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Article

Wood Frame Walls Designed with Low Water Vapour Diffusion Resistance Wind Shields

by
Nickolaj Feldt Jensen
1,
Torben Valdbjørn Rasmussen
1,* and
Lars Hofmann
2
1
Division of Building Technology, Management and Indoor Environment, Aalborg University, 2450 Copenhagen, Denmark
2
JCN BOLIG A/S, 8210 Aarhus, Denmark
*
Author to whom correspondence should be addressed.
Buildings 2025, 15(5), 706; https://doi.org/10.3390/buildings15050706
Submission received: 14 January 2025 / Revised: 11 February 2025 / Accepted: 14 February 2025 / Published: 23 February 2025
(This article belongs to the Section Building Energy, Physics, Environment, and Systems)
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Abstract

In response to concerns over resource shortages and environmental impacts, biobased materials are increasing in popularity. This includes an interest in replacing traditional vapour control systems, including polyethene (PE) membranes. However, the susceptibility of these materials to moisture-related degradation poses challenges. This study examines the water vapour diffusion resistance of the vapour retarder and the wind shield as key properties. Examining wood frame walls designed with low water vapour diffusion resistance wind shields, this study analyses the necessary properties of the vapour retarder as a function of the properties of the wind shield. We evaluated exterior wood frame walls that were thermally insulated with materials including mineral wool and biobased options such as flax, grass, wood fibre, straw, and cellulose. Using WUFI Pro software, we determined the relations between properties necessary to prevent mould growth. Hygrothermal simulations determined the necessary properties of the vapour retarder as a function of the properties of the wind shield. Analyses were carried out in temperate cold climates. Wind shield diffusion tightnesses ranging from 0.01 to 1 (m2·s·GPa)/kg were evaluated. Assessments were performed for walls with a U-value of 0.15 and 0.10 W/(m2·K). The indoor humidity classes 1 to 3, as defined in EN ISO 13788, were used for the simulations. The results indicate that the necessary properties of the vapour retarder depend on the properties of the wind shield, as well as the insulation material, the indoor humidity, and the U-value. As the wind shield diffusion tightness decreases, the necessary vapour retarder diffusion tightness also decreases, eventually reaching a fixed value determined by the insulation material, the indoor humidity, and the U-value.

