consumption, making it imperative to decrease energy use in the building sector [3].In addition, this sector is also regarded as one of the most cost-efficient fields in which to reduce energy use [4].Thus, many researchers have focused on building energy consumption in recent years [5][6][7].
In existing studies, building energy simulation has proven to be an important method for predicting building energy consumption [8][9][10].To obtain building energy prediction results, accurate envelope parameters need to be entered into the simulation software.The thermal transmittances (U-values) of building envelopes are crucial thermal parameters [11,12].
The U-value is the rate of heat transfer across a building envelope.As shown in Equation (1), Φ is the heat transfer, A is the area in square meters, and ∆T is the temperature difference between the interior and exterior sides of the building envelope.
The U-value shows the thermal insulation property of the building envelope.It is important for predicting building energy consumption and understanding the impacts of buildings on the environment.There are two types of U-values in existing studies: theoretical U-values obtained by formulas and measured U-values obtained by experiments.

Theoretical U-Values of Building Envelopes
The theoretical U-values of envelopes are calculated according to the ISO 6946 method [13].Under this method, the U-value of a building envelope can be estimated by related envelope parameters, as shown in Equations ( 2) and (3): where R se and R si are thermal resistances of the external and internal surfaces of the building envelope, correspondingly.R sum is the sum of the thermal resistances of all layers within the building envelope.R is the thermal resistance of each layer, D is the thickness of each layer, and λ is the thermal conductivity of each layer in the building envelope.
Theoretical U-values may be reasonable for the initial phase of design.However, detailed parameters about some existing buildings are not available or are not maintained.The U-values of many existing building envelopes are difficult to calculate using the theoretical method.

Measured U-Values of Building Envelopes
In order to obtain U-values of envelopes in actual conditions, both laboratory and in situ measurements can be reasonable approaches.The common U-value measurement methods are shown in Figure 1.For laboratory measurement, the Hot Box Test (HBT) is a common method.Both ASTM C1363 and ISO 8990 standards are used to regulate measurement equipment and procedures [14,15].Guarded hot box (GHB) and calibrated hot box (CHB) are two common types in HBT, as shown in Figure 1a,b.Both methods require steady-state conditions and are suitable for full-scale building components.A Uvalue measurement system includes (1) several heat flow sensors; (2) several temperature sensors; and (3) a data logger.The measured U-value can be calculated by Equation (4).
where U is the U-value of the tested envelope during the measurement period, q j is the heat flow density at time j, and T ij and T ej are the temperature in the indoor and outdoor environment at time j, respectively.n is the number of recorded samples during the measurement period.Many researchers have used these methods to measure the U-values of different types of envelopes [16][17][18].For example, Yang et al. used the GHB method to investigate the U-values for straw bales with different structural details.The results showed that straw bales with plastering had lower U-values [16].Chen et al. applied a CHB to study the Uvalues of double-glazing units.Comparing the measurement results with simulation results revealed a difference of less than 5%, which can be considered negligible [19].However, the temperature and relative humidity are fixed values in laboratory measurements, and laboratory conditions are different from the actual conditions of buildings.Many researchers have used these methods to measure the U-values of different types of envelopes [16][17][18].For example, Yang et al. used the GHB method to investigate the U-values for straw bales with different structural details.The results showed that straw bales with plastering had lower U-values [16].Chen et al. applied a CHB to study the U-values of double-glazing units.Comparing the measurement results with simulation results revealed a difference of less than 5%, which can be considered negligible [19].However, the temperature and relative humidity are fixed values in laboratory measurements, and laboratory conditions are different from the actual conditions of buildings.
The in situ U-value measurements have been conducted to investigate U-values in actual situations.Four methods are commonly used: the heat flow meter (HFM), the simple hot box-heat flow meter (SHB-HFM), the thermometric (THM), and the quantitative infrared thermography (QIRT).As shown in Figure 1c, the HFM method is a standardized method for in situ U-value measurement, and it is governed by ISO 9869-1 and ASTM C1155 standards [20,21].The equipment in the HFM method includes several heat flow sensors, several temperature sensors, and a data logger.After the data have been collected, the U-value can be calculated by Equation (4).Many studies have measured the U-values of building envelopes by the HFM method on various occasions [22][23][24].For example, the U-values of seven masonry envelopes were measured by this method [25], and the results showed that there was a discrepancy between the measured and theoretical U-values.The shortcoming of this method is that when the temperature difference is unstable, there may be a large error in the measured U-values.
To avoid this drawback, the SHB-HFM method was developed.In this method, a simple hot box is attached to one side of the test envelope, as shown in Figure 1d.This box has heating equipment to control the temperature difference between indoor and outdoor environments.The U-value can be calculated by Equation (4) as well.Meng et al. proposed this method and verified its feasibility through an in situ U-value measurement.The results showed that the test error of the U-value by the SHB-HFM method was only −5.97% relative to the theoretical U-value [26].This method requires additional specialised equipment; hence, the applications of this method are limited.
The THM method is a low-cost method which needs less equipment than other methods.It requires several temperature sensors and a data logger, as shown in Figure 1e.The calculation method of measured U-values in the THM method is shown in Equations ( 5) and ( 6) [27].The in situ U-value measurements have been conducted to investigate U-values in actual situations.Four methods are commonly used: the heat flow meter (HFM), the simple hot box-heat flow meter (SHB-HFM), the thermometric (THM), and the quantitative infrared thermography (QIRT).As shown in Figure 1c, the HFM method is a standardized method for in situ U-value measurement, and it is governed by ISO 9869-1 and ASTM C1155 standards [20,21].The equipment in the HFM method includes several heat flow sensors, several temperature sensors, and a data logger.After the data have been collected, the U-value can be calculated by Equation (4).Many studies have measured the U-values of building envelopes by the HFM method on various occasions [22][23][24].For example, the U-values of seven masonry envelopes were measured by this method [25], and the results showed that there was a discrepancy between the measured and theoretical U-values.The shortcoming of this method is that when the temperature difference is unstable, there may be a large error in the measured U-values.
To avoid this drawback, the SHB-HFM method was developed.In this method, a simple hot box is attached to one side of the test envelope, as shown in Figure 1d.This box has heating equipment to control the temperature difference between indoor and outdoor environments.The U-value can be calculated by Equation (4) as well.Meng et al. proposed this method and verified its feasibility through an in situ U-value measurement.The results showed that the test error of the U-value by the SHB-HFM method was only −5.97% relative to the theoretical U-value [26].This method requires additional specialised equipment; hence, the applications of this method are limited.
The THM method is a low-cost method which needs less equipment than other methods.It requires several temperature sensors and a data logger, as shown in Figure 1e.The calculation method of measured U-values in the THM method is shown in Equations ( 5) and ( 6) [27].
where U j is the U-value of the tested envelope at time j, and T ij and T ej are the temperature of the indoor and outdoor environment at time j, correspondingly.T sij is the internal surface temperature of the tested envelope at time j.h i is the heat transfer coefficient of the internal surface of the tested envelope.U is the U-value of the tested envelope during the measurement period.n is the number of recorded samples during the measurement period.Bienvenido analysed eight tested envelopes to evaluate the advantages and shortcomings of this method.The results showed that the U-values obtained through the THM method were valid in winter, while it was difficult to obtain valid results in warmer seasons [27].This method needs very stable indoor conditions.Thus, it is less adapted.
The QIRT method has been used widely in recent decades.This method can be conducted according to ISO 9869-2 and ASTM C1060 standards [28,29].This method is expensive and requires specialist training.In the QIRT method, an infrared camera, temperature sensors, heat flow sensors, and a data logger are needed, as shown in Figure 1f.Compared to other methods, the QIRT method is a newer method that has been developed in recent years.There is no universal equation for calculating the U-value in this method [30].Each new method of calculating the U-value is related to the actual conditions of in situ measurements [31].Mahmoodzadeh et al. used the QIRT method to study the U-values of timber-framed building envelopes.They found that estimated U-values were not identical on different days due to variations in outdoor environmental parameters [32].Climate conditions and air pollution can also influence the results obtained by the QIRT method.
In the existing literature, the theoretical U-values of building envelopes were different from the measured U-values [32][33][34][35].The type of U-values entered into the building energy simulation affects the building energy prediction results and the energy management in the building sector.Thus, it is important to understand the differences between theoretical and measured U-values for different building envelopes.In this paper, U-values of both inorganic and bio-based envelopes (including concrete, brick, timber and straw bale envelopes) will be examined.In Section 2, theoretical and measured U-values of these four types of envelopes will be shown according to the data from existing related studies.The environmental factors affecting the U-values of building envelopes will be analysed in Section 3, while in Section 4, the studies related to energy impacts caused by U-values will be reviewed.A comparison of the theoretical and measured U-values and research gaps will be analysed in Sections 5 and 6, respectively.

Theoretical and Measured U-Values of Inorganic and Bio-Based Envelopes
Inorganic and bio-based envelopes are two important types of envelopes.Concrete and brick envelopes are inorganic envelopes, which are the most common worldwide [36,37].Both timber and straw bales have been paid more attention due to the lower environmental impacts in recent years [38][39][40][41].Using life cycle assessment (LCA), the carbon emissions of bio-based material buildings are lower than inorganic material buildings [42][43][44].In this section, concrete, brick, timber, and straw bale envelopes will be the objects of the study, and existing studies will be examined to assess both the theoretical and measured U-values of these four types of envelopes.

Concrete Envelopes
As shown in Table 1, the theoretical U-values of concrete envelopes range from 0.12 to 1.61 W/m 2 K with most concentrated around 0.15-0.50W/m 2 K.In U-value measurements for concrete envelopes, more research applied in situ measurements than laboratory measurements, and most used the HFM method.Reinforced concrete (RC) envelopes have been studied more than other concrete envelopes.The measured U-values of concrete envelopes range from 0.14 to 5.45 W/m 2 K with most concentrated around 0.15-0.60W/m 2 K.The measured U-values in a few studies are much larger than the theoretical U-values.For example, O'Hegarty et al. found that the measured U-values of the concrete envelopes were around twice their theoretical U-values [33].Some researchers conducted in situ measurements in winter.Because of the larger temperature difference between indoor and outdoor environments in winter, a steady heat flow can be generated in the envelopes, which can improve the accuracy of the measurement results.Several researchers mea-sured the U-values of concrete envelopes with different orientations [23,45], and the results showed that the U-values of the north envelopes were smaller than the U-values of the envelopes with other orientations.

Brick Envelopes
In existing studies related to the U-values of brick envelopes, the main types have included clay, limestone, hollow, perforated, red, solid, ceramic, and silica brick envelopes.As shown in Table 2, the theoretical U-values of brick envelopes range from 0.22 to 2.09 W/m 2 K.The thickness of the insulation is an influencing factor.For example, Albatici et al. calculated the theoretical U-value of the brick envelope as 0.225 W/m 2 K.The thickness of the insulation is 8 cm [60].Marshall calculated the theoretical U-value of the brick envelope as 2.09 W/m 2 K.This envelope is a 222.5 mm solid brick envelope without insulation [61].
In U-value measurements for brick envelopes, more researchers applied in situ measurement methods, the most common of which was the HFM method.The measured U-values of brick envelopes range from 0.15 to 5.26 W/m 2 K.The majority of studies found that the theoretical U-values were close to the measured U-values with a deviation of less than 20%.However, a minority of studies found theoretical U-values to be much larger or much smaller than the measured U-values.For example, Evangelisti et al. found that the measured U-value of a tuff brick envelope was only 40% of the theoretical U-value [62].Ratnieks et al., on the other hand, found that the measured U-value of a ceramic brick envelope was twice the theoretical U-value [52].

Timber Envelopes
As conventional building materials have serious impacts on the environment, biobased building materials have been given more attention in recent decades [78][79][80].Such materials can store carbon and reduce carbon emissions [42].As an important bio-based building material, timber envelopes have gradually re-emerged because timber envelopes not only have a low environmental impact but also have a simple manufacturing process and can have high prefabrication rates [81,82].
Due to the popularity of timber envelopes, there have been a growing number of studies investigating the U-values of timber envelopes in the last decade, as shown in Table 3.In existing studies related to the U-values of timber envelopes, the main types of timber envelopes include cross-laminated timber (CLT) envelopes, oriented strand board (OSB) envelopes, light timber envelopes, plywood panel envelopes and timber frame envelopes with different insulations.The theoretical U-values of these timber envelopes range from 0.15 to 1.10 W/m 2 K with most concentrated around 0.15-0.20 W/m 2 K. Theoretical U-values were not calculated in some of these studies.This may be due to the lack of information on the thermal conductivity of some bio-based building materials, such as wood-hemp insulation panels and wheat chaff insulation panels [83,84].Future studies on thermal conductivity will need to increase the variety of tested bio-based building materials.
In measuring the U-values of timber envelopes, in situ measurements were used more than laboratory measurements with the QIRT and HFM methods being adopted the most often.The measured U-values of these timber envelopes range from 0.04 to 0.98 W/m 2 K with most concentrated around 0.20-0.25 W/m 2 K.There are deviations between the theoretical U-values and measured U-values with the ratio of measured Uvalues to theoretical U-values ranging from 25% to 165%.For example, Williamson et al. investigated the thermal performance of two residential buildings using the HFM method to measure the U-values of two OSB external envelopes.The results showed that the measured U-values could be up to 1.65 times the theoretical U-values [85].

Straw Bale Envelopes
Although straw bale has been used widely as a construction material since the 20th century, the benefits of this material have been recognised in the last decade.The notable advantages are impressive physical properties, including thermal and acoustic insulation, an energy-efficient manufacturing process and a carbon storage capacity [91][92][93].In recent years, more researchers have focused on the U-values of straw bale envelopes.In related studies, the most common structure of straw bale envelopes is the straw bale envelope with a timber frame, as shown in Table 4.An alternative to this is a metal frame structure, such as a light-gauge steel frame [94].There are also many different types of straw, such as wheat straw, rice straw, oat straw and corn straw.
There is limited research on the theoretical U-values for straw bale envelopes.In existing studies, Miljan et al. studied the theoretical and measured U-values of a straw bale envelope [95].The results showed that the measured U-value (0.125 W/m 2 K) differed from the theoretical U-value, which was 0.092 W/m 2 K.As a new building envelope type, there is limited information related to the thermal conductivity of different straw bales.Thus, it is difficult to calculate theoretical U-values of straw bale envelopes with different structures and materials.More quantitative research on the thermal conductivity of different straw bales needs to be conducted to fill this research gap in the future.
Most studies have focused on the measured U-values for straw bale envelopes.Laboratory measurements were applied in the majority of these with the remainder applying in situ measurements.The measured U-values range from 0.12 to 1.30 W/m 2 K with most concentrated around 0.2 W/m 2 K.The wide range of measured U-values may be related to the envelope materials.For example, Sun et al. applied the CHB method to explore the measured U-values of the straw bale envelopes with light-gauge steel frames.The results showed the measured U-value of the envelope with the paper straw board (0.669 W/m 2 K) was lower than that with the wheat straw strand board (0.912 W/m 2 K) [94]. Figure 2 summarises the theoretical and measured U-values for four types of envelopes in Tables 1-4.Compared with inorganic envelopes, bio-based envelopes have lower Uvalues, and the differences between the measured and theoretical U-values of bio-based envelopes are smaller.It indicates that the thermal performances of bio-based envelopes are close to expectations.However, related data are not sufficient.More related measurements need to be conducted in the future.Among the two types of inorganic envelopes, the ranges of both theoretical and measured U-values are large for brick envelopes due to the greater variety of brick envelopes.Concrete envelopes have a small range of theoretical U-values and a large range of measured U-values, suggesting that the actual insulation performances of concrete envelopes may be lower than expected.

Environmental Factors Influencing U-Values
As the U-value represents the heat transfer capacity of the envelope in the actual environment, the environmental factors influencing the U-value are complex.The main environmental factors include temperature, relative humidity and solar radiation.There is limited research on the factors influencing U-values, and there is a positive correlation between the U-value of the envelope and the thermal conductivity of the envelope material.This section will also summarise articles related to the thermal conductivities of

Environmental Factors Influencing U-Values
As the U-value represents the heat transfer capacity of the envelope in the actual environment, the environmental factors influencing the U-value are complex.The main environmental factors include temperature, relative humidity and solar radiation.There is limited research on the factors influencing U-values, and there is a positive correlation between the U-value of the envelope and the thermal conductivity of the envelope material.This section will also summarise articles related to the thermal conductivities of building materials, as shown in Table 5.
The thermal conductivities of building materials can be affected by changes in the thermal conductivities of air and water due to temperature changes.When temperature increases from 10 to 40 • C, air and water conductivities will increase by 10% and 8%, respectively.Regarding the high amount of air with water vapour in the envelopes, the increase in air and water conductivities is not negligible.Existing studies have quantified the effects of temperature on the thermal conductivities of both conventional and biobased building materials.Wang studied the thermal conductivity of aerogel-incorporated concrete (AIC).The results indicated that the thermal conductivity of AIC increased by 15.5% from 20 to 90 • C [102].Danovska studied dynamic thermal conductivity functions dependent on temperature for some timber materials.The results indicated that the thermal conductivities of CLT, woodchips, and wood-fibre panels increased 10.2%, 26%, and 21% from 10 to 50 • C [103].However, there is limited research examining the quantitative relationship between temperature and the U-values of various envelopes.
In terms of relative humidity effects, relative humidity can change the moisture contents of building materials, affecting the thermal conductivities of building materials and thus the U-values of envelopes.The moisture content of a building material in an actual situation is related to its hygroscopicity, which is mainly related to the composition, porosity and pore characteristics.Several studies have quantified the effects of relative humidity and moisture contents on the thermal conductivity of building materials and U-values of envelopes [104].Various types of building materials and envelopes are studied.There is more research on insulation materials.This may be because insulation materials are usually porous, and their thermal conductivities are more susceptible to the effects of relative humidity.For example, Wang et al. investigated the relationship between the relative humidity and the thermal conductivity of common insulation materials.The result showed that the thermal conductivities of these materials increased by more than 100% when relative humidity increased from 0% to 100% [105].Boukhattem et al. analysed the influence of moisture content on the thermal conductivity of the date palm fibre (DPF) insulation board.The result showed that the thermal conductivity of the DPF board could increase four times from a dry stage to its saturation state [106].
In addition, solar radiation is also a factor influencing U-values.It has been found that solar radiation can increase the heat flow through envelopes and thus increase the U-values of the envelopes.Evangelisti et al. conducted in situ U-value measurements in both summer and winter in Italy.As the building being tested was located in the northern hemisphere, the solar radiation intensity and sunlight hours were higher on the south envelope than on the north envelope.The results showed that the U-values of the south envelope were approximately 25% higher than the U-values of the north envelope in both winter and summer [23].Ahmad et al. also found that the U-values of the east envelope were approximately 23% higher than the U-values of the north envelope in Saudi Arabia [45].This is because the north envelope is exposed to solar radiation for a shorter time than the east envelope, which leads to a lower heat flow through the north envelope.However, limited studies have been conducted to quantify the relationship between U-values and solar radiation.This needs to be systematically studied in the future.CLT panels; woodchip insulation panels; wood-fibre insulation panels The thermal conductivity increases by 10.2% for CLT, 26% for woodchips and 21% for wood-fibre from 10 to 50 • C. [103]

Relative humidity Mineral wools
The thermal conductivity increases from 0.10-0.14W/m K to 0.7-0.9W/m K (from low moisture contents of 5-20% to saturation).
[104] Stone wool panels; hemp panels U-values of both stone wool panels and hemp panels increase in 56-90% RH.
[86] Hemp concrete, flax concrete and rape straw concrete The thermal conductivity is proportional to the water content.
[107] Solid brick envelopes The transient U-values achieve higher values within the moist stage.
[108] Insulating building materials made from DPF mesh Thermal conductivity increases with water content. [

102] Common insulation materials
The thermal conductivity increases by 171.9% from 0% to 100% RH. [105] CLT panels; woodchip insulation panels; wood-fibre insulation panels The thermal conductivity increases by 12% for CLT, 18% for woodchips and 8% for wood-fibre from low to high moisture content. [103]

Solar radiation Hollow-reinforced precast concrete envelopes
The U-value of the north envelope was 37.3% lower than that of the east envelope, because the north envelope was exposed to solar radiation for a shorter time than the east envelope.
[45] RC envelopes The obtained U-value can be heightened by solar radiation. [23]

Impacts of U-Values on Building Energy Consumption
The life cycle energy of buildings includes embodied energy and operational energy, as shown in Figure 3.As an important thermal parameter of the envelope, the U-value affects the building's operational energy, especially energy consumption for cooling and heating.In recent years, the number of studies on the relationship between operational energy consumption and U-values of envelopes has increased rapidly, as shown in Table 6.The types of buildings in these studies are mainly office and residential buildings.The types of envelopes examined include inorganic envelopes (such as concrete envelopes and brick envelopes) and bio-based envelopes (such as timber envelopes and straw bale envelopes).
According to existing studies, the U-values have a large impact on building operational energy consumption, and the relationship between the U-value and operational energy consumption varies in different climatic conditions.Lower U-values can save operational energy in cold climates.For example, Fernandes et al. used dynamic simulation to study the U-value impact on the thermal performance of residential buildings.The results showed that operational energy consumption decreased as U-values decreased in cold climates [109].However, the relationship between U-values and operational energy consumption needs to be dependent on the specific situations in relatively warm climates.On the one hand, some researchers have found that lower U-values can lead to higher operational energy consumption.For example, Ihara et al. investigated envelope properties in their study of energy efficiency in Tokyo office buildings.The results showed that the decrease in the U-value of the non-transparent parts of RC envelopes was observed to increase the yearly energy use of some high-rise buildings [110].On the other hand, some researchers have reached the opposite conclusion.For example, Suleiman found that when U-values of external envelopes are 3.03 W/m 2 k and 5.26 W/m 2 k, the corresponding estimated annual energy consumption is 40.26 kWh/m 2 and 69.93 kWh/m 2 in North Africa [64].
Buildings 2024, 14, x FOR PEER REVIEW 13 of 20 6.The types of buildings in these studies are mainly office and residential buildings.The types of envelopes examined include inorganic envelopes (such as concrete envelopes and brick envelopes) and bio-based envelopes (such as timber envelopes and straw bale envelopes).According to existing studies, the U-values have a large impact on building operational energy consumption, and the relationship between the U-value and operational energy consumption varies in different climatic conditions.Lower U-values can save operational energy in cold climates.For example, Fernandes et al. used dynamic simulation to study the U-value impact on the thermal performance of residential buildings.The results showed that operational energy consumption decreased as Uvalues decreased in cold climates [109].However, the relationship between U-values and operational energy consumption needs to be dependent on the specific situations in relatively warm climates.On the one hand, some researchers have found that lower Uvalues can lead to higher operational energy consumption.For example, Ihara et al. investigated envelope properties in their study of energy efficiency in Tokyo office buildings.The results showed that the decrease in the U-value of the non-transparent parts of RC envelopes was observed to increase the yearly energy use of some high-rise buildings [110].On the other hand, some researchers have reached the opposite conclusion.For example, Suleiman found that when U-values of external envelopes are 3.03 W/m 2 k and 5.26 W/m 2 k, the corresponding estimated annual energy consumption is 40.26 kWh/m 2 and 69.93 kWh/m 2 in North Africa [64].
The fluctuation of U-values can also have impacts on the predictions of building operational energy consumption.U-values are dynamic because they are influenced by environmental factors, such as fluctuations in temperature and relative humidity.If an average U-value or a theoretical U-value is used to simulate building operational energy consumption throughout the year, errors will arise.Bruno et al. applied the WUFI software to study dynamic U-values of three different inorganic envelopes in the Mediterranean climate.The results showed that the changes in the U-values each month were significant, providing deviances of as much as 9.2% in quarterly energy consumption when compared to the results obtained from a steady-state U-value [12].The fluctuation of U-values is an important issue for building energy prediction.However, related quantitative research focusing on bio-based building envelopes is limited and needs to be investigated in detail.[64]

All use Timber envelopes
There is a linear relationship between the average U-value of the envelope and the cooling and heating energy consumption.
[112] 2015 Office RC envelopes The energy use decreased due to the reduction in the U-values of windows.The energy use increased due to the reduction in the U-value of the non-transparent envelopes in high-rise buildings.[110] 2015

Residential RC envelopes
In cold areas, the yearly heating energy use of buildings modelled with the 3D dynamic method is 8-13% higher than that modelled with the average method.In warm areas, the yearly cooling energy use is underestimated by 17% with the average method.
[ Due to the variation in U-value, the yearly total heating load increased by 26%, and the yearly total cooling load increased by 13% in Beijing.
[120] All use Brick and concrete envelopes In the Mediterranean climate, the change in U-value each month is significant, providing deviances as much as 9.2% in quarterly energy consumption. [12] The fluctuation of U-values can also have impacts on the predictions of building operational energy consumption.U-values are dynamic because they are influenced by environmental factors, such as fluctuations in temperature and relative humidity.If an average U-value or a theoretical U-value is used to simulate building operational energy consumption throughout the year, errors will arise.Bruno et al. applied the WUFI software to study dynamic U-values of three different inorganic envelopes in the Mediterranean climate.The results showed that the changes in the U-values each month were significant, providing deviances of as much as 9.2% in quarterly energy consumption when compared to the results obtained from a steady-state U-value [12].The fluctuation of U-values is an important issue for building energy prediction.However, related quantitative research focusing on bio-based building envelopes is limited and needs to be investigated in detail.

