Whether or Not to Insulate a Slab
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.
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