Is it OK to insulate under basement floor slab with rigid foam if there is rigid foam on exterior of basement walls?
ZZ54e3arn5
| Posted in Energy Efficiency and Durability on
The slab will be isolated with foam but the foundation will not have a completely closed envelope since the basement walls have rigid foam on the exterior. How much of a concern is the thermal bridge at the footing? This is in Columbus, OH.
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Replies
Jason,
1. "Is it OK to insulate under basement floor slab with rigid foam if there is rigid foam on exterior of basement walls?"
Yes. It's not only OK, it's very good.
2. "How much of a concern is the thermal bridge at the footing?"
Not much of a concern. The thermal performance of your foundation will be slightly improved if you (a) Insulate the exterior of the footing (the short vertical leg as well as the short horizontal leg) with XPS or closed-cell spray polyurethane foam, and (b) Insulate the perimeter of the slab with vertical insulation (for example, vertical XPS insulation, 2 inches thick and 4 or 5 inches high).
Jason: Kudos on working to complete your thermal evelope. Foam beneath the slab is rarely a bad thing. If foam is installed to a thickness of four inches or more, you might start to lose the summertime cooling advantages of having this slab in your thermal mass.
If (like most people) you intend to insulate to just two inches or so, you should do quite well. The thermal bridge at the foundation is important. The only way that I know of to calculate the ideal thickness of insulation and impact of the Thermal Bridge is to model the home in the Passivehouse Planning Package (PHPP). If you're truly serious about low energy homes, it might not be a bad idea to contact PassiveHouse Institute US (PHIUS) about finding a PH Consultant in your area. Tony Olaivar-HERS/BPI Rater, PH Consultant Candidate
Thanks for the input.
Jason,
The sub-slab foam board can extend over the footings, such that with slab edge insulation the slab is isolated from the footings and there is no thermal bridge at all.
25 PSI standard XPS foam board is strong enough to build an entire house upon, so there is no reason not to continue it over the footings to support the slab. I would never do this any other way.