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Community and Q&A

Insulating walkout garage lower level

RedFalcon6 | Posted in General Questions on

I’m building a foundation and garage which has a garage on the top floor, and a “walkout” level below which I am using as a shop.  The question is on how to best insulate and detail the shop portion.  It is probably important to note that the shop also has a garage door on the one freestanding wall, and will not permanently be kept at 70 degrees.  So the solution doesn’t need to be perfect – just “good enough”.  I also live in NH, so it is cold!

I have the below slab insulation squared away.  The question is on the insulation for the concrete walls that will be below grade and the shop ceiling (which will be 6 inches of concrete — the floor of the garage above). 

My plan for the ceiling was foam board or spray foam.

For the walls, the question is do I use external insulation and bring the shop wall thermal mass inside the envelope, or do I go ahead and do internal insulation all the way around?  If I go external, there would seem to be a thermal bridge in that inevitable section of shop wall / floor interface?  But if I go internal, I am losing some floor space because of the framing….

Thoughts appreciated!!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Red Falcon.

    There are a number of ways you could go with the basement walls, exterior insulation, interior insulation, or a combination. As you noted, insulating the interior eats up your floor space. However, if you build stud walls, you could sheath them with plywood or OSB which offer excellent fastening for cabinets and shelves and wood storage, which are nice to have in a shop. If you won't be insulating the garage above, working from the inside also means that you can make the insulation continuous from the walls to the ceiling.

    As far as materials go, there are two camps. Some builders won't use fibrous insulation at all in basements and will only use rigid foam and spray foam. Others will use rigid foam or spray foam against the concrete and then fibrous insulation in the framed walls. In my experience with basements, I tend to agree with the no fibrous insulation folks on this one. But this is a new building so you have the potential to manage water and moisture issues with propper assemblies so maybe you'd feel comfortable with some non-foam products down there.

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