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

Insulating a Concrete Foundation Wall

mmtackmier | Posted in Energy Efficiency and Durability on


I am soon to be building a garage in Wisconsin (Climate Zone 6b). The framing will sit on 4’ frost walls with a 6” slab on the inside. I will be running a radiant heat system in the entire building (37’ x 66’). I am looking for guidance as I choose how to insulate the foundation and slab most efficiently. Here is what I am thinking:

Foundation: 2” of eps on the outside of the foundation wall

Slab: 2 layers of 2” eps (4” total) under slab

I am also considering using either 1” or 2” eps on the outside of all the exterior walls. The exterior walls will be blown in fiberglass along with the attic.

My main question is weather or not I should have the foundation insulation layer on the inside or the outside of the 8” concrete foundation wall? Also is 4” of insulation overkill under the slab?

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
  2. dickrussell | | #2

    One thought that occurs to me, assuming the described assembly has the slab poured up against the frost walls, is that the heated slab will lose some heat at the perimeter by conduction to the wall and down to the soil under the insulation and footing. You could put the insulation all on the inside of the frost walls, from footing up to the top, and place the sub-slab insulation and slab inside that. In the outside location, the frost wall insulation will have to be protected from UV and physical ("weed whacker") damage. If on the inside, any projection above slab level must be protected from physical damage as well as for thermal/ignition reasons. If that is the case, such protection can be provided by cement board (eg. Durock), placed before the slab is poured, projecting maybe 1/2" above the top of the foam. A 2x layer can be placed over the top of the frost wall/foam board, with a dado cut into the under side of the 2x to capture the top of the cement board. This arrangement was used inside my own garage.

    With a heated slab, 4" of insulation beneath it certainly is not too much, considering the huge surface area. You could estimate the loss for any thickness of the insulation, using a slab temperature somewhat above inside air temperature and an assumed ground temperature below the insulation. But there is some uncertainty in that ground temperature, and it won't be quite what deep ground temperature is. The ground will be heated to some extent by heat loss to it, and that heat gained will be passed on to the ground further down, according to the thermal conductivity of the soil.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |