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

Whether or Not to Insulate a Slab

dpilot83 | Posted in General Questions on

Building a 2,200 square foot home in far NW KS so I’m right on the boundary between 5a and 5b.

It will have a full walkout basement under it.

Site prep is done.

My builder is very focused on quality and him and his son do the vast majority of the work but he hasn’t done an extreme level of focus on air sealing or high end windows or external insulation in the past. I have researched a lot of this stuff and I have talked with him about them and he has been willing to learn and adapt. In my early talks with him I thought he might and overall I’ve been very happy with him thus far.

We are now planning on:

– ICF basement
– 2×6 walls with ZIP sheathing
– normal vented attic
– 3” of Rockwool externally
– rockwool batts internally
– hoping to achieve less than 1 ACH 50
– will be using ERV

One thing that I have not studied is insulation under the slab. I have briefly talked with him about it and he says he hasn’t done that in the past. Soil temps are generally around 55 degrees around here at a basement depth year around.

Initially I was OK with that but now I’m beginning to wonder if we’ll have a lot of regrets not getting some sort of insulation under the slab. The walkout basement especially means that parts of the basement will be be very close to ground level so the floor will likely be colder there.

He already has rock at the site for radon mitigation. I think a small amount has been spread. I don’t want to jerk him around but I wonder if there will be a noticeable comfort issue in the basement in the winter time, especially towards the shallow end of the basement.

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. dpilot83 | | #1

    Must read another thread and it’s replies that was recommended to me as a similar thread by the forum software after I posted:

    They talk about insulation for the frost wall which I assume they mean the footers?

    Sheesh. I researched everything else to death but not the foundational aspects of the building and here we are building…uhhhg.

  2. Expert Member
    Akos | | #2

    Typically for a walcout basement in cold climate, my recommendation would be to insulate the slab. With an ICF foundation, you have insulation down to your footing, so the slab is much warmer, closer to a bellow grade basement. In this case you can skip the insulation but I think an inch or so of rigid under the slab would still not hurt. Really depends on how much pushback you get from your contractor.

    I'm in the north edge of zone 5 and I find in a reasonably sealed place with a fully bellow grade basement and with a slab with no insulation covered with LVT is not cold.

    Bare concrete would always be cold unless heated.

  3. Jon_Harrod | | #3

    I agree with Akos' points. Another thing to consider is the possibility of spring/summer condensation. Insulating the slab will bring it closer to interior temperature and reduce the chance that it will become a condensing surface when interior air is warm and relatively humid.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |