Garage wall insulation–detached garage
Golly there’s a lot written about attached garages, but so little about detached garages!
Current project description: 24’x24′ 2-car detached garage, 10 ft. ceiling, 2×6 24′ o/c framing, Zone 7, natural gas available, planned heater is 45,000 BTU Hot-Dawg HDS-45 sealed combustion. Wall insulation: R-19 fiberglass, ceiling R-60 cellulose, slab R-8 rigid foam under. Use: will probably keep heat on much of the winter to maintain 45°-55°F for woodworking, vehicle work, etc. (With our current, similar garage in Zone 6, I have noticed that it is more efficient to keep the heat constant than to let the slab cool and then re-heat it. )
Question: Would it be worth adding EPS rigid insulation to the walls? The house will have 6″ of rigid on the outside, but, knowing that the primary heat loss in the garage will be the leaky 8×8 overhead doors, would the extra wall insulation be worthwhile or wasted? Each 2″ layer of EPS would cost about $600.
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You have about 720sqft of walls. Say it is -10F outside, you are trying to maintain 45F, so 55F delta.
With R19 batts, your walls are about R16. So you loose ~2500btu. Bumping the wall up to R24 with the rigid insulation you loose 1600btu. I doubt the additional heating cost is worth the $600 in insulation.
For workshops, it is much better to have the floor insulation above the slab, this is hard with cars though. Without the thermal mass, you could keep the place much colder and quickly heat it up.
One thing commonly forgotten is a thermal break at the garage door and slab. Don't extend your slab out into the cold as a giant heatsink.
Thanks for the thoughts and calculations. We need to come up with a good air-sealing method for garage doors. In regard to the thermal break, I assume we are talking about some kind of gasket on the bottom of the door?
You can extend the foam across the garage door, you can drive over it for years and it won't hurt it. I always intended to make some sort of threshold and never did. A nice piece of thin stainless over the foam between the floor and the apron would work
Doors are always a problem, I don't know why they don't make better ones, starts with the wheels and tracks not controlling the door well enough to make decent seals work.
I think the real temptation is to put the appropriate amount of foam on the outside and heat it with a mini split. Maybe the gas heater has a better ability to pull up from 45 to 65 but either way your regular energy use will be minimal even in a cold climate.