Requirements of the Vapour Barrier in Wood-Frame Walls

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The research on the Z-value requirements of the water vapor and wind barriers in wooden frame walls is of great practical significance, as it is closely related to building energy efficiency and durability, ensuring the airtightness of the building, reducing water vapor diffusion, and preventing mold growth.But the following modifications still need to be made:

1.The use of WUFI Pro for hygrothermal simulations, considering various parameters and factors such as internal moisture loads, thermal insulation materials, wall thickness, U-value, external climate exposure, etc., makes the research results reliable and practical.

2.The paper mentions that the Z-value of new wind barrier materials (such as biogenic wind barriers) is lower than 1 GPa s m²/kg, but the discussion on the specific performance, characteristics, application prospects, and compatibility with other materials of these materials is relatively limited.

3.Some sentences are rather cumbersome, with some repetition and redundancy, affecting the readability and fluency of the article.

4.The citation format of the references should be consistent throughout the article, and the annotation of some references may need to be further checked and corrected.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Optimize the language expression to make the article more concise and lucid.

Author Response

Review 1:

The research on the Z-value requirements of the water vapor and wind barriers in wooden frame walls is of great practical significance, as it is closely related to building energy efficiency and durability, ensuring the airtightness of the building, reducing water vapor diffusion, and preventing mold growth. But the following modifications still need to be made:

 

Comments 1

The use of WUFI Pro for hygrothermal simulations, considering various parameters and factors such as internal moisture loads, thermal insulation materials, wall thickness, U-value, external climate exposure, etc., makes the research results reliable and practical.

Response 1:

We strongly agree and have good experience using WUFI Pro for hygrothermal simulations. We carry out many WUFI calculations and consider the results to be reliable and practical when we bother with the assumptions of the calculations.

 

Comments 2

The paper mentions that the Z-value of new wind barrier materials (such as biogenic wind barriers) is lower than 1 GPa s m²/kg, but the discussion on the specific performance, characteristics, application prospects, and compatibility with other materials of these materials is relatively limited.

 

Response 2:

The paper has been adjusted, so that it does not directly describe wind barrier below 1 GPa s m²/kg, but describes the trend.

 

Comments 3

Some sentences are rather cumbersome, with some repetition and redundancy, affecting the readability and fluency of the article.

 

Response 3:

The article has previously been proofread by Cambridge Proofreading & editing LLC, and we have reviewed the article for redundancy.

 

Comments 4

The citation format of the references should be consistent throughout the article, and the annotation of some references may need to be further checked and corrected.

 

Response 4:

References in the article, as well as the reference list, have been reviewed.

Reviewer 2 Report

Comments and Suggestions for Authors

In general, the paper addresses an interesting topic, and the overall approach is good. However, there are quite a few aspects that should be improved. There are some methodological issues, but also in the analysis one would expect more in-depth discussion explaining the physics. Even though the vapour tightness of the wall is the key element, it is not clear whether drying to the inside in summer is a relevant aspect. What exactly is going on in the wall is not discussed.

The paper should also clearly indicate the limitations of the study. 1D simulations. No water ingress. No increased initial moisture content. Perfectly airtight. One location. One orientation. These assumptions can perhaps be acceptable, but the papers needs to highlight these elements more clearly.

The paper does not state how the cavity was modelled. It is a “ventilated cavity” but the air change rate is not mentioned, even though this is crucial for the hygrothermal behaviour. The calculation of Z-values is unclear.

The result section only reports the output without any type of interpretation. The discussion section seems to be a combination of analysis, but does not comprise a discussion on limitations, extrapolation, or comparison to the state of the art. Hence, perhaps this section should be called “analysis” instead of “discussion”.

 

More detailed remarks can be found below.

Title: it is not clear what is meant by “Water Vapour Barrier” in the title.

1.       Introduction

Both the abstract and introduction implicitly lead to think that the paper addresses moderate climates, whereas only a single location in Denmark is studied.

The Z-value is expressed as GPa s m²/kg. Please elaborate on the relation to mu-values and Sd values.

