Insulating garage with 6” floating slab
Hi all.
Just purchased a home in Maine that has a garage I’d like to convert into a wood shop. The garage is 24×24 and has no footings. It’s just a 6-8” thick pour on grade with no raised stem wall. I’m not familiar with this style slab, but the surface is in really good shape (no cracks, no heaves) The double sill plate and siding are really low to finished grade and there are rotted corners I’ll need to repair. But before I get into that my question is, if I ultimately want to insulate and condition this space, is there any insulation I should be doing at the slab if there is no frost wall? Is it worth the effort and material? Like I said I plan to insulate the rest of the space, and I’d like to keep it dry in there.
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Dig down around the outside an install the required skirt of rigid insulation to make it a frost protected shallow foundation. Besides protecting the slab, this will also take the edge of the cold slab in the winter.
Energy efficiency with these types of buildings is mostly driven by how tight you can make it. Unless designed to be air tight from the build, this is very hard to do after the fact without spray foam.
If you don't want SPF, you can do a decent job by installing a solid warm side air barrier, just make sure you handle all the transition properly (slab to wall, wall to ceiling and back down).
P.S. Make sure to figure out how to vent the roof properly, again not easy after the it is built.
Thanks. I modified my question to be more clear. It’s a floating slab, only about 6” thick at the edges. Not sure if that changes your thinking, but there are no down turned edges.
Do you think I get more bang for the buck by just focusing on air sealing? There shouldn’t be much interior humidity.
Even without the thickened edges, the physics of heat loss is the same. The dirt next to the building will be pretty close to ambient air temperature. This means, without any insulation, the slab will be at the same temperature as the soil which is very cold.
The solution is the same as proper foundation, perimeter slab edge insulation plus the correct sized wing insulation. Quick google came up with this as a link with how to figure out sizing:
https://framebuildingnews.com/below-grade-insulation-part-1/
If you want the place comfortable, you need to do both, insulate and air seal. Insulation by itself if the wind is blowing through the place won't get you much comfort.
Do up a quick sketch of what the building structure is like as is and where you plan on insulating. Can work from there to figure out what is possible. Does the place have a WRB or sheathing under the metal skin (both walls and roof)?
I am also looking for the same answer.
Thinking of putting most effort towards airsealing and interior insulation. With regard to foundation, might be leaning towards insulating over the slab in the interior. Plan is still evolving.
I think the point being made to you is that in MAine one should have at least 4 foot footings. So this currently has no frost protection, which can be problematic in a finished, heated structure. The insulation 'skirt' protects the structure as well as providing insulation
You have to watch with a heated structure. As is, the dirt under the whole structure freezes during the winter so the whole place moves about the same. Once you heat it, the edges will still freeze but the center won't. There is a pretty good chance of cracking the slab.
The exterior rigid+frost wings will are needed to prevent the edges from freezing when you heat the place
Dropped everything into Sketchup. Based on Frost Protected Shallow Slabs, here's a PDF showing one insulation direction I'm thinking about. Only showing the insulation plan at slab level (not showing interior details, or vapor control). This will be a woodworking shop, and only conditioned to about 45-50 in the winter. Myy primary concern is controlling interior humidity and obviously not rotting out my framing.