Wall assembly – 2X6 with Spray Foam
R values of components
Air film – inside .68
2 layers gypsum board 1.12
5 1/2” spray foam 37.4
2X6 stud 6.875
Sheathing 1.3
Exterior cladding .5
Air film – outside .17
Calculation using Type 1, conventional method:
R value through cavity – add components
Air film – inside .68
2 layers gypsum board 1.12
5 1/2” spray foam 37.4
Sheathing 1.3
Exterior cladding .5
Air film – outside .17
Total R 41.1
R value through stud – add components:
Air film – inside .68
2 layers gypsum board 1.12
2X6 stud 6.875
Sheathing 1.3
Exterior cladding .5
Air film – outside .17
Total R 10.6
Calculate total R of the wall assembly – multiple the above by the percentage of each in the wall.
This will assume that the wall has 18.75 percentage stud area
10.645 X .1875 + 41.17 X .8425 = 36.68
Calculation using Type 2, ASHRAE method:
Calculate R value of components not in cavity:
Air film – inside .68
2 layers gypsum board 1.12
Sheathing 1.3
Exterior cladding .5
Air film – outside .17
Total 3.77
Calculate the wall cavity using the weighted average of the U values of the components:
((1/37.4) X (.8125)) + ((1/6.875) X (.1875)) =.049 = 1/.049 = R20.4
Add the two together 3.77 + 20.4 = 24.17
Difference in methods
Two equations to consider:
2 + 2 =4
½ + ½ = 1/1 = 1
AND
2 X .1875 + 2 X .8125 = 2
(1/2 X.1875) + (1/2X.8125) = .5 = 1/.5 = 2
But with unequal numbers things change:
4+2= 6
¼ + ½ = ¾ = 1/.75 = 1.33
AND
2 X .1875 + 4X.8125 = 3.6
1/2X.1875 + ¼ X.8125 = .296875 = 1/.296875 = 3.368
The long and short of this is that using the ASHRAE method of the weighted average of U values, weights the smaller number over the bigger number. That’s why ASHRAE does it, as more heat will go through the wall where there is less resistance holding that heat back. In other words, more and more heat with go through the stud as more and more resistance is put in the cavity, without increasing the heat on the inside.