The introduction discusses the common approach to specify a ratio of the Z-values of the vapour barrier and the wind barrier. Reference is made to guidelines in Denmark, but only information is provided for classes 1-3. First of all, please highlight what is described for classes 4-5. Secondly, it would be good to check literature on this topic: are there other references that also adopt such rules of thumb? For Belgium the Belgian Building Research Institute uses a ratio of “(minimal) 6 to (preferably) 15”. See p13, paragraph underneath table. Contact 37 (1-13) NL (buildwise.be)

2.       Methods and materials

Please highlight that only 1D simulations were done. Discuss the impact and validity of this simplification and it’s repercussions on extrapolation potential.

2.1   Wall construction.

Please highlight that the insulation thickness was adjusted to obtain the specific U-values, making abstraction of the standard dimensions of wood studs.

“the number of additional studs installed due to the durability of the windows, doors, and corners” It is unclear what the link is between durability of windows and the number of studs in the wall.

Table 1 comprises a “thermal conductivity with wooden frame”. Please highlight that this is an “equivalent” thermal conductivity, and specify how this was calculated. Perhaps this is just area-based, but there are at least 3 common ways to calculate an equivalent thermal conductivity of such a configuration. In that case: also specify the thickness of the wooden stud, as this is not indicated in figure 1.

“In Figure 1, for the exterior wooden frame wall, the required Z-value ratio between the wind and vapour barriers was calculated, creating a wall with no mould growth risk”. The figure only provides a sketch of the wall. Hence, it is strange to say “In figure 1 … was calculated”. Next to that, I would suggest not to describe any details about the evaluation criteria at this point, and limit that to section 2.3.

2.2   Material properties

One would expect that Z-value of the vapour barrier and wind barrier is varied. Table 2 only provides fixed values for both. Next to that, it only specifies the water-vapour diffusion resistance factor. Underneath the table it is indicated that the gypsum board has a Z-value of 0.88 GPa s m²/kg. Please ensure that this conversion is explained in the text. Tabel 2 indicates that the vapour diffusion resistance of gypsum board is 7.03m. Hence the Sd-value is 7.03 * 0.025m = 0.17575m. To calculate the Z-value, Sd needs to be divided by the vapour permeability of air (1.87 10^-10 s at 20°C). Hence, the Z-value is 0.17575 * 18.7 * 10^9 m/s = 3.29 10^9 m/s. Given that 1Pa = 1 kg/m.s², m/s is equal to Pa.m/Pa.s = Pa.s.m²/kg. Hence 3.29 10^9 m/s = 3.29 GPa.s.m²/kg. Underneath the table it is indicated that the Z-value of the gypsum board is 0.88 GPa.s.m²/kg. How was this calculated? In general it is unclear how Z-values are calculated. Underneath table 2 it is also mentioned that the wind barrier has a thickness of 9mm?

There is no text in section 2.2, only a table. Please rearrange: either elaborate on the material properties and explain how the Z-values are varied (which is unclear now), or merge this section with another one.

2.3   Models for Mould risk analysis

Please clarify how this graph was developed. Compared to paper 12, that is presumably published in Journal of THERMAL ENV.&BLDG. SCI., Vol. 25, No. 4—April 2002, figure 2 does not correspond exactly to figure 5 top. Hence, perhaps a different reference was used, but the bibliographic reference is not entirely clear.

A reference is made to Table 4. But it is a bit strange that table 4 is part of the results, and it is part of the results section. It is strange to refer to table 4, when table 3 is not mentioned yet.

If I understand correctly, a running average of 4 days, 8 days, and 16 days is plotted on figure 2, and not a single point should supersede the corresponding line? For the SBi it is not clear which time resolution is used.  

The Z-value of the wind barrier is varied between 1 and 8. With a step function of 1 unit per step? Then “numerous” simulations are done with varying Z-value for the vapour barrier. Please indicate how this was done. Was this a manual system where compliance was evaluated? What interval was used to ‘optimize’ the Z-value? Was the goal to find the lowest possible Z-value that did not entail problems? Please elaborate on the methodological approach that was adopted.

Why is interstitial condensation not considered? Why is yearly drying not evaluated (mandatory in EN 15026)?

3.       Results

Please analyse and discuss the results in table 3.

Table 3: the note underneath the table is redundant.

Table 4: please put “Z-value ratio – wind barrier: vapour barrier” above “thermal insulation”.

Table 4: the first part of the note underneath the table is redundant. Then it states: “Values in parentheses indicate the Z-value ratio of the wind barrier to vapour barrier”, whereas the value in front of the parentheses is “Required Z-value ratio of the wind and vapour barriers”. Please clarify the difference more clearly.

Please analyze and discuss the results in table 4.

Figure 3 reports “the multiplication factor for the Z-value of the vapour barrier”. Isn’t this exactly the same as the aforementioned “Z-value ratio of the wind barrier to vapour barrier”? It is a pity that no complete results are included for straw, grass, and hemp.

A factor of 10 is reported as criterion in SBi. In the result section reference is made to [13], in the introduction reference was made to [11]. It would be logical to refer twice to the same source, as exactly the same criterion is mentioned. Only on the graph this is introduced as DK-factor.

Figure 3: please analyze and discuss the results: what is the implicit logic of what is reported? Why are certain values low/high, what is the impact of the Z-value of the wind barrier and why?

Figure 4: the Y-axis “Z-value of the vapour barrier” is actually identical to “Z-value ratio of the wind barrier to vapour barrier” (but for one case Z-value vapour barrier = 1). Hence you report the same thing but with different names every time.

4.       Discussion

4.1   Properties

Why does it all of a sudden mention that material files were changed? Based on section 2 I had the impression only standard material files were used.

“Mineral wool, hemp and flax results are given based on the SBi moisture instructions [13].” Unclear. The results can be compared with the SBi instructions, but not “given”. Or did the authors want to say that the SB-criterion was the most stringent one compared to Sedlbauer’s model for 3 time resolutions?

4.2   Insulation

“Detailed hygrothermal simulations must be conducted for moisture load classes of 4 and 5.” Please include this in the introduction where the requirements are discussed.

“An increase in the wind-barrier Z-value lowered the required Z-value ratio between the wind and vapour barriers for an exterior wooden frame wall” Please explain why.

4.3 Moisture classes

Line 290: reference is missing

4.3   Z-value ratio

The paper states that wind barriers have typical Z-values of about 0.3 to 3 GPa s m²/kg. In that case: why do the simulations only consider 1 as the lowest value? Based on figure 3, these values may entail very specific requirements.

5.       Conclusions

“Using the requirements that make up the standard is considered good practice in designing exterior walls is not adequate.” From the conclusions it is not cleat that this refers to SBi standards. Please include, as the conclusions should be clear without reading the paper.

“However, moisture that penetrates the thermal insulation of the exterior wall must have less resistance towards the exterior side of the wall.” This sentence is rather vague.

 

Comments on the Quality of English Language

None

Author Response

Review 2:

In general, the paper addresses an interesting topic, and the overall approach is good. However, there are quite a few aspects that should be improved. There are some methodological issues, but also in the analysis one would expect more in-depth discussion explaining the physics. Even though the vapour tightness of the wall is the key element, it is not clear whether drying to the inside in summer is a relevant aspect. What exactly is going on in the wall is not discussed.

The paper should also clearly indicate the limitations of the study. 1D simulations. No water ingress. No increased initial moisture content. Perfectly airtight. One location. One orientation. These assumptions can perhaps be acceptable, but the papers needs to highlight these elements more clearly.

The paper does not state how the cavity was modelled. It is a “ventilated cavity” but the air change rate is not mentioned, even though this is crucial for the hygrothermal behaviour. The calculation of Z-values is unclear.

The result section only reports the output without any type of interpretation. The discussion section seems to be a combination of analysis, but does not comprise a discussion on limitations, extrapolation, or comparison to the state of the art. Hence, perhaps this section should be called “analysis” instead of “discussion”.

 

More detailed remarks can be found below.

 

Comments 1:

Title:

It is not clear what is meant by “Water Vapour Barrier” in the title.

 

Response 1:

We remove “water” in the title, so the title is “Requirements of the Vapour Barrier in Wooden Frame Walls”

 

Comments 2:

1. Introduction

Both the abstract and introduction implicitly lead to think that the paper addresses moderate climates, whereas only a single location in Denmark is studied.

The Z-value is expressed as GPa s m²/kg. Please elaborate on the relation to mu-values and Sd values.

 

Response 2:

Materials have a µ value which is a dimensionless value. µ values can be converted to Z and Sd values. The µ, Z and S_d values is determined by the following formula, from DS_EN ISO 12572.

