GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter 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

Frost-Protected Slab Foundation

Nest_Builder | Posted in General Questions on

Greetings All
I’m trying to use a FPSF for my addition.

I understand that as designed the house is supposed to help with heating the earth so there’s no freezing. Also more recent articles show insulating the slab itself. I see this done both using sleepers and rigid foam above the slab below the subfloor or below the slab itself. I have two questions:


Should I insulate under the slab?

What are the dimensions of the footings?
I saw some styrofoam forms that come 16 inches deep so I assume that should be the base of the footing  and it says 18” below the soil for height plus up to a foot above….

thanks for any help!!

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. GBA Editor
    Kiley Jacques | | #2

    Hi Trevor,

    I know it is not exactly the frost-protected shallow-foundation approach you are planning, but if you haven’t seen this article by Mike Maines, I think the details would interest you. It specs 2 in. of 25 psi ESP around the footing: Minimizing Concrete in a Slab-on-Grade Home.

  2. Nest_Builder | | #3


  3. Nest_Builder | | #4

    So the drawings do not show any insulation under the slab in a heated space. I’m Zone 5 in Putnam CT.


    It will have sleepers and ridged insulation under subfloor on top of slab but NO radiant heat.

    Thanks so much!!

  4. Expert Member


    There are two approaches to FPSFs. One uses the heat from the building above to ensure the ground doesn't freeze, the other uses more insulation to do the same thing. This is a comprehensive guide to both types:

  5. Nest_Builder | | #6

    Thanks Malcom!
    That’s what the image I posted comes from!

    But I see a lot of people saying I should have R10 under the slab and that’s what I’m finding confusing.

    If I insulate over and under the slab then it doesn’t let heat come into earth from house. That’s seems like a plus but it doesn’t fulfill the idea of the FPSF.

    But I could just pretended it was I heated and go that route...


    1. Expert Member
      Deleted | | #7


    2. Expert Member
      MALCOLM TAYLOR | | #8


      If you go the route of insulating the slab so that you don't rely on heat from the house keeping the earth from freezing, you are instead relying on that insulation (and that which extends out at the perimeter) keeping the warmer earth below the house from losing it's heat. The two advantages of going that route are that you save the energy you would have used heating the soil, and the house can be left without heat without suffering damage.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |