GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Foam Under Slab Foundation

mpsterner | Posted in GBA Pro Help on

I have an unconventional side project going on at my all-electric Pretty Good House. I am building an insulated hot tub constructed from poured concrete, stone, tile, etc. I am not sure where else to turn for help other than my trusty GBA community. 

I’ve been working to get help with the energy calcs here:

I can see that adding foam underneath the slab foundation cuts heat loss by about 41% (I think).

I have a packed bed of gravel on top of undisturbed hard-packed sand. 

I have read this article and it seems like this shouldn’t be much of a concern but I could use some experienced feedback:

Can I put something of this weight on top of 25psi foam?
(alternatively, I could skip the foam underneath and lay wings of foam out to the sides under soil like a shallow frost protected foundation)

I have this weight distributed over 116sf (or 16704 square inches):
216 cu ft of concrete (x 150lbs/cu ft) = 32,347.5 lbs
161 cu ft water (x 62.4/cu ft) = 10046.4 lbs.
Estimated 10 people = 1,800 (generous)
Total = 44193.9 / 16704 (sq in) = 2.65 psi

With 25 psi foam and a reasonably flat packed bed, am I correct in saying that this is well within the range of no worries for cracking my 6″ slab foundation?

Should I go thicker on the slab or add more rebar than 2′ on center?

Thank you! 

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Patrick_OSullivan | | #1

    I'm no hot tub expert (other than enjoying a drink in one), but the math seems reasonable.

    I would seek the advice of those that build such hot tubs, but I'd also be curious about preloading the structure before finish coat.

    To put it another way: after the slab and walls are formed and poured, can you temporarily fill the whole thing with water to further compact/load the intermediary layers? The usefulness is dependent on the plasticity of the overall structure, but perhaps it would help "get out of the way" some future stresses that could lead to cracking after finishes are installed. Just a thought.

  2. wastl | | #2

    if you have access to that - this is an application calling for a bed of glass foam gravel. (if you have a high water level then forgot what I said).

  3. gusfhb | | #3

    I had a radiant heated garage. 2 inches of standard blue foam under. point loaded four spots with up to a 4000 pound SUV

    Never a crack

    I would think the biggest risk is at the edge of the tub due to differential compression

    Steel is cheap
    In fact come to think of it, if you bent steel into the walls it might become a structural part of the building.
    Quick tornado coming, everyone into the hot tub!!!

  4. mpsterner | | #4

    What I've decided to do is 1.5" foam underneath the slab set on a very smooth surface, then I compacted the foam over the base to push it further into the stone and sand. Then, I increased the foundation slab to 8" and the rebar to 12" on center.

    I can't imagine that this will result in cracking with those steps taken.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |