Condensation Issue

[Masoome Haghani]

Hi all,

Working with Therm 7.8, I wanted to analyze condensation issue in interior side of wall and based on the wall detail's info (Gypsum board + cavity wall with 2x6 metal studs @ 16" OC + rigid insulation + CMU), I got the below result:

Based on the dew point location which is in the cavity area of the metal stud layer, I am wondering if it will cause any issue for interior side of wall where I have gypsum board adjacent to the metal stud. I mean do I need to increase insulation thickness or the result that I already have will not cause any condensation on interior surface?

I would appreciate if anyone could help me.

[Georg Reichard]

You are misunderstanding what the "Dewpoint" is — i.e. it's not a iso-line per se.

The dewpoint is a hygrothermal metric of air — now what "air" you are referring to is the important point.

If you talk about your interior air — which I assume you do — then any interior surface (along your gypsum board) that is below that temperature will see condensation. So in this case, you should be good ...

However, the air in your cavity does NOT have the same hygrothermal condition as your interior air (unless you have air leaks) and is defined by the vapor diffusivity of your gypsum board and vapor pressure difference between interior and exterior air ... something THERM does NOT calculate!

Now, if you have an air leak from the interior side, you can assume that this air would mix with cavity air and soon have similar hygrothermal conditions (of course depending on the size of the leak/hole) — but it doesn't hold true that the dewpoint "line" is in the middle as shown in your image as this air will simply condensate on any surface in the cavity that is below the dewpoint ... so any surface to the right of your line, but not start raining in "mid-air" ..

Hope this makes sense ... it's not that easy and I'd recommend to do some reading on vapor diffusion vs air leaks as it leads to different results!

Dr. Georg Reichard, P.E.
  Professor and Head, Department of Building Construction
  Associate Director, Myers-Lawson School of Construction
  Director, Building Enclosure & System Technologies Lab

  (540) 818-4603  |  reic...@vt.edu  |  www.bc.vt.edu
 

Department of Building Construction
the MYERS-LAWSON SCHOOL of construction

College of Engineering | Virginia Tech
430C Bishop-Favrao Hall (0156)  |  1345 Perry St  |  Blacksburg, VA 24061

On 7/24/2023 3:03:39 PM, "'Masoome Haghani' via THERM" <LBNL-...@googlegroups.com> wrote, copying :

Hi all,

Working with Therm 7.8, I wanted to analyze condensation issue in interior side of wall and based on the wall detail's info (Gypsum board + cavity wall with 2x6 metal studs @ 16" OC + rigid insulation + CMU ), I got the below result:

Based on the dew point location which is in the cavity area of the metal stud layer, I am wondering if it will cause any issue for interior side of wall where I have gypsum board adjacent to the metal stud. I mean do I need to increase insulation thickness or the result that I already have will not cause any condensation on interior surface?

I would appreciate if anyone could help me.

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[Daniel Bettenhausen]

Therm does not consider the diffusivity of materials to water vapor.  In order to use it in the way you are describing there needs to be a distinct vapor barrier which separates the interior and exterior environment.  For example, if the insulation in your model was foil faced, polyethylene sheet (or some other strong vapor barrier) it's likely that the vapor pressure (water) in the wall cavity would be close to the interior vapor pressure (the gypsum board is vapor permeable). 

My first observation is that your dew point temperature appears to be close to your interior temperature.  I would not expect interior humidity levels in most applications to be that high if the design temperature is indeed -5 F.  That would be challenging to design for.  My second observation is that the insulation layer is relatively thin.  Thus, I wouldn't be particularly surprised if you had a condensation problem assuming the vapor barrier is placed at the interior face of the insulation.  You are correct to assume that increasing the insulation thickness might mitigate the problem. 

If there is no distinct vapor barrier you would be better off using WUFI or some other software program that can consider the vapor permeability of each layer and how the vapor pressure changes throughout the wall.

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