Foam Under Slab - Needed?

[Bolson32]

I've recently added on a 26x30 3rd stall and am looking at eventually heating it. I got a screaming deal on 4" used roof foam that I put around the interior of the frost walls. Should I do the slab too? Or would poly be enough? Budget is getting tight wondering if I'll ever make that $1500 back.

 

[Dagny]

do the foam. its mn.

 

[billconner]

If you care about building code, 2" of foam or equivalent - Rockwool, Glavel, etc. - under slab is required if heating. Tough to add later.

 

[Firebrick43]

If you are keeping it heated you will make it back

 

[ConCretin]

Underslab insulation should meet ASTM C578. Your roofing insyulation may not be appropriate for this use.

In any case, unless you are installing in-slab radiant heat, I don't think you'll recover your investment in underslab insulation.

 

[Bolson32]

Underslab insulation should meet ASTM C578. Your roofing insyulation may not be appropriate for this use.

In any case, unless you are installing in-slab radiant heat, I don't think you'll recover your investment in underslab insulation.

It's 4" 25lb psi EPS, pretty sure that would qualify.

 

[rancherbill]

It's 4" 25lb psi EPS, pretty sure that would qualify.

It's a long time since I researched, but I think there several types.

 

[iagsxr]

After having a shop with foam under the floor I wouldn't even think about not having it if I were to build again.

As mentioned, it's pretty hard to add later.

 

[rayra]

Lake Elmo, MN?

How is this even a question?

 

[Bolson32]

Lake Elmo, MN?

How is this even a question?

Because the frost walls already have 4" of foam on them from the top of the slab down 48". This means 4ft of soil and slab are effectively already insulated.

Based on this 2021 IECC handout it doesn't look like both are needed.

An installation guide to slab edge insulation that meets the 2021 ... https://www.fcgov.com/building/files/cfc-slab-edge-insulation-v2.3.pdf

 

[Moss]

I'm sure it would take a while to get the cost back in dollars but I put foam under my slab and it was one of the best things I did. I'd do it again 100 percent. The slab stays so warm when I heat it and it's much more comfortable on my feet. Worth every penny.

 

[600SL]

I didn't heat my floor or put in insulation under the slab. I regretted that later as my feet always were cold. My new slab uses high density 60 PSI rated 2" foam. Corning Formular 600. I went with the Formular 600, 60 PSI rated foam because I'm putting a lift on it.

FOAMULAR® NGX™ 400/600/1000 | Owens Corning Insulation

FOAMULAR 400/600/1000 products are high strength XPS insulation boards designed for use in engineered applications requiring additional load-bearing capability.

www.owenscorning.com