Comparison of Theoretical and Measured U-Values
In recent years, many countries have established the range of U-values of envelopes in building codes to save building energy use [121][122][123].The U-values in these codes are theoretical U-values.More existing studies have used theoretical U-values, and fewer studies have applied measured U-values to simulate building operational energy.To predict building operational energy accurately, it is important to obtain U-values of envelopes in actual situations, so it is vital to understand whether the theoretical and measured U-values correspond to the real-life scenario.
Firstly, theoretical U-values of envelopes are different from actual situations of envelopes in most situations because they can be affected by environmental factors, especially temperature and relative humidity.The temperature and relative humidity are constantly varying in the real environment.Several researchers have focused on dynamic U-values [124][125][126].Their findings suggest that using theoretical U-values for building energy simulation may lead to errors to some extent.Secondly, whether the measured U-values are close to the actual conditions of envelopes needs to be verified.Lots of research results showed that the theoretical U-values of both inorganic and bio-based envelopes were different from their measured U-values.The measured U-values may be closer to the actual situations of envelopes than the theoretical U-values because the measured U-values take the influence of the environment into account.However, errors in measured U-values may be caused by incorrect installation of equipment and unstable measurement conditions [127].Thus, it cannot be stated conclusively that the measured U-values correspond to the actual situations of envelopes.It is worth noting that most of the existing studies do not verify the measured U-values.

Conclusions
This study provides a systematic review of the existing studies related to the U-values for envelopes of different materials.Both theoretical and measured U-values of four types of envelopes (including concrete, brick, timber and straw bale envelopes) are introduced.Environmental factors influencing U-values and the impacts of U-values on building energy consumption are analysed.This study also discusses the accuracy of both theoretical and measured U-values.Three research findings are summarised as follows: (1) The relationship between environmental factors and U-values needs to be studied in detail.Some studies have focused on the relationship between the environmental factors and thermal conductivities of building materials.However, there is limited research examining the quantitative relationship between important factors (such as temperature, relative humidity and solar radiation) and the U-values of various envelopes.(2) The gaps between theoretical and measured U-values are significant, especially for concrete and brick envelopes.The theoretical U-values of concrete envelopes range from 0.12 to 1.61 W/m 2 K.Meanwhile, the measured U-values of concrete envelopes range from 0.14 to 5.45 W/m 2 K.The theoretical U-values of brick envelopes range from 0.22 to 2.09 W/m 2 K.Meanwhile, the measured U-values of brick envelopes range from 0.15 to 5.26 W/m 2 K. (3) The accuracy of both theoretical and the measured U-values needs to be verified.In building energy simulation, it is also necessary to verify which type of U-value to input can make the simulation results more accurate.

Figure 1 .
Figure 1.Schematic diagrams of laboratory and in situ U-value measurement methods.

Figure 1 .
Figure 1.Schematic diagrams of laboratory and in situ U-value measurement methods.

Figure 2 .
Figure 2. Theoretical and measured U-value distribution of four types of envelopes.

Figure 2 .
Figure 2. Theoretical and measured U-value distribution of four types of envelopes.

Figure 3 .
Figure 3.The life cycle energy of buildings.

Figure 3 .
Figure 3.The life cycle energy of buildings.
values of external envelopes are 3.03 W/(m 2 •K) and 5.26 W/(m 2 •K), the corresponding estimated annual energy consumption are 40.26kWh/m 2 and 69.93 kWh/m 2 in North Africa.

Table 1 .
Existing studies related to U-values of concrete envelopes.

Table 2 .
Existing studies related to U-values of brick envelopes.

Table 3 .
Existing studies related to U-values of timber envelopes.

Table 4 .
Existing studies related to U-values of straw bale envelopes.

Table 5 .
Environmental factors influencing U-values of envelopes.

Table 6 .
Impacts of U-values of envelopes on building energy consumption.
This envelope achieves ultra-low U-values.It is responsible for a 10% reduction in space heating and cooling energy.