Foam board options for basement walls
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For insulating basement walls in climate zone 5a, I am considering using foam board alone without any batt insulation. I was thinking about going with R15 polyiso foam board.
Anyone have any example product they would use? Are there any options that are 2.5 inches thick that are at least R15?
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Allison Bailes at Energy Vanguard (he is very respected around here) used InsoFast insulated panels in his own basement. He has some blog posts and You Tube videos about the product and the install that may be of interest to you.
https://www.energyvanguard.com/blog/insofast-makes-basement-insulation-easy/
I did InsoFast in a basement powder room, mostly just to see how it works. Pros are it's fast and easy. Cons are it's expensive and it doesn't meet energy code, it's just R8.
They have multiple thicknesses. 2" (R-8), 2.5" (R-10), and now they have a 3.75" (R-15.9) as well. I love the product.
To get R15 with 2.5 inches you need R6 per inch, which means polyiso.
Do you have links to any examples? Having a hard time finding 2.5 inch polyiso.
Highest I can find is 2 inches: https://www.homedepot.com/p/RMAX-Pro-Select-R-Matte-Plus-3-2-in-x-4-ft-x-8-ft-R-13-1-Foam-Insulation-Board-637898/313501510
You may have to go to a roofing supply store or a building supply store.
2.5" material is going to be rated R16 in most cases, and is the closest standard size thickness to what you need R value wise. It's fine to go over code minimum if you want, but you need to use AT LEAST code minimum for your project.
Common sizes are 2", 2.5", then 3". 3" is usually rated R19. The box stores usually only carry up to 2" thick, if even that, but the thicker stuff IS commonly available. Try commercial insulation or roofing suppliers (commercial roofs are often insulated with polyiso), and also try the "Pro desk" at the box stores, which will probably be able to order in the 2.5" thick material without any trouble.
Bill