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.
Best Answer
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.