Flooring options for a top-insulated monolithic slab
Hi all,
My partner and I live in rural MA, climate zone 5B, in an old gas station that I have been converting/restoring for the last 4 years. We are hoping to put a 900 sqft addition on in the summer that would house a bedroom, bathroom, storage, and my wood shop. The existing building is CMU, with a monolithic slab poured directly over ledge in some areas–no vapor barrier or insulation, no crawlspace or basement.
We are considering using a monolithic slab for the addition as well because it is the technique of choice for most of the concrete contractors in our area, and the quotes we are receiving for monolithic are less than half the cost of footing-stem wall-slab construction. We insulated the existing floor with an assembly similar to what is detailed here: https://www.greenbuildingadvisor.com/article/installing-rigid-foam-concrete-slab (taped vapor barrier (stego wrap), taped 2.5″ recycled polyiso, and two layers of 3/4 plywood (staggered, glued and screwed), and have been very happy with the results. The floor is warm and comfortable to walk on, and the floating wood slab helps to isolate noise from the wood shop.
If we go with the same method for the addition, and insulate the floor on the interior of the building, what flooring options do we have for the bathroom in the addition? Can we put down cement board over the floating subfloor and tile over it?
Any potential pitfalls to be aware of when using an over-slab insulation approach more generally?
Thanks!!
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Replies
egt,
I don't think that stack-up precludes any type of flooring.
Something to consider is that monoslabs for houses are often designed to be frost protected shallow foundations with exterior insulation to keep the ground under the slab warm and unfrozen. With interior insulation on a monoslab, that sort of frost protection will not be possible. So, I don't have the answers, but joining 2 monoslabs without frost protection on either of them together may have some special considerations. Like, at a minimum, dowel the crap out of the junction between them and put down alot of well draining material underneath.
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tjanson,
Having an insulated slab doesn't preclude using a FPSF:
https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf
Thanks for the correction, in fact I built a FPSF with complete insulation, below the slab, footers, and exterior sides. Having the insulation on top of the slab confused me.
tjanson,
I’ve only read the guide, and have never had any direct experience with FPSFs. After your build, what was your impression of them?
If you don't want an exposed concrete floor, you don't need to use concrete: https://www.greenbuildingadvisor.com/?s=concrete+free+slabs
Thanks all! I'm trying to convince our concrete contractor about the virtues of FPSFs, but have yet to make any headway. Michael Maines, thanks for the reference to this article, we may just go concrete-free slab.