S_d = µ x d

S_d = d_air x Z

µ = δ_air / δ.

Z = µ * d / d_air

Where:

• Sd is water vapor diffusion equivalent air layer thickness (in meters).
• Z is the water vapor resistance (in GPa·s·m²/kg or Pa·s/m).
• ρ_air is the density of the air.
• δ is the water vapor diffusion coefficient for water vapor in air

The relation between the µ, Z and S_d values above, is added in section 2.2 Material propeties

 

Comments 3:

The introduction discusses the common approach to specify a ratio of the Z-values of the vapour barrier and the wind barrier. Reference is made to guidelines in Denmark, but only information is provided for classes 1-3. First of all, please highlight what is described for classes 4-5. Secondly, it would be good to check literature on this topic: are there other references that also adopt such rules of thumb? For Belgium the Belgian Building Research Institute uses a ratio of “(minimal) 6 to (preferably) 15”. See p13, paragraph underneath table. Contact 37 (1-13) NL (buildwise.be)

 

Response 3:

There are no guidelines for interior moisture load classes 4 and 5 in Denmark, which is added in the article.

The text below are also added in the article:

Other countries have different ratio between vindbarrier and vapour barrier e.g. Belgien has from 1;6 to 1:15 [22], and Findland has from 0 to 1:80 [23] etc.

 

Comments 4:

2. Methods and materials

Please highlight that only 1D simulations were done. Discuss the impact and validity of this simplification and it’s repercussions on extrapolation potential.

 

Response 4:

We have added the following text to the article: We use the calculation program WUFI Pro for simulations, which performs one-dimensional hygrothermal calculations on the cross sections of building parts.

An article from 2005, describes the variation from 1D to 2D simulation, which indicates that the results are very comparable.

Article from NRC-CNRC in 2005 :

Hygrothermal performance of building envelopes: uses for 2D and 1D simulation. 

 

Comments 5:

2.1   Wall construction.

Please highlight that the insulation thickness was adjusted to obtain the specific U-values, making abstraction of the standard dimensions of wood studs.

“the number of additional studs installed due to the durability of the windows, doors, and corners” It is unclear what the link is between durability of windows and the number of studs in the wall.

 

Response 5:

We do not change the U-value, in relation to wood studs around doors and windows in this article, but we have to do this when we design buildings.

The text “However, the thermal transmittance of the wall depends on the insulation type and thickness and the number of additional studs installed due to the durability of the windows, doors, and corners” is deleted from the article.

 

Comments 6:

Table 1 comprises a “thermal conductivity with wooden frame”. Please highlight that this is an “equivalent” thermal conductivity, and specify how this was calculated. Perhaps this is just area-based, but there are at least 3 common ways to calculate an equivalent thermal conductivity of such a configuration. In that case: also specify the thickness of the wooden stud, as this is not indicated in figure 1.

 

Response 6:

The wooden studs are 45mm, and the thermal conductivity is from DS418 (EN6946).

Figure 1 is changed with the thickness of wooden studs.

The text below is added:

The wood frame makes up 7.5% of the total construction, and have af thermal conductivity on 0,12 W/mK, where the insulation makes up 92.5% of the construction.

 

Comments 7:

“In Figure 1, for the exterior wooden frame wall, the required Z-value ratio between the wind and vapour barriers was calculated, creating a wall with no mould growth risk”. The figure only provides a sketch of the wall. Hence, it is strange to say “In figure 1 … was calculated”. Next to that, I would suggest not to describe any details about the evaluation criteria at this point, and limit that to section 2.3.

 

Response 7:

The text “In Figure 1, for the exterior wooden frame wall, the required Z-value ratio between the wind and vapour barriers was calculated, creating a wall with no mould growth risk. The analysis results indicate combinations of Z-values with no mould growth risk behind the wind barrier, when the Z-value ratio between these barriers was met.” is deleted.

 

Comments 8:

2.2   Material properties

One would expect that Z-value of the vapour barrier and wind barrier is varied. Table 2 only provides fixed values for both. Next to that, it only specifies the water-vapour diffusion resistance factor. Underneath the table it is indicated that the gypsum board has a Z-value of 0.88 GPa s m²/kg. Please ensure that this conversion is explained in the text. Tabel 2 indicates that the vapour diffusion resistance of gypsum board is 7.03m. Hence the Sd-value is 7.03 * 0.025m = 0.17575m. To calculate the Z-value, Sd needs to be divided by the vapour permeability of air (1.87 10^-10 s at 20°C). Hence, the Z-value is 0.17575 * 18.7 * 10^9 m/s = 3.29 10^9 m/s. Given that 1Pa = 1 kg/m.s², m/s is equal to Pa.m/Pa.s = Pa.s.m²/kg. Hence 3.29 10^9 m/s = 3.29 GPa.s.m²/kg. Underneath the table it is indicated that the Z-value of the gypsum board is 0.88 GPa.s.m²/kg. How was this calculated?

 

Response 8:

The materials with varied Z value, was change by another µ values, where the materials have the same thickness.

The value of 7.03 is µ value, and not S_d value.

The formula is: µ * d / δ_air = Z and thus:

((7,03 * 0,025) / 2 ^ 10)/1000000000 = 0,88 Gpa * s * m² / kg

 

Comments 9:

In general it is unclear how Z-values are calculated. Underneath table 2 it is also mentioned that the wind barrier has a thickness of 9mm?

 

Response 9:

The thickness of materials is the same, but we change the water-vapour diffusion resistance factor (µ), to get the right Z value. The method applies to both wind barrier and vapour barrier.

 

Comments 10:

There is no text in section 2.2, only a table. Please rearrange: either elaborate on the material properties and explain how the Z-values are varied (which is unclear now), or merge this section with another one.

 

Response 10:

We have added the text below:

“The water vapour diffusion resistance factor has been changed to be able to achieve the required Z-value for wind barriers and vapor barriers. The specified values show the standard value for the reference construction, as well as the value for the first calculation for other constructions.”

Comments 11:

2.3   Models for Mould risk analysis

Please clarify how this graph was developed. Compared to paper 12, that is presumably published in Journal of THERMAL ENV.&BLDG. SCI., Vol. 25, No. 4—April 2002, figure 2 does not correspond exactly to figure 5 top. Hence, perhaps a different reference was used, but the bibliographic reference is not entirely clear.

 

Response 11:

It is a polynomial, formed from readings of the curve from Sedelbauer's article. Both 1 and 2 days and LIM are excluded from the graph.

See the picture below.

Comments 12:

A reference is made to Table 4. But it is a bit strange that table 4 is part of the results, and it is part of the results section. It is strange to refer to table 4, when table 3 is not mentioned yet.

 

Response 12:

We have added the text below:

"Table 3 shows the results for all insulation materials, with a Z-value of 1 GPa s m²/kg for the wind barrier, in moisture load class 1-5, as well as the U-values 0.15 W/m²K and 0.10 W/m²K."

 

Comments 13:

If I understand correctly, a running average of 4 days, 8 days, and 16 days is plotted on figure 2, and not a single point should supersede the corresponding line? For the SBi it is not clear which time resolution is used.  

 

Response 13:

The single point is to get the polynomial line, from Sedelbaur´s graph. SBi guidelines does not have any indication of days of excess, where we have assessed that an 8-day continuous period is representative of the formula, as seen from Sedelbaur's graphs. We have added the text below:

“The limit value for the number of days with risk of mold growth according to the formula from the SBI instructions regarding moisture, 8 days with risk of mold growth are chosen on the basis of the comparison with the isopleth curve limits. Isopleth limit values for the 8-day curve agree well with the formula from the SBI instructions regarding humidity when comparing the calculated limit values for relative humidity.”

 

Comments 14:

The Z-value of the wind barrier is varied between 1 and 8. With a step function of 1 unit per step? Then “numerous” simulations are done with varying Z-value for the vapour barrier. Please indicate how this was done. Was this a manual system where compliance was evaluated? What interval was used to ‘optimize’ the Z-value? Was the goal to find the lowest possible Z-value that did not entail problems? Please elaborate on the methodological approach that was adopted.

 

Response 14:

We changed the value manually, and the reason for the study was to see the development in the Z-value ratio when the Z-value of the wind barrier is increased. The target was still the limit value in relation to the risk of mould.

 

Comments 15:

Why is interstitial condensation not considered? Why is yearly drying not evaluated (mandatory in EN 15026)?

