Calculating Dew Point in Wall Assembly
Posted: 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.
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.