Insulating Detached Garage Slab
I will be building a detached 24′ x 24′ garage which I don’t plan on heating. At the moment I only see myself possibly heating the space for a project or car/tractor repair and likely would use a propene heater.
I am wondering if it’s worth the expense of putting 1-2 inches of EPS foam under the slab just in case down the road I end up doing heating the space a lot more than I think?
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"Worth it" is subjective, and the right answer depends on your climate zone, cost to install, energy cost, and of course how often you heat it. It's a good idea regardless to continuously insulate the stem wall and slab edge, but the juice may not be worth the squeeze to insulate under the slab when it won't be constantly heated.
If you are working on cars, isolating the slab from the wall is worth it even if you do not heat it at all. Insulating underneath is the next step. The slab now retains heat rather than wicking it to the outside
The increase in the cost of the project is minimal compared to the benefits.
A similar question resulted in a good discussion you might be interested in. See this this Q&A thread.
An excerpt from Martin Holladay:
"I faced a similar decision when building my own garage in Vermont. The garage is unheated. I decided to install both vertical insulation at the slab perimeter and horizontal insulation directly under the slab, guessing that the advantages of the horizontal insulation outweigh the disadvantages."
Thanks for the response everyone. I think I will go ahead and install at least 2 inches EPS foam (reclaim if I can find it) under the slab and in the future I may insulate the vertical and walls.
When our house (superinsulated) was built in 2010-11, I had 4" of foam placed under the basement slab. But when it came to the unheated (and attached) garage, I did go with 2x6 wall framing (insulated with FG), blow cellulose over the ceiling to R30, and 2" foam on the inside of the frost walls, from sill down to footing. But I did not put foam under the garage slab. My intent was to take advantage of low-level ground heat coming up to "warm" up the garage space. In the coldest weather (zero to single digits outside), the coldest I have seen the garage get is just above freezing. Normally the garage stays in the upper 30s, and any snow on or under a car brought inside melts off quickly. I have never seen any water on the slab freeze, even just a couple of feet inside the garage door.