 

Response 15:

Initially, we checked that the water content does not increase in the construction and is therefore stable over time. In using wufi, we have applied their instructions regarding period stability for water content, convergence failure, balance, etc.

 

Comments 16:

3. Results

Please analyse and discuss the results in table 3.

 

Response 16:

The text below is added underneath table 3:

"Most of the results in the table 3 can be predicted when one has done the calculation for a moisture load class, except for a few. That is the results with light green marking. We cannot explain these results, but when you look at the results, it seems that smaller periods of risk for mold in other moisture load classes turn into larger continuous periods of risk for mold growth."

 

Comments 17:

Table 3: the note underneath the table is redundant.

Table 4: please put “Z-value ratio – wind barrier: vapour barrier” above “thermal insulation”.

 

Response 17:

The note underneath table 3, is deleted.

Table 4 has been corrected according to comments.

 

Comments 18:

Table 4: the first part of the note underneath the table is redundant. Then it states: “Values in parentheses indicate the Z-value ratio of the wind barrier to vapour barrier”, whereas the value in front of the parentheses is “Required Z-value ratio of the wind and vapour barriers”. Please clarify the difference more clearly.

Please analyze and discuss the results in table 4.

 

Response 18:

The note underneath table 4 has been corrected.

Some text has been deleted, an the text below has been added:

"Wood fibre and flax has the same increase of Z value, where wood fibre goes from 6 to 18 GPa s m²/kg, and flax goes from 12 to 24 GPa s m²/kg. Mineral wool has only an increase of Z value from 10 to 16 GPa s m²/kg, and thus has the largest change in Z value ratio (from 1:10 to 1:2)."

 

Comments 19:

Figure 3 reports “the multiplication factor for the Z-value of the vapour barrier”. Isn’t this exactly the same as the aforementioned “Z-value ratio of the wind barrier to vapour barrier”? It is a pity that no complete results are included for straw, grass, and hemp.

 

Response 19:

That is right. Figure 3 shows the same as table 4, and figure 3, 4 and 5 have been removed to limit repetition in the article, as previously mentioned.

We have chosen to carry out calculations for wood fibre, mineral wool and flax as examples. We have chosen a material that has a Z value ratio below the starting point with mineral wool, as well as one that is above it.

 

Comments 20:

A factor of 10 is reported as criterion in SBi. In the result section reference is made to [13], in the introduction reference was made to [11]. It would be logical to refer twice to the same source, as exactly the same criterion is mentioned. Only on the graph this is introduced as DK-factor.

 

Response 20:

The reference [11] and [13], is now in both sections.

 

Comments 21:

Figure 3: please analyze and discuss the results: what is the implicit logic of what is reported? Why are certain values low/high, what is the impact of the Z-value of the wind barrier and why?

 

Response 21:

Figures 3, 4 and 5 have been removed to limit repetition in the article, as previously mentioned.

Figure 3 show results from table 4, and when the Z value increases from 1 to 8, the Z value ratio decreases.

 

Comments 22:

Figure 4: the Y-axis “Z-value of the vapour barrier” is actually identical to “Z-value ratio of the wind barrier to vapour barrier” (but for one case Z-value vapour barrier = 1). Hence you report the same thing but with different names every time.

 

Response 22:

Figure 4 is deleted.

 

Comments 23:

4. Discussion

4.1   Properties

Why does it all of a sudden mention that material files were changed? Based on section 2 I had the impression only standard material files were used.

 

Response 23:

See also response 9, were the text below, are added underneath table 2.

"The thickness of materials is the same, but we change the water-vapour diffusion resistance factor (µ), to get the right Z value. The method applies to both wind barrier and vapour barrier."

 

Comments 24:

“Mineral wool, hemp and flax results are given based on the SBi moisture instructions [13].” Unclear. The results can be compared with the SBi instructions, but not “given”. Or did the authors want to say that the SB-criterion was the most stringent one compared to Sedlbauer’s model for 3 time resolutions?

 

Response 24:

The text below is added in the sentence:

"and our assessment on a 8 days timeline"

Underneath an example from our master project, where the results from mineral wool, wood fibre and cellulose insulation is appear.

e.g. it is the SBi formula that indicates the limit value for mineral wool, and for wood fiber it is the LIM curve for 16 day limit value. These results show that it is not the same formula that specifies limit values ​​for the various insulation materials.

