Insulated slab – shallow or not? Maybe PWF?
Getting ready to build a smallish (1000 sq. ft.) home for myself in climate zone 6B and am trying to walk the line between green, cost, and ease of build. I’ve decided to build on a slab and am trying to come up with the best way to insulate it.
Building a FPSF uses quite a bit of foam running at least 2 vertical feet outside the slab and 2 horizontal feet away from it (over $400 worth of 2″ XPS). Then I also have to detail that 2″ of exterior foam to the exterior wall.
Trying to think of ways to use less foam and less money I had the thought of building a PWF stem wall 4′ deep to get below the frost line and then only using enough foam to create a thermal break between the stem wall and slab, as well as full sub slab insulation. This would save me $400 in foam and maybe $500 in concrete. I figure the PWF stem wall would cost me about $750 to build.
So cost savings aren’t huge but I’m more comfortable working with wood than concrete so there would be a labor advantage. Excavation cost would be higher having to dig the full 4′ footings but I’m really not sure how much more it would be.
I’ll be building with a double stud wall so that will cover up the foam thermal break between the stem wall and slab. If I used exterior foam on the slab I feel like I’d probably need to construct the outer wall from 2×6 so I could hang it over the foundation enough so that it would end up flush with the foam. With no exterior foam I’d feel good building it with 2×4, which I have a lot of after tearing down my old house.
Any thoughts? Is this a mistake? Should I just quit being a weeny and pour concrete and foam the outside? Should I still use exterior foam even with a full stem wall?
I’ve done a lot of research on PWF and am pretty comfortable with them. They’re somewhat common in my area and I’ve talked to a couple people that have had them installed for 35+ years with no issues. I’m also blessed in this regard to be building on very well drained soil with a low water table.
Any thoughts appreciated,
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