Need Whole Wall R-Value Estimate
jackofalltrades777
| Posted in GBA Pro Help on
Looking to get a whole wall R-Value for the following setup (commercial application):
10″ I-studs with 3/8″ OSB web (does this reduce thermal bridging?)
16″ o.c. framing
Icynene foam insulation under the exterior siding
What would the total whole wall R-Value be for this setup? Any STC ratings on this wall system?
The option is between the above or utilizing an ICF wall (2.5″ EPS x 8″ concrete x 2.5″ EPS).
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Peter,
I'm not sure that I understand your wall assembly. It sounds like you will be using I-joists for studs, and will be filling the 10-inch deep stud cavities with open-cell Icynene spray foam. Is that correct?
The confusing part is your statement that you will be installing "Icynene foam insulation under the exterior siding." I assume that you meant to write, "on the interior side of the sheathing."
You can assume that the Icynene has an R-value of R-3.7 per inch, so 10 inches would have an R-value of R-37. I'm not sure what type of de-rating is necessary to account for the (small) value of the thermal bridging through the webs of the I-joists and the volume of the I-joist flanges. But if you de-rated by 10%, you would end up with R-33.3.
Before the decide to go this route, you should check with your foam installer to be sure that the manufacturer allows this type of spray foam to be installed at this depth. Not all types of foam have obtained a fire rating for deep installations of foam.
Needless to say, many building inspectors balk at the use of I-joists as studs, since I-joist manufacturers haven't tested their joists under compression when installed vertically. That means that you may need a separate 2x4 or 2x6 wall to support your roof loads.