Insulate the slab in a shop building?
I’m in Central NY, Zone 5. I will be putting up a 30X52X12 shop this year with two rooms – one 24X30 and the other 28X30. The building will have R19 insulation in the walls and R32 in the ceiling. The building will have one 10X10 insulated overhead door. The floor will be 5″ concrete. I’m planning to use two minisplits for heating and cooling.
I plan to keep the building at 50 most of the time in the winter, with occasional forays into the low 60’s. In the summer on real hot days, I might cool it a little, but most of the time I plan to open the overhead door and windows and run some fans.
From a dollars and cents perspective, what type of insulation on the slab will pay for itself over a 25 year building life?
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Andy, that's a tough building to model accurately, but over a 25-year period I bet 2" of EPS would pay for itself. It will also improve comfort and reduce risk of condensation that can lead to mold and musty odors. I recently priced out 2" EPS at about $20/sheet, or about $1K for your building. That's $40/year spread over 25 years.
How is the building supported? If it is a frost protected shallow foundation, most of your energy savings comes from the exterior perimiter insulation. You best dollars are best spent adding insulation there.
Since you are heating the shop only to 50F, which is pretty close to sub soil temperature, there is very little energy saved in sub slab insulation.
In terms of comfort, different story. You want some above slab insulation with plywood. Unlike a concrete slab (or insulated concrete slab), this surface is quick to heat up when you use the space and much easier on feet in terms of impact. Even 3/4" of rigid makes a big difference. I have this setup for a garage to studio conversion and adding in the rigid+plywood made a very noticeable difference in how cold the place felt.
P.S. No matter which way you go, make sure there is poly vapor barrier under your slab.
It will be a pole building. The slab is not a structural element. Regarding comfort, I plan to have either wooden slats or rubber mats by the work benches or stationary tools.
The ground under the shop should stay at about the sub-soil temperature, which is the year-round average temperature, which is probably in the 50's where you are. I would argue that in the winter there is minimal heat loss from a minimally heated building, and in the summer you get significant cooling, so that an uninsulated slab might actually have a better energy use than an insulated one.
However, soil is a poor insulator and it might get cold around the edges in the winter. A common thing to do is to put insulation under the floor for a couple of feet around the outside perimeter.
In your climate I would expect condensation in the summer. In your climate the only way to prevent that is to run air conditioning or a dehumidifier.
Regarding condensation, in my current shop, which is five miles down the road from my new location, condensation is common in the spring as the slab warms up. It pretty much disappears in the summer.