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Makeup for "sand cushion" under slab
I'm having a hard time figuring this out. I met with a customer the other day with a new 32x48 pole shed, wanting to install a boiler to heat it. He laid the tubing himself, stapling 6 loops of 1/2" (250ft) down to 2" XPS, but for some reason dumped 9" of coarse sand on top of the foam and then 5" of concrete over that. There were a few builders around here back in the 90's that pushed this as a "heat sink", but they also ran 180* SWT to everything. I did a heat loss on this building and came up with just over 30k, about 20 BTU/ sq ft. It's well insulated, including 2" XPS around the perimeter but has tall sidewalls and big overhead doors. My original thought was a UFT-80W would serve it nicely, but I'm having trouble negotiating the sand. My math says the thermal resistance of 9" of sand and 5" of concrete ends up equivalent to an R value of roughly 6.5. Seeing as there is R10 below and R6.5 above, would it be accurate to pump up the BTU needs by 155% to overcome the downward loss? I feel like I should know this already but am drawing a blank; I know one of you fine gentlemen know the answer though, anyone willing to lend some info please?