1. Introduction

Traditional construction practices rely on materials like concrete, bricks, mineral wool, and polystyrene. These materials are derived from non-renewable resources such as sand, gravel, and steel and require significant energy for production—typically oil, coal, and natural gas. Local Danish studies have warned of potential raw material shortages for materials such as gravel and sand within a few decades [1]. In contrast, biobased materials, such as wood, straw, flax, seaweed, and grass, are renewable resources that offer sustainable alternatives for several building components [2]. Additionally, these materials absorb carbon dioxide (CO2) during growth, store carbon (C), and release oxygen (O2). They enable carbon storage during a building′s lifespan as they partially replace conventional materials like concrete, bricks, mineral wool, and steel [2].
Buildings offer significant societal value and are designed and built to endure for many years. Unlike conventional materials, biobased materials are more susceptible to biological degradation, mould growth, and wood decay exacerbated by high moisture content, oxygen, and moderate temperatures. As such, structures utilising biobased materials must be designed to maintain sufficiently low moisture levels. The material moisture content can increase due to the conditions arising from building usage. Indoor relative humidity (RH) is influenced by outdoor RH, building use, and ventilation patterns. Indoor moisture loads can be categorised using the five indoor humidity classes, defined as classes 1 to 5 in EN ISO 13788 [3,4]. A shift in the construction industry away from traditional methods necessitates the development of substantial new knowledge and experience to ensure the safe application of biobased materials.
Traditional vapour retarders, such as polyethene (PE) membranes, and wind shields are typically produced from non-renewable resources. In cold temperate climates, vapour retarders play a crucial role as they ensure adequate airtightness and reduce the amount of water vapour penetrating into cold zones of thermally insulated building envelopes [5,6]. Airtightness is critical for reducing energy consumption and is often mandated by local building regulations (e.g., [5]). Limiting vapour diffusion prevents excessive moisture accumulation within the thermally insulated building envelope, which is essential for maintaining structural integrity [6].
There is extensive knowledge and practical experience regarding the use of PE-membranes joined by using adhesive tape. However, recent studies [7,8] have identified durability issues with joints and have reported a lifespan of 30–40 years for some membranes in accelerated ageing tests. Membranes are also prone to mechanical damage during construction and building operations. In addition, studies indicate that the lifespan of these PE-based vapour-retarder systems is often shorter than that of the exterior wall, highlighting their limitations for long-term use [7,8]. This necessitates the development of improved vapour-retarder systems [9]. Such alternatives may include sheet or rigid board materials tailored to specific applications within buildings. However, the use of biobased materials for vapour-control systems may challenge existing guidelines, particularly regarding the necessary water vapour diffusion resistance for vapour retarders and wind shields, which ensures the sufficient hygrothermal robustness of the building envelopes. According to [9], a potential solution is the use of biobased hardboard, which aligns with climate mitigation objectives. Hardboards can be used to regulate water vapour diffusion in exterior wood frame walls, and they may also contribute to the structural stabilisation of a building.
Rasmussen and Nicolajsen [10] studied wood frame walls insulated with biobased materials and PE vapour retarders, finding no mould growth. This suggests that moisture-safe wood frame walls can be constructed using biobased thermal insulation materials, at least when using traditional PE vapour retarders. Vinha [11] examined the wind-shield vapour-retarder water vapour diffusion resistance ratios in wood frame walls with hygroscopic and non-hygroscopic thermal insulation materials, showing that highly hygroscopic materials with lower water vapour diffusion resistance ratios are difficult to dry if wind shields are too vapour-tight, regardless of the internal vapour control layer′s water vapour diffusion resistance. Similar conclusions were obtained in [12,13], where the hygrothermal behaviour of wood frame walls thermally insulated with mineral wool and cellulose was examined, determining the relation between wind-shield and vapour-retarder water vapour diffusion resistance. Borodinecs et al. [14] examined the hygrothermal performance of wood frame walls in Latvia through field measurements and simulations. They studied walls insulated with mineral wool, wood fibre, and loose-fill cellulose in combination with different vapour-retarder sd-values. Mould growth-free structures were fabricated using a wind-shield sd-value to vapour-retarder sd-value ratio of 1:3.5 for mineral wool and 1:1.75 for wood fibre and loose-fill cellulose. The authors concluded that the biobased materials performed better than the mineral wool due to the higher moisture capacity and better than the wood fibre due to the higher vapour diffusion resistance of the insulation itself. In contrast, other studies [15,16] suggest that the moisture capacity of the thermal insulation material has a limited effect. Pihelo and Kalamees [17] examined wood frame walls and U-values between 0.17 and 0.08 W/(m2·K), finding higher RH levels with lower U-values. In addition, walls with cellulose insulation exhibited lower RH levels than those with mineral wool, which they attributed to cellulose′s higher moisture capacity. Vanpachtenbeke et al. [18] studied the hygrothermal performance of wood frame walls with brick veneer cladding using OSB boards as a vapour retarder in Belgium. They varied the sd-value of the vapour-retarder, finding increasing mould growth risk with decreasing wind-shield sd-value to vapour-retarder sd-value ratios. Korsnes et al. [19] similarly studied the performance of wood frame walls using OSB boards as a vapour retarder in four locations: Oslo and Bergen in Norway, Munich in Germany, and Stockholm in Sweden. The authors found that the RH and temperature conditions in the wall construction did not reach levels sufficient for mould growth in any of the four locations.
Danish best practice guidelines recommend that the water vapour diffusion resistance, denoted by the Z-value, of a vapour retarder in exterior wall construction should be at least 10 times higher than that of the wind shield. These guidelines are intended to ensure “moisture-safety” and minimise the risk of mould growth behind the wind shield when exposed to the upper limit of indoor humidity class 3 [4]. For indoor humidity class 1 or 2, mould growth behind the wind shield is not considered a risk if the wind-shield vapour-retarder Z-value ratio is at least 1:5 [19,20]. However, the guidelines do not provide specific recommendations for exterior walls exposed to indoor humidity class 4 or 5, as these represent high moisture loads typically associated with facilities such as sports halls, indoor swimming pools, or commercial kitchens rather than standard residential buildings. Moreover, different countries provide varying recommendations for the wind-shield vapour-retarder Z-value ratio. For example, in Belgium, the recommended ratios range from 1:6 to 1:15 [20], and in Finland, they range from 0 to 1:80 [11]. In other countries, such as Sweden, the building code does not directly specify a recommended ratio but instead states that the construction should be airtight and that it should be evaluated using dynamic hygrothermal simulation software [21]. Simonson, Ojanen, and Salonvaara [22] studied the performance of wood frame walls insulated with mineral wool and loose-fill cellulose in cold climates exposed to humidity class 1. The authors concluded that a moisture-safe construction could be achieved with a wind-shield sd-value to vapour-retarder sd-value ratio of 1:3. They also noted that the vapour retarder layer could allow considerably more water vapour diffusion than a typical PE-membrane.
In this study, hygrothermal simulations were conducted using the moisture modelling software WUFI Pro. The software was used to reveal the relation between the water vapour diffusion resistance, denoted by the Z-value, between the wind shield and the vapour retarder of an exterior wood frame wall. The focus of the simulations was evaluating the wind-shield vapour-retarder Z-value ratio in cold climates. As this study investigated wind-shield Z-values ranging between 0.01 and 1 (m2·s·GPa)/kg, the analysis complements a previous study examining wind-shield Z-values ranging from 1 to 8 (m2·s·GPa)/kg. In both studies, the necessary vapour-retarder Z-value for indoor humidity class 3 as a function of the wind shield property was identified. The findings suggest that the 1:10 recommendation may not ensure moisture safety in all cases, particularly when using biobased thermal insulation materials [23]. To further explore the potential for replacing conventional vapour-retarder systems, such as PE-membranes, a parameter analysis was performed of the necessary vapour-retarder Z-value for low water vapour diffusion resistance wind shields.
Z-values ranging between 0.01 and 1 (m2·s·GPa)/kg for the wind shield are of particular interest due to the desire to use biobased alternatives for both the wind shield and the vapour retarder, as many biobased materials to be used as vapour-control systems allow more water vapour diffusion than traditional PE-membrane systems. Hence, the wind shield must allow even more water vapour diffusion to prevent critical moisture levels from developing behind the wind shield. This interest in biobased materials is further supported by their commercial availability, combined with the findings by Hansen et al. [23], who observed that the recommended 1:10 in Z-value ratio might not be valid below a wind-shield Z-value of 1 (m2·s·GPa)/kg. However, since this finding has not been fully confirmed, further investigation into wind shields with a low Z-value is essential.
Conversions between parameters describing water vapour resistance as the factor µ, the sd-value, and the Z-value for materials are described in DS/EN ISO 12572:2016 [24].
The necessary relation between parameters was examined for two wall U-values exposed to different indoor climate conditions thermally insulated with biobased materials as well as mineral wool. The objective was to determine the feasibility of replacing conventional PE-membranes as vapour retarders with boards or membranes made with biobased materials.
The following research questions were examined:
  • What is the minimum required wind-shield to vapour-retarder ratio for biobased wall constructions when the wind-shield Z-value is lower than 1 (m2·s·GPa)/kg?
  • How much do these minimum requirements change in relation to the indoor moisture load from humidity class 1 to 3 and for different wall U-values?
  • Are there significant differences between the use of various biobased insulation materials in relation to the effect of the indoor moisture load and wall U-value?

2. Materials and Methods

This paper evaluated the hygrothermal behaviour of a typical Danish exterior wood frame wall, as detailed in [14,15,16]. The wall assembly included a ventilated cladding and a wind shield mounted to the thermal insulation, with a vapour retarder mounted on the interior side. This study examined variations in several important parameters: internal moisture loads represented by the indoor humidity class; different thermal insulation materials; different wall thicknesses, defined by two wall U-values; dynamic exterior climate conditions representative of Denmark; and combinations of wind-shield and vapour-retarder Z-value, focusing on low wind-shield Z-values between 0.01 and 1 (m2·s·GPa)/kg.
Hygrothermal behaviour was analysed using WUFI Pro 6.7 [25], a software tool for one-dimensional hygrothermal simulations of multi-layered building elements under dynamic external and internal boundary conditions. The simulation results were subsequently post-processed using the Isopleth model to evaluate the theoretical risk of mould growth in the biobased materials.

2.1. Wood Frame Wall

Figure 1 illustrates a horizontal cross-section of the wood frame wall in this study. The placement of the wind shield and the vapour retarder is highlighted. Additionally, the analysis area of interest is highlighted, which was the outermost 10 mm of the thermal insulation layer. This exterior wood frame wall was thermally insulated with different insulation thicknesses to reach a thermal transmittance of 0.10 and 0.15 W/(m2·K), the latter representing the minimum insulation requirement specified by the Danish building code [7].
The wood frame wall designs featured U-values of 0.15 and 0.10 W/(m2·K). Mineral wool, flax, grass, wood fibre, straw, and cellulose were utilised as thermal insulation materials.
As this is a purely theoretical study, the examined insulation thickness may not always correspond to typical commercially available insulation thicknesses. Table 1 outlines the design parameters for wood frame wall designs with U-values of 0.15 and 0.10 W/(m2·K). Table 2 lists the material properties of the thermal insulation products, boards, and other materials used in the hygrothermal simulations and analysis. The wood frame comprises 7.5% of the wall section, with a thermal conductivity of 0.12 W/(m·K), while the insulation accounts for 92.5% of the wall section. U-value corrections were applied in the inhomogeneous layers, including wood frame studs and battens.

2.2. Material Properties

The hygrothermal simulation models were created based on the specified material layers and thicknesses. The horizontal cross-section of the wood frame wall is shown in Figure 1. The most relevant material properties are provided in Table 2, which were obtained from the WUFI Pro materials database.
The Z-value is widely used in Denmark. However, the WUFI Pro tool uses the factor μ for the calculations of water vapour diffusion in materials. Parameter conversions were conducted by adjusting the material µ-value to achieve the necessary wind-shield and vapour-retarder Z-values. The model variations are detailed in Section 2.4.

