Slab Insulation for Unconditioned Garage for EV Car
I am building a garage attached to the house. The architect specified some under slab insulation but I knew builders in my area would be reluctant and I am not sure if I really need it. I had the architect remove it as a requirement but I may reconsider. On the “pro under slab” side, our Massachusetts climate is very humid as of late. The combination of 55 degree concrete and June humidity leads to moisture. While I know the theory, I don’t know the risks if I don’t do this, so I lean on you all for advice. The basement, existing, is at the same level of the proposed garage and we manage moisture through a dehumidifier in the summer. Actually we have just used a heat pump water heater — since the basement was AIR SEALED we have been able to just rely on our showering to dry the basement and the dedicated dehumidifier has been kept off all summer. If we go on a vacation and the forecast is for humid weather, we do put that dehumidifier into action. The new garage will have a connection to the basement but a door will prevent the more humid air in the garage from coming in. When I poured the new basement slab 10 years ago, I did an experiment. I decided to forego full under slab insulation, however I DID INSULATE A 2 FOOT PERIMETER OF MY NEW BASEMENT SLAB WITH 2 INCHES OF EPS FOAM and I put 1/2 inch foamular around the outside, vertically, as a thermal break with the cold, thick fieldstone walls. This has proven rather effective. The walls can get damp, especially the area 1 to 3 feet below grade that can freeze on the outside. But the slab, 6 feet below grade, is much warmer and stays dry. I have not seen any mold on the bottom of cardboard boxes stored in the basement since the new slab was put in (with vapor barrier). For the new addition, the garage (which will have a room on top that is fully insulated), is going to be 7 feet below grade. My options are: A) old school, pour concrete directly on the crushed stone with a vapor barrier B) foam around the perimeter only, as I did in 2013. C) full 2″ under slab foam. I am leaning towards option B. The garage walls will be concrete for the first 2 to 6 feet so I also seek advice on managing any potential condensation on the walls which, in the back, will expose a full 7 feet below grade. Thank you.
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Replies
Perimeter IMO is 90 percent of the deal.
In 1986 I insulated the perimeter of my barn, concrete guys shook their heads. When it was unheated a little solar gain always kept it above freezing
I would go so far[and have done so] as to insulate across the door. It can be hidden under a thin plate.
I have insulated under a working garage floor, and it caused no problems, but that was radiant heated, I would see no reason to do so in an unheated space, as insulating the perimeter reserves you the ability to heat the space in the future
Hi Gusfhb - are you talking about insulating the edge of the slab on the interior, or a couple feet under the perimeter? And when you say "insulate across the door" what exactly do you mean.
Thanks.
Insulating the perimeter of the slab including across the aperture of the doorway
The way I see it insulating an unheated space is ungreen.
You are forcing the manufacture of more petroleum base foam insulation.
You are forcing the transportation of that petroleum from the other side of the earth to your garage.
It is selfish and wasteful to divert useful products from the supply chain and not put them to their best possible use.
What is the real up side? For 3-5 day each spring the floor of you garage will not be damp. My guess is the slab might condensed 2-3 pints of water over the 5 days. I think if you park a car driven in the snow in the garage once you could bring in 10 times more moisture.
I think if you insulate the floor the garage will be colder in the winter and warmer in the summer.
If you have an extra 3K burning a hole in your pocket give it to a local charity.
Walta
The upshot is that the slab was poured (very old school) on a thick bed of crushed stone. I did put an insulation layer; see the photo. It is on the outside of the garage, it is 3 inches thick reclaimed EPS foam. The cost of the foam was only $270 with tax for 16 sheets.
I used 12-inch wide strips of Vycor Env-S to cover the seams of the EPS and I fastened it with one or two Tapcon screws, with metal or plastic 1" washers to grab the foam. The Vycor is not intended for use below grade of course but it was leftover, so it was free and will last long enough for the clay soil to pack into place. I plan to fasten the tops of the EPS to the concrete with adhesive and use zip tape to protect the top of the foam. There is not much exposed concrete but I have the option to add 2" foam for the above grade portion -- covering the sill. Then I could stucco it.
I these modifications result in a 45 degree or 50 unheated garage (say, in January) where it would otherwise be 32 to 40 degrees in the garage, then this is a significantly better environment to store an electric car. It is also a better environment for a home battery.