What is the best exterior rigid foam insulation for under a deck?

Andrew Cherubini
Updated: March 22, 2021

I live in Calgary, Alberta and bought a house built in 2007. I noticed this past winter when I started renovating my basement that the rim joists were poorly insulated and we're damp. I insulated the rim joists with R10 rigid board with spray filling the gaps and then R12 batt insulation. I then noticed that under our front deck, the hole for the foundation wasn't back filled so approximately 4 feet of concrete was left exposed with the sil plate. 

I'm thinking of taking Rigid board and installing it against the concrete wall and will plate to hopefully help with preventing dampness and further heat loss. Is this recommended? Do I need to put a protective barrier over the rigid board, or will leaving it exposed once it's secure to the foundation wall be enough? The porch is covered and has skirting so the rigid board is somewhat protected from the elements. 

Any help/advice is appreciated

Responses (3)

Andrew Cherubini
Andrew Cherubini 3 years ago

Thank you for the response!! I'm glad to know I was on the right track with the rigid foam board. The front porch is covered by an overhang and deck boards. The side of the porch will be skirted with 2x6's. So the rigid board will be protected from the elements. My plan is to clean the concrete and the put it up using foam board adhesive and if need be tapcons. I was just curious to see if I had to cover it with anything. Might just attach it to the foundation and put a bit of back fill to help keep it in place. 

Emmanuel B. Cosgrove 3 years ago

Adhesive should be okay, if for any reason it starts to come unadhered you can always put a few tapcons in after. If it's backfilled then it's not going to go far anyway.