Uninsulated radiant slab
I have a customer who inherited a 20 year old house with hydronic heating in a concrete slab on grade. The word is that there’s no insulation under the slab. What I’d like (for my own understanding) is an informal sense of the severity of the heat loss in a situation like this, compared to a typical slab with R-10 XPS under it. This is zone 4a, ~5500 HDD climate, the site is level and well-draining (a foot of topsoil on top of a couple of feet of coarse sand/gravel till with lots of small rocks), the slab fill is apparently quarried sand from nearby, and with a little luck there’s some poly under the ‘crete. Footing depth for us is a hefty 12″.
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Built after the first great oil shock of the 70s and no slab insulation under a heated floor?
Your deep ground temperature is in the 55°-60° range, so the delta-T to the shallow earth is probably not as severe as to wintertime air temperatures, but the warmer the slab temperature the greater the heat loss downward. And, if the tubing is near the bottom of the slab (which is often the case), then there will be greater heat flux down than up.
Most slab heat loss is at the perimeter and edges, so if you could add exterior rigid insulation you'll dramatically improve the floor's heating efficiency.