Comments 25:

4.2   Insulation

“Detailed hygrothermal simulations must be conducted for moisture load classes of 4 and 5.” Please include this in the introduction where the requirements are discussed.

“An increase in the wind-barrier Z-value lowered the required Z-value ratio between the wind and vapour barriers for an exterior wooden frame wall” Please explain why.

 

Response 25:

We added some text in the introduction. See also response 3.

The Z value ratio decreases when the wind barrier's Z value increases. If you look at the value of the vapor barrier, then the construction will be exposed to less moisture, and thus less moisture that must diffuse through the construction, and further out through the wind barrier.

 

Comments 26:

4.3 Moisture classes

Line 290: reference is missing

 

Response 26:

The reference is deleted.

 

Comments 27:

4.3   Z-value ratio

The paper states that wind barriers have typical Z-values of about 0.3 to 3 GPa s m²/kg. In that case: why do the simulations only consider 1 as the lowest value? Based on figure 3, these values may entail very specific requirements.

 

Response 27:

We deleted 0,3-3 GPa s m²/kg, and change it to 1 GPa s m²/kg or higher.

In Denmark, are the wind barrier typical between 1 GPa s m²/kg and 8 GPa s m²/kg, which is the reason for the choice, in our parameter study.

We added the text below:

"The tendency for new wind barrier products is that they are more open to diffusion (lower than 1 GPa s m²/kg) than the current wind barriers, which indicating that the 1:10 ratio is not representative of these wind barriers."

 

Comments 28:

5. Conclusions

“Using the requirements that make up the standard is considered good practice in designing exterior walls is not adequate.” From the conclusions it is not cleat that this refers to SBi standards. Please include, as the conclusions should be clear without reading the paper.

“However, moisture that penetrates the thermal insulation of the exterior wall must have less resistance towards the exterior side of the wall.” This sentence is rather vague.

 

Response 28:

SBi is not a standard, but a guideline in Denmark, with suggestions on how to construct buildings in Denmark.

We change the sentence, as below:

“However, it is absolut necessary that moisture penetrate into the thermal insulation of the exterior wall must have less resistance leaving towards the exterior side of the wall.”

We have added the text below:

In Denmark, we have a guideline to Z-value ratio on 1:10 (for moisture class 3), which is placed in the middle of the results, with af wind barrier of 1 GPa s m²/kg. The guideline for the Z-value ratio is a conservative ratio with wind barrier that are on the high end compared to the parameter study, where wind barrier with low values ​​(especially below 1-1.5 GPa s m²/kg) will have a risk of mold growth if they are not analyzed before execution.

 

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for addressing the comments. Sometimes the authors explain something to the reviewer, whereas the goal is the explain it to the reader. 

I understand that the authors presume that the U-value of a wall can be calculated as the surface-averaged U-value (92.5% / 7.5%). This simplification that neglects the interaction between studs and insulation is not allowed in a number of countries. However, now reader will probably be able to derive that indeed this is calculated in a simplified way.

I did make an error in the calculation of Z-values - apologies for that! However, I still find it strange to talk about "wind barriers of 9mm": one may presume that you refer to an Sd value of 9mm instead of thickness.

Next to that, I do feel the paper fails to explain the logic behind the simulations. A number of general concepts are missing. It is unclear why one should not just opt for an extremely vapour tight barrier. It is unclear whether there is any significant drying to the inside. It is unclear whether there might be interstitial condensation and runoff during winter... The paper explains WHAT was done, but little consideration is given to the HOW and WHY. The paper neglects the existence of intelligent vapour barriers, it fails to address moisture tolerance (initial moisture content, air leakage, rain ingress). Even though this could easily be addressed in the discussion section. 

Hence, the paper lacks a certain maturity, as the WUFI model is mainly used as a black-box model. Whether the output makes sense, or is logical, is hardly addressed. Furthermore, the paper discusses the minimal Z-value of the vapour barrier, but does not discuss whether there might be a maximum value. 

Author Response

Thank you for addressing the comments. Sometimes the authors explain something to the reviewer, whereas the goal is the explain it to the reader. 

