I have a house that has a garage below the main floor. The house is built into a hill, and so the entry way, the basement, and the garage are all on the same level, and the entry way has a stair case that leads to the main floor which is above everything.

The house is very old (~1890s) and leaks hot air worse than a politician. It's so bad that when it's ~19F outside, the furnace can't keep up, even though the furnace is properly sized and in good repair.

Among the many things I want to do to this house to deal with the poor insulation and air leaks, I'd like to put an air barrier on the bottom of the garage ceiling to prevent air infiltrating through the garage into the first floor, which I have good evidence is significant.

The structure of the main floor/garage ceiling, from top to bottom:

  • Rugs
  • Hardwood floor
  • Floor joists (about 6 inches of room)
  • Lath and plaster (1 inches total)
  • 2 inches XPS rigid foam, loosely attached via furring strips screwed to floor joists.

I was considering attaching plastic sheeting to the bottom of the rigid foam over the entire ceiling to seal it as much as possible.

I also am curious about injecting blown-in insulation in the joist space, but I don't want to worry about that until I create a worthy air seal.

One major concern I have is that this is a violation of proper vapor barrier/air barrier design - that I could get condensation on either side of the air barrier and cause mold.

So, is this a bad idea?

I should note the house is in Rochester, New York, so very cold. The house is not air-conditioned; only upstairs bedrooms are via through-window ACs.

  • 2
    The first thing you do is install a CO detector in the room above the garage! Commented Dec 16, 2013 at 18:07
  • The XPS foam is a vapor barrier assuming the seams are properly sealed with the appropriate tape.
    – Steven
    Commented Dec 16, 2013 at 19:02
  • Unfortunately, the foam blocks are not sealed. They're held in place against the lath by furring strips, and that's not snug. The seams between them are sometimes a half-inch apart. The furring strips holding the foam aren't perfect, and so air can leak through the joint between the furring strips and foam pretty easily.
    – antiduh
    Commented Dec 16, 2013 at 19:18

2 Answers 2


Yep, the 120 year old houses do make life more interesting (and expensive and/or colder.)

Use an air barrier that is NOT a vapor barrier. More commonly known as housewrap. Vapor moves through, but bulk air movement is reduced.

For insulating inside the floor joists, either blown-in cellulose (which supposedly has very little issue with vapor, due to having very little air movement through it) or a sprayed foam product. Getting good fill blind (without blowing out the ceiling, in the case of the spray foam) is tricky.

I presume you don't want to rip out the ceiling for better access - but you really need to think hard about getting some drywall below the XPS (or XPS and added air barrier), or you have (and have now) a major fire hazard - plastic materials exposed in a garage that's below occupied space. The plaster ceiling above it may not burn through for a good long time, but the smoke from it will do you in even if the structure is unaffected by by a fire.

  • Thank you, this is a great response. I wasn't aware that house wrap was able to allow vapor to diffuse out while still acting as an air barrier, but after reading, you're right. As to the fire problem, as I was writing the question it occurred to me that this entire situation might be a fire hazard, so thank you for confirming.
    – antiduh
    Commented Dec 16, 2013 at 19:36
  • I think, based on your advice, I will rip out the existing lath and plaster, put up housewrap under the joists, put in blown-in cellulose above the housewrap, and finish it with new drywall.
    – antiduh
    Commented Dec 16, 2013 at 19:38
  • 1
    That will work, but if you are ripping out the ceiling, you can use vapor barrier (you'll need a bit more, since it's going to end up wrapping around joists) or you can use a spray foam that is a vapor barrier (need not be for the whole fill - just an inch or so to be the barrier and seal, then switch to less expensive insulation below it - given the labor on getting a sheet product into the joist spaces, it's probably a win unless you like that particular activity.) You can also re-use your XPS in the joist spaces unless you have a better use for it elsewhere.
    – Ecnerwal
    Commented Dec 16, 2013 at 19:46

This may be way late but keep it simple. You have lath and plaster, remove the foam, recycle it, cut a nice attic access hatch, blow in the insulation. The ground paper should fill all you can. Seal access hatch with the foam. Next mud any cracks on lath n plaster. Last but not least, paint it with a concrete wall paint made to keep out wetness, like a leak control it is plastic base and will also act as your vapor barrier and give a good look. Get a good co2 and smoke alarm for area above, use the foam you have left over on the wall adjoining to home, and recover with drywall.

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