Frost-protected slab on grade with radiant floor heat
I’m looking at a slab on grade, using frost protected shallow foundation design and have a concern about incorporating radiant floor heat.
IRC 2015 Figure R403.3(1) shows a frost protected slab on grade but has no insulation under or above the slab, i.e. no insulation between the conditioned space and the earth underneath. If I incorporate radiant floor heat into or above the slab, best practice is to have insulation between the heat source and the slab or earth below. However this means warmth from the house will not be warming the earth under the house.
Does insulating under the heat source conflict with the intent of the frost-protected design, the goal of which is to maintain a warmer temperature under the house?
If I insulate under the heat source am I at risk of having cold enough temperatures in the earth under the footings that frost heave becomes a factor again?
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Replies
I can't answer your question regarding frost protected slabs, but I suggest looking into insulated raft slabs. It addresses both of your concerns.
You want the insulation to kick out like the lower sketch and then 2" continuous EPS under the entire slab. The Kick out outside the slab will prevent the cold from going under the slab and ground temp will be maintained as normal under the frost line.
So I needn't be concerned about creating a thermal break between the conditioned space and the earth beneath? Normally I would want a break, of course, but if I'm depending on heat flow from the house to maintain above freezing temperatures under the foundation then insulation in the floor could cause problems. That's what I'm trying to get at.
Antonio,
There are two distinct designs for FPSFs. One relies on the heat from the structure above to keep the earth below warm. The other relies on more insulation to stop the earth below from freezing.
https://www.homeinnovation.com/~/media/Files/Reports/Revised-Builders-Guide-to-Frost-Protected-Shallow-Foundations.pdf
It sounds like the latter is what I want. Thanks for the link. I'll study the document.
+1 on what Malcolm said. Almost everyone should use an "unheated building" FPSF design.