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Community and Q&A

Insulating my garage from my house

mpi | Posted in Green Building Techniques on

New member here. I’ve read lots of interesting articles here but none really answers my questions so here’s my first post.

I have a double garage under my house in Massachusetts and the bedrooms above it are cold in the winter, especially the floors. I initially thought about having dense pack cellulose blown into the joist bays but there is already 40 year old fiberglass in there, broken down to around 1” thick and full of mouse poop and I was told I’d have to pull those out first.

That’s still an option, but I was looking for an excuse to rip the ceiling and walls down anyway as they are in poor shape, so I’m now thinking about doing that, filling the bays and then re-boarding. It’s about 550 sq.ft and I had a quote for spray foam but it was more than I can manage, so on finding this site I checked into recycled foam board and I can get 5” EPS for $18 per sheet from one of the recyclers often mentioned on here. I am thinking this might be my best way. Build myself a hot wire cutter and, if I can do it carefully enough, cut pieces to interference-fit into the bays. Including delivery, it’s $500 or so of foam.

The garage is not well insulated but has decent doors and it doesn’t go much below 50 in the winter. It is possible that this is only because the insulation to the house is poor though, so I’m heating it indirectly, but I don’t want to go crazy, and R-20 for the 5” EPS is so much better than what I currently have, especially if I seal all the drafts, that I don’t know if it’s worth the extra cost and effort to go higher. The spray foam quote was $3k and would have been R-30.

While I’m open to any suggestions at all, my main question is: is it OK to just push the EPS until it’s far enough in to clear the bottoms of the joists, fill any imperfections with Great Stuff and cover with suitably rated drywall, or do I need a vapor barrier or anything else?

Other note: my ceiling is only 7’8” high so I can’t afford to drop the ceiling to mitigate thermal bridging of the joists, and I think that would be overkill anyway.

Related question. The front bedrooms overhang the garage by a couple of feet so along that strip above the garage doors the floors are above outdoor space and are even colder still. I just drilled a hole to look inside there and it’s the same 2×8 joists with ~1” fiberglass, so I could do the same thing with 5” EPS, but is the answer the same or different regarding the vapor barrier? For this area, would it make sense to fill the whole bay and get better than R-20? Should I do the 5” EPS and something else for the other 3”, or do something different entirely? For this section, I could look at extending it downwards to break the bridging on the joists.


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  1. GBA Editor
    Martin Holladay | | #1

    Martin Pitwood,
    When it comes to the cantilevered overhang, it's essential that you not only pay attention to R-value, but that you do an impeccable job of air sealing. I'm sure that outdoor air is circulating in your joist bays, and that's the main reason for your discomfort.

    For a full discussion of this issue, see How to Insulate a Cold Floor.

    If you can afford a slight decrease in ceiling height, you won't regret the decision to install a continuous layer of rigid foam on the underside of your floor joists. Even 1 inch of rigid foam can make a difference.

  2. mpi | | #2

    Thanks for replying so quickly and for that link, and from there I found an Energy Star document about Thermal Bypass. It is pretty clear that I should be meticulous about air sealing both for the cantilever and for the garage ceiling, and making sure the insulation is pushed up to the subfloor.

    I assume that the rigid foam, if I do it, would be the air barrier for this construction, and I'd only need to pay attention to the edges and the seams?

    I am starting to see why spray foam is popular for this kind of work.

  3. GBA Editor
    Martin Holladay | | #3

    Martin Pitwood,
    Q. "I assume that the rigid foam, if I do it, would be the air barrier for this construction, and I'd only need to pay attention to the edges and the seams?"

    A. That's correct. If you decide to skip the rigid foam, the air barrier would be the ceiling drywall in the garage, and the soffit material (for example, plywood) under the cantilever.

  4. DIYJester | | #4

    Martin Pitwood,
    You can get some cans of spray foam and use that to fill the seams, edges, and any nail holes etc. since it is recycled.

    As for cutting, I have had very good success cutting XPS using a putty knife with a sharpened edge. I can only sharpen pocket knives to about a spoons usefulness, but the putty knife goes right through with a gentle sawing action after lightly touching up the edges with a hand file. EPS may be a bit more or a pain to cut, but it may be worth a shot.

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