2.3. Boundary Conditions

External boundary conditions were simulated using climate data for Sjælsmark, Zealand, Denmark (20 km north of Copenhagen). The Sjælsmark dataset is a reference year based on measured climate data collected by the Danish Meteorological Institute (DMI). Climate data were collected between 2000 and 2019, with the dataset being released in 2023 [25,26]. The relevant characteristics of the reference year are the following: average RH = 82.1%, average temperature = 9.2 °C, average wind speed = 3.2 m/s, annual horizontal precipitation = 648 mm, and annual direct normal radiation and diffuse horizontal radiation = 1012 and 538 kWh/m2, respectively. The prevailing wind direction in Denmark is southwest. The internal boundary conditions were simulated as indoor humidity classes 1 to 3 [4], covering the moisture load of most residential buildings. Hansen and Møller [27] conducted a study on the moisture supply in Danish single-family houses. Of the 500 owner-occupied homes that were included in the study, the authors found that 40%, 28% and 32% fell into indoor humidity classes 1, 2, and 3, respectively. However, the authors also found that these variations could not be directly attributed to any family or social relations [28]. Simulations were carried out using initial temperature and relative humidity values of 20 °C and 80% RH, respectively (default settings in WUFI Pro).
The wood frame wall was oriented facing north, as this orientation was determined to have the highest risk of moisture accumulation, despite southwest being the prevailing orientation for wind-driven rain (WDR) in Denmark. Since the wall structure is designed with a two-step sealing system, including a rain screen and a ventilated cavity, it is assumed that this provides sufficient protection against WDR, provided that no errors occur during the fabrication. Experimental investigations by Vinha (2008) on wood frame houses in Finland demonstrated that the two-step sealing system provided adequate protection against WDR [11]. In addition, wood frame walls with southern orientations in the northern hemisphere benefit from higher solar radiation, leading to increased drying compared with walls with northern orientations [29].
The simulations for the wood frame walls were performed for a 3-year period or until the model results were shown to be period-stable. Subsequently, the final simulated year was post-processed.

2.4. Model Configurations

The initial simulation model was created using a wind-shield to vapour-retarder Z-value ratio of 1:10 (wind-shield Z-value = 1 (m2·s·GPa)/kg and vapour-retarder Z-value = 10 (m2·s·GPa)/kg) to match the most diffusion-open wall models examined by [23]. The ventilated cavity behind the rain screen was simulated as air without additional moisture capacity and with a fixed air change rate of 10 times per hour.
To examine the minimum Z-value requirements of the vapour retarder, various combinations of Z-values for the vapour retarder and wind shield were simulated. Table 3 displays the different Z-values simulated, with corresponding sd-values and µ factors for reference. In general, the different Z-values for the vapour retarder were simulated with a corresponding wind-shield Z-value equal to 1.0, 0.5, and 0.01 (m2·s·GPa)/kg, starting from the highest vapour-retarder Z-value and reducing its value until the simulation results failed the specified mould growth criteria. The examined Z-value ratios ranged from 1:1 up to 1:1000. Parametric variations were carried out for a wall with a U-value of 0.15 W/(m2·K) with indoor humidity class 1 to 3 applied and for a wall with a U-value of 0.10 W/(m2·K) with indoor humidity class 2 and 3 applied.

2.5. Mould Growth Evaluation

The risk analysis for mould growth utilised data from the thermal insulation layer located directly behind the wind shield (indicated by red dashed lines in Figure 1). The data, obtained from WUFI Pro, consisted of hourly values, which were then converted into a 24 h average. The 24 h average values were used to analyse and evaluate the risk of mould growth using the Isopleth model. This approach effectively removed peak values from the dataset.
The isopleth model described by Sedlbauer in 2001 was used [30]. This method was chosen as an approval criterion for evaluating whether the construction is moisture-safe, following Johanson, Lång, and Gapener [31], who demonstrated that static methods, such as the Isopleth model, are more reliable than dynamic models. The static models were found to be accurate in 80% of cases, and where discrepancies occurred, the static models tended to predict more mould growth [31]. Moreover, the Isopleth model by Sedlbauer is generally regarded as a conservative choice of mould model [30,31].
Hygrothermal simulations providing cohesive temperature and RH data were post-processed and evaluated against the lowest isopleth model, shown in Figure 2 below. The risk assessment was carried out using the Isopleth Substrate Group I criteria [12], which encompass biologically degradable construction materials, aligning with the biobased materials examined, including the wood frame.

3. Results

The necessary vapour-retarder Z-value in relation to the wind-shield Z-value presented in this section was found by conducting a series of calculations for a region in which no risk of mould growth should be present within the wall construction in the area of the red dashed lines shown in Figure 1. The risk of mould growth was evaluated utilising the Lowest Isopleth Substrate Group I criteria. The necessary vapour-retarder Z-value should therefore fulfil the criteria for all three curves given by 4, 8, and 16 days.
Table 4 shows the necessary vapour-retarder Z-value and its relation to the wind-shield Z-value. A necessary vapour-retarder Z-value increase is shown for increasing indoor moisture load (illustrated by the indoor humidity class) and decreasing wall U-value.

