Discuss slab on grade insulation on smaller building

fastline

We just finished smaller of two building slabs. The smaller mechanical shed slab had to get done quickly due to weather and no insulation was added, but 280ft of 1/2" PEX was put in the slab, 1" below the surface. The smaller shed is 18x21x12high. It's primary purpose is to house pressure tank, water softener, solar inverters, batteries, compressors, etc. The intent is to just stay above freezing but probably a target of about 50-55F. Because of all the electrical equipment, the waste heat from that may be plenty to keep the building warm, BUT we need to explore the PEX if there is not enough waste heat. This is in the midwest and temps can drop in the teens but design usually for 0F, though we have not seen it in years. Because there is no insulation under the slab, I am wondering if you guys feel perimeter insulation installed vertically would be needed? I think we will have a few different ways to heat the slab, and I realize this should have been part of the design, but we got in a hurry. Our ground water temps hover around 58F so I considered a minor pump/dump but wanted to see what you guys think.

heating degrees days here are about 4600.

Jamie Hall

You definitely need perimeter insulation -- at least 4 inches of blueboard or whatever, on the outside and go down if possible to the footings. Otherwise you will lose a lot of heat out of the edges of the slab.

With your relatively high groundwater temperature the main effect of not having insulation under the slab will be that the system -- and floor -- will be astonishingly slow to heat up -- or cool down. You'll just have to be really patient with it (it may take days -- literally -- to change by 10 degrees!).

heathead

I f the slab is not insulated, and it is a shed for equipment I would one not use a different heat emitter source than the slab. Why not put baseboard in the shed, then you are not heating slab and loose heat to the ground in the process. What do others think? If it was you house and you wanted to be comfortable it would be worth the heat loss for comfort.

GroundUp

There isn't a lot of value in trying to heat a slab with no insulation under it; more than half of the heat introduced to the slab is wasted to the soil below. You will still want to insulate the perimeter with a minimum of 2" of rigid foam, but the most efficient course of heating the building is not going to be via radiant slab.

hot_rod

My well house is a tad smaller, concrete block with 2" foam inside. A 100W light bulb keeps it above freezing into the single digits.
Heat will travel down to the earth below without a good thermal break, at least 2" of foam. If the goal is just to keep it safe, above freezing, fin tube or a small forced air unit heater would work fine.