Should I insulate the top of a block wall with spray foam? And a question about attic R value

Ken Traefald
Updated: April 19, 2022

Two questions

My house is a 1920's, 48x24, 960sq ft with a 4/12 hip roof in Minneapolis. It has 2x4 rafters notched to the top plate. I have a R 48 insulation in the middle of attic. Would you know the effective R valve for the whole ceiling as the outer 3 feet of the perimeter progressively fall off in R value to the bare top plate? It's also subject to wind washing.

I'm thinking of having my basement walls spray foamed on the inside with the new low VC foam. Should the top of the block be foamed also? In one video, a Univ. researcher said not to but gave no reason why. One theory I have would be that by foaming the top of the blocks, any moisture working up thru the blocks could only pass the floor joists. Which could cause early failure.


Responses (1)

Emmanuel B. Cosgrove 2 years ago

If your floor joists are embedded in concrete then they are at risk of staying wet, and leaving space around them will provide some release of moisture in close proximity to them, that may be the reasoning behind that recommendation and we wouldn’t disagree with that. 

As for your attic insulation, you could make such a calculation, but do you need it? It is without a doubt insufficient for your climate, The problem with a roof such as you describe is that it is very difficult to insulate properly to the edge. Are you hoping to further insulate it more?