I'd like to insulate an exterior 2x4 wall to R-21. It looks like I'll need to use Rigid Foam of some sort. I'm not finding any products at my local store which are 3.5 inches thick. At the moment, I'm finding:
- Rmax Thermasheath-3 Polyisocyanurate at 2 inches which provides R-13.1
- Owens-corning Foamular XPS at 1.5 inches which provides R-7.5
(that adds up to 20.6, which after considering the exterior wood and siding and the interior sheer wall, I should technically be at > R-21)
Is this my best bet? Simply put these two foams one on top of the other? Is there a better system or single product I can buy in 3.5 inch thickness? Should one or the other be on the outside?
Thanks for any advice, I'm new to insulation work.