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

Insulation under gravel in slab?

ranson | Posted in Green Building Techniques on

I’m looking at an engineer’s drawing (research, not from my own house), and I noted an interesting detail for a frost protected monolithic slab. Working up from the bottom, there’s a flat layer of crushed gravel and then a flat layer of EPS. The turn-down footing sits directly on the EPS, and the slab sits on a thick layer of compacted stone over the EPS. The vapor barrier is located in contact with the concrete.

Why don’t I see this detail more frequently? It seems well suited to locales with thick topsoil, where you have to remove 12+ inches of dirt anyway. There’s no thermal bridge, and the insulation is placed flat and weighed down naturally by the gravel.

Any thoughts?


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
    Martin Holladay | | #1

    As long as the builder remembers that the layer of polyethylene belongs directly under the concrete, the system you describe can work.

    When I built my garage, which has a thickened-edge slab, I found it easy to slope the crushed stone to create the thickened edge, and then to install the rigid foam directly under the poly and concrete. Sloping the rigid foam wasn't that tricky -- you can always use a little bit of housewrap tape to hold the rigid foam in position.

    For more information, see Polyethylene Under Concrete Slabs.

  2. GBA Editor
    Martin Holladay | | #2

    One example of your approach is the foundation built by Rob Myers. Here is a link to a GBA article describing his experience: A Timber-Frame House for a Cold Climate — Part 1.

    Below, I will attach a relevant photo.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |