What is the best foam board insulation under drywall?

Wesley Pracht Jan. 11, 2020Updated: Sept. 4, 2020

I live in Climate Zone 6 in a low humidity, cold, and snowy environment. Heating only no cooling. Currently have T1-11 siding, tar paper, R13 FB insulation, and the interior finish is 1/4 plywood panels.

The siding is in OK shape but I really want the plywood panel interior gone and replaced with drywall. My idea is to tear out the plywood and remove the old FB insulation. I would like to replace with R15 Rockwool and about 1" of foam board insulation attached to the stud and seams taped. Then I would set 1/2" drywall on top of the foam and attached with long screws.

Does this sound like a good plan? Would anyone suggest against using Polyiso insulation as it does not do good in cold environments? Please let me know your thoughts

Responses (3)

H Vargas
H Vargas 7 months ago


Hopefully this article isn't too old to receive comments and questions.  I see a couple of characteristics discussed but one is missing.  Or perhaps it's just persumed and I'm not catching it.  Polyiso foam board can address water vapor and insulating performance, as discussed, but what about compressive strength?  Will the polyiso crush or deform in some manner when you drive the drywall screw through the sanwiched drywall/Polyiso/stud configuration?  If not initially, what about over several years?  Thanks for sharing your knowledge.

Hugo V.

Mike Reynolds 7 months ago

Hi Hugo,

I would not worry at all about it compressing while attaching it to a wall, it behaves pretty much the same as EPS rigid foam in that cirmcumstance. Any compression would be right where the screws are, it would be insignificant, and once it's attached to a wall it would not compress any further.