## Calculating Dew Point in Wall Assembly

### Calculating Dew Point in Wall Assembly

Has anyone seen any official calculations for this? I know it's a concept that is talked about in a couple study guides, but they never seem to explain it. I found this http://www.greenbuildingadvisor.com/blogs/dept/musings/are-dew-point-calculations-really-necessary which seems to give an explanation, but then discredits it further down..

The calculation is: Temp at sheathing = Indoor Temp - (Delta T * % of Insulation at Interior)

The example explains that average exterior temp for the area is 30.3F and the given interior temp is 70F with 35% Relative Humidity, with a dew point of 40F (taken from psychometric chart)

Wall assembly is Interior GWB, 2x6 Wood Studs with Cellulose, OSB, 1" Rigid Insulation, Rain screen gap and Wood Siding
Delta T is difference between interior and exterior temp: 39.7F
% of Insulation at interior is R-value inside/Total R value for the wall: R19 (Cellulose) / R24 (Overall Wall construction) - These Are Given. = 0.79, 79%

Tempt at sheathing= 70F -(39.7*0.79) = 38.6F < 40F Dew point. This means that at the average coldest temp, the sheathing would have condensation accumulation on the interior side. Solution would be to add additional exterior insulation.
rgharvey

Posts: 50
Joined: Tue Jun 23, 2015 6:22 pm

### Re: Calculating Dew Point in Wall Assembly

I too am wondering about this...I came across the exact same article which explained it to a point of the concept making sense, but the calculation is still a bit unclear. Anyone have a good source to help understand the calc process?
adamgidaszewski

Posts: 18
Joined: Wed Sep 20, 2017 10:58 am

### Re: Calculating Dew Point in Wall Assembly

Just wanted to bring this question up again and seeing whether you guys or anyone has found an answer?
bdaniel_877

Posts: 6
Joined: Sat Sep 15, 2018 1:00 pm

### Re: Calculating Dew Point in Wall Assembly

I don't think that you'll need to calculate the dew point, but you may need to calculate the temperature at a point in the wall. The dew point would be in addition to this and is probably more than the exam will ask.
thd7t

Posts: 381
Joined: Mon Aug 22, 2016 1:23 pm

### Re: Calculating Dew Point in Wall Assembly

There is a formula in the "Thermal Basic" chapter in MEEB
Anob

Posts: 60
Joined: Tue Feb 21, 2017 6:12 am

Return to PDD - Project Development & Documentation

### Who is online

Users browsing this forum: No registered users and 2 guests