Calculating Dew Point in Wall Assembly

Calculating Dew Point in Wall Assembly

Postby rgharvey » Sun Jan 28, 2018 11:04 am

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
 
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Re: Calculating Dew Point in Wall Assembly

Postby adamgidaszewski » Fri Mar 02, 2018 12:29 am

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?
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