Quote:
Originally Posted by AC_Hacker
However, when a radiant floor heating is contemplated, the floor becomes the heat source and it is in contact with the world's biggest heat sink. So slab insulation is of the greatest importance if the radiant floor is to be efficient.
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I've heard this is heavily dependent on ground water. If there isn't ground water near the soil beneath the slab, then the earth acts as a huge insulation bank. Earth has a low R-value, something like 1 per foot, so you see the thermal gradient traverse deeply underground, unlike a short gradient through typical insulation in a wall. The large volume of heated earth has much thermal mass, so it takes a while to heat up.
But I've heard without ground water, the heat will just sit there in the soil. This is how a frost protected shallow foundation (FPSF) can resist frost heaving at low depths, the soil is heated. The heat loss occurs at the perimeter of the foundation, where it finds its way around the insulation up to the cold surface, where it radiates away.