Condensation Location in an Assembly

[MSM HGN]

Hi all,

I am trying to figure out where the potential condensation might happen in an assembly and am wondering if Therm 7.8 is good enough to run such analysis or it needs to be Therm 8.1 which has Water Vapor Diffusion Resistance Factor of the materials. 

Your help is appreciated.

Thanks

[MSM HGN]

Below shows the analysis I did with the two versions (and two runs with version 8.1; with and without vapor resistance data of the materials, middle image and right image respectively). The results show the same location for the potential condensation (dew point temperature). Please help me if I am on the right track for what I want which is to make sure where the condensation will happen in my assembly.

Thanks 

[Jerzy Bogdan Zembrowski]

This program won't solve this problem. You need to use WUFI 2D because it requires a dynamic analysis of heat and water vapor flow over a period of at least 24 months, using meteorological data for a given building location.

[Collin Robinson]

I agree that WUFI is best to solve this problem. However the typical version of WUFI is a 1-dimensional analysis, so you must select some 1D slices through the assembly you showed to run. If your concern is condensation leading to mold, WUFI has a really great mold evaluation tool from Viitanen et al where you can select various materials.

You can still learn things from the THERM model, though. It looks like most of the materials are not storing much water, so that helps the accuracy of the Therm model. The other thing Therm doesn't tell you about is air movement. Assuming winter conditions, if you can keep the warm humid interior air out of the assembly via air barrier construction, surfaces within the assembly at condensing temperatures are low hazard. Example would be installing a vapor-permeable air barrier behind the interior finish. Although Therm is at a steady state condition you can use 99% design values for exterior conditions, similar to what's in the US NFRC 500 guide shown here, designed for fenestration not clear wall assemblies: https://nfrccommunity.org/resource/resmgr/2026technicaldocs/NFRC_501-2026_E0A0.pdf. 

There is also a fantastic resource from Canada, called the Building Envelope Thermal Bridging Guide. In that they did 3D modeling of common assemblies, perhaps yours is one. They show the coldest point and you can put in your exterior and interior conditions to find what that temp would be: https://research-library.bchousing.org/Home/ResearchItemDetails/722

Best of luck!

Collin

On Monday, April 20, 2026 at 2:45:00 PM UTC-4 zembr...@bdb.com.pl wrote:

This program won't solve this problem. You need to use WUFI 2D because it requires a dynamic analysis of heat and water vapor flow over a period of at least 24 months, using meteorological data for a given building location.

poniedziałek, 20 kwietnia 2026 o 17:36:22 UTC+2 MSM HGN napisał(a):

Below shows the analysis I did with the two versions (and two runs with version 8.1; with and without vapor resistance data of the materials, middle image and right image respectively). The results show the same location for the potential condensation (dew point temperature). Please help me if I am on the right track for what I want which is to make sure where the condensation will happen in my assembly.

[Jerzy Bogdan Zembrowski]

I referred to WUFI 2D, and this is not a one-dimensional analysis, but a two-dimensional one. I should emphasize that the program is not for everyone, as the interpretation of the results must be performed by a structural physics specialist. Best regards.

[Collin Robinson]

Yes WUFI 2D is 2D, sorry. It looks like a good software. I haven't used it yet, curious to check it out!

For this particular assembly, I think I found something close in the Canadian resource: https://thermalenvelope.ca/catalogue/5.1.13/v1.7.3

In that study, they show the lowest temp on sheathing to be next to the studs. This is shown as Ti1 in the heat map diagram. With R10 exterior insulation, that factor is 0.42, so if exterior conditions are 0°C and interior conditions are 20°C, that spot on the sheathing would be 20°C x 0.42 = 8.4°C. If interior air can reach that and its vapor pressure is such that it's 80%-100% of that at the 8.4°C dewpoint, condensation is a risk for the hours during the year where that occurs. Whether that's a problem depends on many things of course. 

Collin