The authors thank you for the thorough review. We have endeavoured to fill in the review round two to give equally adequate answers and additions to the paper. Many thanks for a fruitful corporation.

I understand that the authors presume that the U-value of a wall can be calculated as the surface-averaged U-value (92.5% / 7.5%). This simplification that neglects the interaction between studs and insulation is not allowed in a number of countries. However, now reader will probably be able to derive that indeed this is calculated in a simplified way.

The authors thank you for pointing out that the U-value of a wall is calculated as the surface average U-value (92.5% / 7.5%). It is a simplified method that neglects the interaction between studs and insulation. It is assumed that the reader can infer that this is actually calculated in a simplified manner.

I did make an error in the calculation of Z-values - apologies for that! However, I still find it strange to talk about "wind barriers of 9mm": one may presume that you refer to an Sd value of 9mm instead of thickness.

Many thanks for repeating and giving the attention regarding the indication of parameters for the water diffusion resistance of a material. It was, in no way our intention to spread uncertainty around the concepts or the specification of parameters. To increase the transparency of the performed parameter study line 127 – 129 have been clarified, as:

‘The wind barrier is specified with a water vapour resistance factor µ of 22.2, corresponding to a Sd value of 0.2 m and a Z-value of 1 GPa s m²/kg having a thickness of 9 mm.’

Next to that, I do feel the paper fails to explain the logic behind the simulations. A number of general concepts are missing. It is unclear why one should not just opt for an extremely vapour tight barrier. It is unclear whether there is any significant drying to the inside. It is unclear whether there might be interstitial condensation and runoff during winter... The paper explains WHAT was done, but little consideration is given to the HOW and WHY. The paper neglects the existence of intelligent vapour barriers, it fails to address moisture tolerance (initial moisture content, air leakage, rain ingress). Even though this could easily be addressed in the discussion section. 

Since the 1960s, it has been good practice in Denmark to use a vapor barrier made of PE foil. Currently produced from recycled PE. Concerns regarding the quality of recycling and new PE foil have led to assessments of the lifetime of PE foils in use as a vapor barrier, which has increased interest in using other materials for vapor barriers. The authors have explained precisely this aspect in the introduction. Allow us to draw attention to lines 51 - 58. For the sake of order and to maintain the reason for the parameter study, the problem is repeated in the discussion.

The following paragraph has been added to the discussion.

This study was conducted as a purely simulation study, to investigate the relation between wind and vapour barrier under different indoor climate conditions and thermal insulation materials, i.e. biogenic insulation materials. In several years the vapour barrier of PE-foil has been the standard in common practice. Today, many practitioners challenge the PE-foil and opt out the PE-foil in favour of other materials e.g. vapour retarders of paper, intelligent vapour barriers and wooden board. One could argue that using PE-foil is the safe solutions. This building practice is given in the publication on common building practice [11] and new knowledge is needed when the PE-foil is changed to other materials. Therefore, this parameter study investigated the relation between the wind barrier and vapour barrier in regards to the vapour diffusion resistance. The aim of the study was not to investigate specific materials, thus contributing to the knowledge of the threshold values under different design considerations which can be used when designing and choosing materials. The study assumed perfect wall designs with no air leakages from the inside or rain ingress from outside. These two parameters are very difficult to determine, however very relevant for the perforance of the structure. In this study rain ingress and air leakage was considered as failure in the structure, as walls with larger air leakage or rain penetration typically fails over time.

Hence, the paper lacks a certain maturity, as the WUFI model is mainly used as a black-box model. Whether the output makes sense, or is logical, is hardly addressed. Furthermore, the paper discusses the minimal Z-value of the vapour barrier, but does not discuss whether there might be a maximum value. 

That the paper is mentioned as having a certain lack of maturity, due to the method using the WUFI program as a basis for the parameter study, gives rise to some wonder. Validation of the WUFI tool itself is not the purpose of the study. The authors would like the assessment and validation of WUFI as a tool to take place elsewhere. The paper is a demonstration of the use of WUFI as a tool for the study shown. WUFI is a reputable tool. The actual correlation between experiments and WUFI as a tool is currently being investigated and will be presented later.

In this paper, it is the minimum Z-value of the vapor barrier that is treated explicitly.

 

Author Response File: Author Response.pdf