4. Discussion

4.1. Water Vapour Diffusion Resistance Ratio vs. Insulation Materials

Figure 3 shows the necessary vapour-retarder Z-value as a function of wind-shield Z-value, as well as results obtained by [23]. The new results from the present study are represented by wind-shield Z-values lower than 1 (m2·s·GPa)/kg. It was observed that, with a decreasing wind-shield Z-value, there is an increase in the necessary wind-shield to vapour-retarder Z-value ratio. In addition, it was observed that the ratio of 1:10 recommended by the Danish best practice guidelines is insufficient when the wind-shield Z-value decreases to less than 1 (m2·s·GPa)/kg. This is illustrated in Figure 3 by the solid curves crossing over the “dash-dot” line representing the DK factor of 1:10.
The Danish best practice guidelines for constructions exposed to indoor humidity class 3 were challenged in this study, thereby confirming the findings by [23], particularly for wall structures using wind-shield Z-values lower than approximately 1 (m2·s·GPa)/kg, as illustrated in Figure 3 and Table 4.
In terms of different thermal insulation materials and their effect on the necessary vapour-retarder Z-value, depending on the wind-shield Z-value, the results vary between the different thermal insulation materials. The lowest necessary vapour-retarder Z-value was observed for wood fibre used as thermal insulation, and the highest necessary vapour-retarder Z-value was observed for flax used as thermal insulation, as reported by [23]. The necessary vapour-retarder Z-value using mineral wool as thermal insulation was roughly similar to that of cellulose as thermal insulation; this was also slightly higher than wood fibre and straw but lower than flax, grass, and hemp as thermal insulation.

4.2. Effect of Indoor Moisture Load and Heat Loss Transmission Through the Wall Structure

Similar to the earlier findings by [23], an increase in the necessary vapour-retarder Z-value was observed for reduced U-values for walls with wind-shield Z-values below 1 (m2·s·GPa)/kg. However, the Z-value increase was smaller for wind shields with low Z-values. This can be seen by comparing the results shown in Table 4 with the findings of [23], shown for comparison in Figure 4 below.
Comparing the results for the individual thermal insulation materials, the present study does not show the same trends observed by [23]. For walls with a wind-shield Z-value of 0.01 or 0.50 (m2·s·GPa)/kg exposed to indoor humidity class 2, the improvement in the wall U-value had little or no effect on the necessary Z-value of the vapour retarder. In contrast, for walls exposed to indoor humidity class 3, the results were more in line with the earlier findings by [23]. The increase in the necessary vapour-retarder Z-value relative to the wind-shield Z-value when improving the wall U-value from 0.15 to 0.10 W/(m2·K) indicates that mineral wool and wood fibre insulation show promising tendencies, as the differences in the necessary vapour-retarder Z-value are small and predictable. In contrast, for walls insulated with flax, grass, and hemp, the increase in the necessary vapour-retarder Z-value is higher and less predictable. This highlights the need for experimental validation of the hygrothermal behaviour of wood frame walls insulated with various types of biobased materials.
The effect of the indoor moisture load on the necessary vapour-retarder Z-value is presented in Table 4. Similar to the findings by [23] for wind-shield Z-values between 1 and 8 (m2·s·GPa)/kg, the results for wind-shield Z-values below 1 (m2·s·GPa)/kg show that the necessary vapour-retarder Z-value increases with increasing indoor moisture load. In addition, for the individual thermal insulation materials, the necessary vapour-retarder Z-value also increases with increasing wind-shield Z-values. These results are similar to the overall tendencies observed by [23] regarding the effect of the indoor moisture load that are shown for comparison in Figure 5 below. Other previous studies [12,13] similarly examined the necessary vapour-retarder Z-value of walls exposed to different indoor moisture loads. The author similarly concluded that, with increasing indoor moisture load, the necessary vapour-retarder Z-value increases; as the water vapour gradient between the outdoor and indoor climate increases, there is a greater need for a more water diffusion-tight vapour retarder.

4.3. Water Vapour Diffusion Resistance Ratio

The results of this study for wind-shield Z-values less than 1 (m2·s·GPa)/kg provide an extension of the earlier results by [23]. Those results showed that, as the wind-shield Z-value decreases, the result is an ever-increasing value of the vapour-retarder Z-value divided by the wind-shield Z-value, where the vapour-retarder Z-value approaches a fixed value, and the wind-shield Z-value approaches 0 (m2·s·GPa)/kg. For example, for a wall insulated with flax (yellow curve in Figure 3), changing from a wind-shield Z-value of 4 (m2·s·GPa)/kg down to 1 and 0.01 (m2·s·GPa)/kg, leads to a decrease in the necessary vapour-retarder Z-value from 20 (m2·s·GPa)/kg down to approximately 12 and 6 (m2·s·GPa)/kg, respectively. This corresponds to an increase in the wind-shield to vapour-retarder Z-value ratio from 1:5 to 1:12 and 1:600, respectively.
The study by [23] concluded that the 1:10 recommendation in Danish best practice guidelines for the ratio between the wind-shield Z-values and the vapour-retarder Z-values was conservative. Since their study focused on wind-shield Z-values higher than 1 and less than 8 (m2·s·GPa)/kg, their study showed ratios of approximately 1:10 and 1:12 for their most diffusion-open wind shield (Z = 1 (m2·s·GPa)/kg) and ratios of approximately 1:2 and 1:3 for their most diffusion-tight wind shield—values that are far from the recommended 1:10 ratio. Similar conclusions were made in [12,13], where the authors examined the hygrothermal behaviour of wood frame walls using mineral wool and cellulose insulation, with varying vapour-retarder Z-values and the wind-shield Z-value fixed at 1 (m2·s·GPa)/kg. The authors found the necessary Z-value ratio to be approximately 1:3 and 1:1.5 for mineral wool and cellulose insulation, respectively, when exposed to indoor humidity class 1 and 2. For indoor humidity class 3, the ratio was found to be 1:5; for indoor humidity classes 4 and 5, the ratio between the wind-shield and the vapour-retarder hygrothermal properties was found to be 1:10. The results of the present study clearly indicate that the recommended Z-value ratio of 1:10 is an inappropriate guideline for wind-shield Z-values less than 1 (m2·s·GPa)/kg. In fact, depending on the insulation material, the 1:10 recommendation is insufficient at a wind-shield Z-value of approximately 1.2 (m2·s·GPa)/kg for flax, grass, and hemp. The guideline becomes even more insufficient for wall U-values below 0.15 W/(m2·K).
The present study confirms the conclusions from earlier studies that the necessary vapour-retarder’s hygrothermal properties depend on the thermal insulation material, the wind shield’s hygrothermal properties, the wall U-value [23], and the indoor moisture load [12,13,23].
It should be noted that the results of earlier studies [12,13,23] and the present results are all based on hygrothermal simulations using climate data applicable to Denmark. However, the hygrothermal behaviour of envelope structures is often dependent on temperature, RH, rain, wind, and solar radiation. The Z-value ratios determined could, therefore, be different for alternative building locations and based on changes to the local climate conditions due to climate change. Projections for Denmark suggest a warmer and more humid climate in the near future, creating an increased risk of moisture-related issues, e.g., mould growth. Further research on this topic should, therefore, include assessments of the hygrothermal behaviour under future climate conditions to ensure long-term constructions adequately address the risk of mould growth.
Regarding practical implementation, the results of this study indicate that a wood frame wall constructed using a wind shield of approximately 0.5 (m2·s·GPa)/kg or lower should be possible if combined with oriented strand board (OSB)-4 with a thickness of 15 mm. OSB can typically have µ-values ranging from 20 to 200, depending on the moisture content, as illustrated in [32]. This corresponds to a Z-value of 1.6 to 15.8 (m2·s·GPa)/kg for a board thickness of 15 mm. As the OBS serves as a vapour retarder mounted on the warm side of the thermal insulation, the RH levels are lower. Assuming RH levels of approximately 50%, this corresponds to an OSB Z-value of 7.8 to 10.7 (m2·s·GPa)/kg. Regarding wind-shield Z-values of 0.5 (m2·s·GPa)/kg or less, several products are available in Denmark that may be suitable. However, the hygrothermal behaviour of wood frame walls, using 15 mm OSB-4 in combination with a low wind-shield Z-value, still needs to be experimentally validated.
Based on this study’s findings, it is recommended that the Z-value of the different layers should decrease moving outwards in the construction, i.e., the vapour retarder should have the highest Z-value, followed by the wind shield. For an unventilated air cavity between the exterior cladding and the wind shield layer at the exterior side of the thermal insulation, the exterior cladding material should have a lower Z-value than the wind shield. For a ventilated air cavity, the Z-value of the exterior cladding material becomes inconsequential.

5. Conclusions

A parametric study examining the necessary vapour-retarder Z-value was performed for wind-shield Z-values less than 1 (m2·s·GPa)/kg and compared with previous research examining wind-shield Z-values from 1 to 8 (m2·s·GPa)/kg. This analysis provides insights into the feasibility of replacing traditional PE-membrane vapour retarders with biobased solutions. Hygrothermal simulations were conducted in WUFI Pro, examining wood frame walls for exterior use that remain free from mould growth. Parametric variations included the wind-shield Z-value and the vapour-retarder Z-value, wall U-value, indoor moisture load, and different types of thermal insulation material.
The results of these analyses support several significant findings.
  • The necessary vapour-retarder Z-value decreases with a decreasing wind-shield Z-value approaching a fixed value, depending on the thermal insulation material used, the indoor moisture load exposure, and the wall U-value.
  • The ratio between the wind-shield Z-value and the vapour-retarder Z-value increases with a decreasing wind-shield Z-value. Moreover, the ratio of 1:10 recommended in Danish best practice guidelines was found to be insufficient for constructions exposed to indoor humidity class 3 using a wind-shield Z-value of 1 (m2·s·GPa)/kg or less, potentially resulting in mould growth.
  • The necessary vapour-retarder Z-value increases with an increasing indoor moisture load.
  • The necessary vapour-retarder Z-value increases with a decreasing wall U-value.
This study shows that exterior wood frame walls can be constructed purely using biobased materials to replace traditional vapour-control systems such as PE-membrane systems and wind shields such as gypsum boards. However, the susceptibility of these materials to moisture-related degradation presents challenges that must be considered.
The risk of mould growth is not sufficiently addressed by specifying a recommended ratio of the water vapour diffusion resistance between the wind screen and the vapour retarder alone. It seems more reliable to evaluate the structures using dynamic hygrothermal simulation software, e.g., WUFI Pro ver. 6.7 or 7.0, supplemented with an analysis of the effect of air infiltration from the indoor environment, which needs to be negligible.
Further research on this topic should involve validation of the theoretical results through laboratory testing (e.g., stress testing using a hox/cold-box setup) and field testing, in which the test walls are exposed to real outdoor and indoor climate conditions. These tests would be valuable to determine the hygrothermal performance of wall constructions and the potential differences between various biobased insulation materials.

Author Contributions

Conceptualisation: L.H., T.V.R. and N.F.J.; methodology: L.H., T.V.R. and N.F.J.; resources and writing—original draft preparation: T.V.R. and N.F.J.; writing—review and editing: T.V.R. and N.F.J.; supervising, writing—original draft preparation: T.V.R. and N.F.J.; validation, formal analysis, investigation, and data curation: L.H., T.V.R. and N.F.J. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding. The APC was funded by the Division of Building Technology, Management and Indoor Environment, BUILD, Aalborg University, and the Division of Education: Bachelor of Architectural Technology and Construction Management, UCL University College.

Data Availability Statement

Data are reported in Danish: 16 February 2025, https://vbn.aau.dk/ws/files/718137712/Krav_til_z_vardi_i_tathedsplanet_i_lette_ydervagge_med_biogene_materialer_ved_forskellige_fugtbelastningsklasser_20240606_MBYG4.pdf.

Conflicts of Interest

Author Lars Hofmann was employed by the company JCN BOLIG A/S. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

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Figure 1. Horizontal cross-section of the wood frame wall examined in this study. This wall design is commonly used in Denmark. The red dashed lines indicate the area of interest, which is the outermost 10 mm of the thermal insulation behind the wind shield.
Figure 1. Horizontal cross-section of the wood frame wall examined in this study. This wall design is commonly used in Denmark. The red dashed lines indicate the area of interest, which is the outermost 10 mm of the thermal insulation behind the wind shield.
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Figure 2. Lowest Isopleth Substrate Group I criteria [12]. Curves representing 4, 8, and 16 days [30] with deduced associate curve equations. Group I encompass biologically degradable construction materials.
Figure 2. Lowest Isopleth Substrate Group I criteria [12]. Curves representing 4, 8, and 16 days [30] with deduced associate curve equations. Group I encompass biologically degradable construction materials.
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Figure 3. Necessary vapour-retarder Z-value as a function of the wind-shield Z-value. The results are obtained for wall U-value of 0.15 W/(m2·K), exposed to indoor humidity class 3, for various biobased thermal insulation materials and mineral wool. The area indicated by the blue dashed line are results from the present study, while the area indicated by the red dashed line represents results obtained by [23]. Of note, hemp was only examined by [23] and is included for comparison.
Figure 3. Necessary vapour-retarder Z-value as a function of the wind-shield Z-value. The results are obtained for wall U-value of 0.15 W/(m2·K), exposed to indoor humidity class 3, for various biobased thermal insulation materials and mineral wool. The area indicated by the blue dashed line are results from the present study, while the area indicated by the red dashed line represents results obtained by [23]. Of note, hemp was only examined by [23] and is included for comparison.
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Figure 4. Effect of wall U-value on the necessary vapour-retarder Z-value for various biobased materials, shown for wind-shield Z-value = 1 (m2·s·GPa)/kg. Original results by [23] visualised for comparison with the present results.
Figure 4. Effect of wall U-value on the necessary vapour-retarder Z-value for various biobased materials, shown for wind-shield Z-value = 1 (m2·s·GPa)/kg. Original results by [23] visualised for comparison with the present results.
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Figure 5. Effect of indoor humidity class on the necessary vapour-retarder Z-value for various biobased materials, shown for wind-shield Z-value = 1 (m2·s·GPa)/kg. Original results by [23] visualised for comparison with the present results.
Figure 5. Effect of indoor humidity class on the necessary vapour-retarder Z-value for various biobased materials, shown for wind-shield Z-value = 1 (m2·s·GPa)/kg. Original results by [23] visualised for comparison with the present results.
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Table 1. The wood frame wall designs feature U-values of 0.15 and 0.10 W/(m2·K). The wall utilises mineral wool, flax, grass, wood fibre, straw, and cellulose as thermal insulation materials.
Table 1. The wood frame wall designs feature U-values of 0.15 and 0.10 W/(m2·K). The wall utilises mineral wool, flax, grass, wood fibre, straw, and cellulose as thermal insulation materials.
Thermal Insulation MaterialThermal Conductivity [W/(m·K)]Thermal Conductivity and Wooden Frame [W/(m·K)]Thermal Insulation Thickness, Main Layer [m]Transmission Coefficient of an Exterior Wall, U-Value [W/(m2·K)]
Mineral wool0.0340.0400.205/0.3300.15/0.10
Flax0.0380.0440.230/0.3700.15/0.10
Grass0.0400.0460.240/0.3800.15/0.10
Wood fibre0.0380.0440.230/0.3600.15/0.10
Straw0.0480.0530.285/0.4500.15/0.10
Cellulose (loose fill)0.0370.0430.220/0.3500.15/0.10
Table 2. Material properties used for the WUFI Pro simulations.
Table 2. Material properties used for the WUFI Pro simulations.
Thermal Insulation MaterialDensity [kg/m3]Porosity [m3/m3]Heat Capacity [J/(kg·K)]Thermal Conductivity [W/(m·K)]Water Vapour Resistance Factor µ [-]Moisture Capacity at 100% RH [kg/m3]
Mineral wool200.9928500.0341.3372
Flax380.9516600.0371.5348
Grass400.9517000.040146
Wood fibre500.9721000.0384300
Straw1000.920000.048295.6
Cellulose (loose-fill)550.9325440.0372494
Air cavity1.30.99910000.1550.510.017
Gypsum board *6250.7068700.167.03430.6
Vapour retarder *1300.00123002.320000.047
Wind shield *11530.52012000.3222.2502
* A 25 mm gypsum board corresponds to a Z-value = 0.93 (m2·s·GPa)/kg. The vapour retarder is represented by µ = 2000, resulting in a Z-value = 10 (m2·s·GPa)/kg, for a thickness = 1 mm. The wind shield has a factor µ = 22.2, which corresponds to a Z-value = 1 (m2·s·GPa)/kg, for a thickness = 9 mm.
Table 3. Model variations for the vapour retarder and the wind shield.
Table 3. Model variations for the vapour retarder and the wind shield.
SimulationThickness [mm]Z-Value [(m2·s·GPa)/kg]sd-Value [m]µ [-]
Vapour retarder1102.02000
Vapour retarder181.61600
Vapour retarder171.41400
Vapour retarder161.21200
Vapour retarder151.01000
Vapour retarder140.8800
Vapour retarder130.6600
Vapour retarder120.4400
Vapour retarder110.2200
Wind shield910.222.2
Wind shield90.50.111.1
Wind shield90.20.044.4
Wind shield90.10.022.2
Wind shield90.010.00030.02
Table 4. Necessary vapour-retarder Z-value in relation to a wind-shield Z-value of 0.01, 0.5, and 1 (m2·s·GPa)/kg. Results represent a wall U-value of 0.15 W/(m2·K) exposed to indoor humidity class 1 to 3 and a wall U-value of 0.1 W/(m2·K) exposed to indoor humidity class 2 and 3. Results in Bold represent wind-shield Z-value = 0.01 (m2·s·GPa)/kg; results in italics represent wind-shield Z-value = 1 (m2·s·GPa)/kg. Indoor humidity class is abbreviated as IHC.
Table 4. Necessary vapour-retarder Z-value in relation to a wind-shield Z-value of 0.01, 0.5, and 1 (m2·s·GPa)/kg. Results represent a wall U-value of 0.15 W/(m2·K) exposed to indoor humidity class 1 to 3 and a wall U-value of 0.1 W/(m2·K) exposed to indoor humidity class 2 and 3. Results in Bold represent wind-shield Z-value = 0.01 (m2·s·GPa)/kg; results in italics represent wind-shield Z-value = 1 (m2·s·GPa)/kg. Indoor humidity class is abbreviated as IHC.
Thermal Insulation MaterialVapour-Retarders Z-Value [(m2·s·GPa)/kg]
U-Value 0.15 W/(m2·K)U-Value 0.10 W/(m2·K)
IHC 1IHC 2IHC 3IHC 2IHC 3
Wood fibre<1/<1/10.05/<2 **/21/3/71/1/31/4/8
Straw<0.5/<1/10.5/2/33/6/81/2/44 */6/10
Mineral wool<1/<1/12/4/55/6/83/4/56/8/10
Flax<1/<1/12/4/66/8/103/5/78/10/13
Grass<1/<1/23/5/66/8/104/5/67/10/12
Cellulose (loose fill)<1/<1/11/2/34/6/71/3/44/7/10
* Necessary vapour-retarder Z-value for wind-shield Z-value 0.2 (m2·s·GPa)/kg. ** Calculations were not performed for vapour-retarder Z-values lower than 2 (m2·s·GPa)/kg for wind-shield Z-value of 0.5 (m2·s·GPa)/kg.
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Jensen, N.F.; Rasmussen, T.V.; Hofmann, L. Wood Frame Walls Designed with Low Water Vapour Diffusion Resistance Wind Shields. Buildings 2025, 15, 706. https://doi.org/10.3390/buildings15050706

AMA Style

Jensen NF, Rasmussen TV, Hofmann L. Wood Frame Walls Designed with Low Water Vapour Diffusion Resistance Wind Shields. Buildings. 2025; 15(5):706. https://doi.org/10.3390/buildings15050706

Chicago/Turabian Style

Jensen, Nickolaj Feldt, Torben Valdbjørn Rasmussen, and Lars Hofmann. 2025. "Wood Frame Walls Designed with Low Water Vapour Diffusion Resistance Wind Shields" Buildings 15, no. 5: 706. https://doi.org/10.3390/buildings15050706

APA Style

Jensen, N. F., Rasmussen, T. V., & Hofmann, L. (2025). Wood Frame Walls Designed with Low Water Vapour Diffusion Resistance Wind Shields. Buildings, 15(5), 706. https://doi.org/10.3390/buildings15050706

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