What’s the best way to insulate a heated room under a concrete garage slab in northern Minnesota?
user-675802
| Posted in Energy Efficiency and Durability on
I have a 1964 rambler home with a heated room under the garage. The garage slab is reinforced concrete in good condition. I’m planning to upgrade the home’s insulation, and this seems like a major area needing attention. There is currently no insulation under the slab, and the room under the garage has a low ceiling (approx 7 ft). The garage has an internal floor drain and is heated, but we keep the temp set just above freezing. The most obvious solution would be to add foam insulation to the underside of the slab, but would this cause problems with condensation? Concerns about potential leaks through the slab in the future?
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Replies
A clarification - the garage thermostat is set at 35 degrees, the room below is 65-70 degrees. Exterior temps can get as low as -30.
Jody,
The best way to insulate the underside of the slab is with closed-cell spray foam. Be sure to insulate the perimeter carefully, and be sure to insulate any thermal bridges (steel beams, etc.).
Jody,
I'm assuming (hoping) your home is a walkout condition in the back.
If so I has seen many similar style homes in the twin cities. Typically the room under the garage originally had a door to the outside and not the rest of the basement. These spaces were used for storage or a workshop and were not originally intended to be conditioned. Later homeowners would remodel and make these spaces part of the basement.
By MN code rooms with ceilings below 7' cannot be used as occupied spaces. Ideally you no longer use this room as conditioned space. I strongly recommend against using this space as a child's bedroom.
In MN we tend to run our cars a bit in the garage in the winter for them to warm up. Garage floors often show signs of oil and antifreeze stains. A concrete slab is a barrier to icky stuff and fumes but not a perfect barrier.
How about adding a layer of rigid insulation above the garage slab, a well-detailed membrane to seal against leakage, and a second slab? That would address the leakage issue and loss of ceiling height below. Of course there would be loss of garage door height unless that were reframed to compensate for the increase in floor elevation.
Dick-
Layering the insulation and membrane as you describe does seem the most thorough solution, and I have detailed it that way in new construction. Maintaining ceiling height in the room below is indeed important.
The downsides you point out are the main problems - and I would also need to rebuild at least part of the driveway to deal with the higher floor. It's a common remodeling problem - one change leads to many more!
J Chesnut -
The house is just about exactly as you describe it, with the space below being mainly a shop and storage. Ceiling height is indeed at a premium here.
Jody,
(Note, I may be over-complicating this.)
If the room in question is used for storage and a workshop then it might note be worth insulating from an environmental impact perspective. High levels of insulation are appropriate for spaces that are conditioned 24/7 because we want to retain heat and not use more energy than necessary for keeping those spaces at a certain temperature.
In small spaces like a workshop that are infrequently and periodically used and not constantly conditioned you are typically not trying to retain heat. Typically you are trying to be comfortable when occupying those spaces. At these times, I hypothesize, it may be less impactful to run a space heater to keep your body warm (as distinguished from conditioning the space.)
Closed cell spray foam has a high environmental impact so there is some question whether any benefit you gain from it recoups its original impact.
See Martin's excellent blog for reference: https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-global-warming-impact-insulation
You could also, I imagine, glue 100 psi XPS to the floor, cover it w/ 3/4" T&G plywood, or more, and drive on it. You'd have to RedHead, or similar, the plywood down, but I'd bet it would be there a long time. That minimizes the driveway re-do, etc.
Jody, leave the slab alone. Both above and below slab temps you describe point me to this conclusion. The heat moving through the slab is heating your garage to the temp you have in the garage. Insulate elsewhere.
AJ -
That's a really good point!