Insulation under hydronically heated slab
I am building a garage in CA zone 16, US zone 4B. Last year I buried temperature recorders at depths of 18″ and 42″. The deeper probe hovered at 33 degrees all winter. The shallower probe occasionally froze to 31 degrees. Well water from 200 ft deep is typically 40 degrees.
The building will have an 8″ structural slab to support some concentrated loads. It is important to me to be able to drill anchor holes anywhere in the slab so I want to put the hydronic tubing at the bottom of the slab.
My current plans call for 8″ aggregate base and 2″ EPS under the slab, with the tubing on top of rebar at the mid height of the slab. If I move the hydronic tubing to the bottom of slab should I be adding more insulation under the slab? Also, in theory, air entrained concrete has a high R value that would limit efficiency of a hydronic system but I have seen opinions ranging from “air entrained R value is same as normal concrete” to “air entrained R value is 10x normal concrete”. I will pour the slab after the building is in place so conceivably I could order non-air entrained concrete.
BTW, slab perimeter insulation is R10 and it is difficult to increase that much. Most I could do is add another R5 to inside of my frost wall.
If it is relevant, my heating objective for the structure is 60 degrees F minimum air temp, with no variation 24×7. I want to keep plumbing and stored equipment from freezing and want to be able to work without my fingers freezing. I will be on time of use electric billing so am hoping to use a time of day thermostat to electrically heat the slab at night and then use its substantial thermal capacity to keep temperature reasonable during the day.
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