Related Advice
Related Guide
Latest posts
Responses (1)
- Discussions
- Foundations and Basements
- New foundations and slab-on-grade
- Slab on Grade
- Spray foam
- Slab on grade
- Insulation
- Insulating
- Basement floor
Related products


ThermaSill PH is an engineered solution for eliminating cold bridge or thermal …
Learn moreThermaSill PH Insulated Threshold Step by Legalett


1 step protection from the ground up
Learn moreHeatlok Soya HFO – Under Slab / Radon Protection


Innovative and patented protection against moisture and mold
Learn moreTEKSILL Vented Sill Plate
Professional services
Related Advice
Would you recommend Legalett over water radiant floor heat for a backyard studio in Toronto?
Posted by Christine Pountney
May 2, 2019, 1:22 p.m.
Delivery: radiant, forced-air and decentralized
1 Responses
Can you insulate an existing slab on grade floor?
Posted by Patrick Berkeley
Dec. 31, 2020, 10:14 p.m.
Foundation and basement improvements
5 Responses
How do you fix a moisture problem in a finished basement?
Posted by Margo MACFARLANE
Oct. 28, 2019, 10:24 a.m.
Foundation and basement improvements
1 Responses
When building a slab on grade, EPS rigid foam insulation is the most common choice of insulation, for performance and cost. I wouldn’t go as far as saying you 'couldn't' use spray foam below a slab, but I’m not sure why you’d want to. Spray foam for sure has uses where it is the best choice by a long shot, but under a slab is not the place. On this page you can see what we like and don’t like about spray foam and why –
4th generation blowing agents for spray foam
If you need it as a vapor barrier, air barrier, or to insulate difficult areas like basement rim joists or to create a radon barrier in a crawl space it’s a great choice, but in most other applications, and certainly with a slab on grade there are better ways. When an installer sprays the polyurethane insulation they really go by experience to get the appropriate depth, and of course a skilled installer could get close, but it would still be an uneven surface to an extent. That would mean your mesh and mesh chairs don’t sit at an even level, nor does your rebar, and then the thickness of concrete may in turn vary as well, so you may need a little more, or it may be too thin in some areas.
The only reason I could even think of to use spray foam would be if you needed to maximize the R value in order to meet height restrictions for your build, but if your allowance is down to a couple of inches that’s cutting it pretty close!
All that said, there may be something I’m not thinking of and I’m curious, why were you thinking spray foam?