Below-grade basement wall insulation & vapor

June 29, 2017 11:07 PM

Experts seem to directly disagree on how to insulate a concrete basement foundation wall. Which insulation prevents mold in your sheetrock wall? EPS, XPS, ISO, wool, fiberglass? Faced or unfaced? Is the bigger risk inside moisture going out and condensing on cold concrete, or outside moisture going in through cracks in the wall (which is either 55 or 115 years old in my case, records are unclear)? Am I trying to block moisture migration entirely, or allow it to dry in the warm inside air of the basement?

I'm in Portland, climate zone 4C, avg. winter temp 41 degrees F, summers very dry. The basement is all heated living space except for one laundry/utility room. I've read a lot of articles and websites on the subject, and the consensus seems to be:

1) rigid foam insulation up against (glued to) concrete wall and sill plates
2) a framed in wall inside of that, with a one inch gap from foam to wall
3) absolutely NO vapor barrier inside of that foam
4) focus on sealing air leaks in and around that foam, that's more important than vapor barrier
5) use semi-permeable foam to allow inward drying, and only permeable insulation inside of that in between the joists of the stud walls (fiberglass or wool foam) with painted sheetrock outside of that. (Do I have that all right?)

Remaining questions:

1) should I seal foundation/basement exterior wall on inside with something like Endur-o-seal? There's no way I could do that on the exterior side except in a few places.
2) XPS, EPS or ISO foam on concrete wall?
3) vapor barrier or no vapor barrier on concrete wall side of foam? ISO would only work if you want a vapor barrier between the foam and the concrete. But would that prevent moisture from cracks in the concrete from drying inward?
4) On one such wall, there is a set of metal joists instead of a wood frame wall, which I've attacked the sheetrock to. Is that OK? it doesn't have any structural strength but it holds the sheetrock and I certainly don't have to worry about it rotting.
5) best way to air-seal the foam and/or the sheetrock?
6) Do I do any of this differently in the unheated utility room vs. the heated living spaces? What about the ceiling in that utility room, which is under the floor of an upstairs bedroom?
6) One website says that "capillary breaks" must be installed at the sill plate, but I can't make heads nor tails of how to do that, and they don't really explain. Thoughts?

Thanks!

posted by msalt to Home & Garden (6 answers total)

Best answer: I went thru the same thing 5 years ago during the gut remodel of our finished basement (previously finished in the 60s). FWIW, I live in the south so humidity is an issue. Basements are rare here but our house is on a hill so we have a walk out basement.

I did rigid polystyrene foam board (3/4") glued to the concrete block wall, small air gap then 2x4 stud walls with fiberglass insulation with no vapor barrier anywhere. I finally concluded that a vapor barrier would just trap moisture and what I really wanted was the air to move within the basement to keep things dry. Also, I really worked hard sealing up the exterior walls as there was some major air leakage because of some things the previous owner did. The basement is much warmer now in the winter now than it used to be.

I used this website for reference.

1. I did not, but our exterior walls are well sealed on the outside.
2. I used polystyrene foam board.
3. Read link above.
4. Metal studs are fine.
5. Focus on air sealing the exterior wall itself.
6. Maybe insulate the ceiling of the unheated utility room.
7. The link above mentions capillary breaks. Reads like something that must be done at time of foundation construction.

As a side note, I used Roxul for some of the insulation. That stuff is awesome to work with. Much better than fiberglass. More expensive though.

If I think of anything else later, I'll post again.
posted by LoveHam at 4:54 AM on June 30, 2017 [1 favorite]

Try to find a spray foam insulation contractor. It's the perfect solution here.
posted by humboldt32 at 8:46 AM on June 30, 2017

I recently found this all-in-one insulation and stud wall system; don't have any experience with it but may be worth checking out, since it seems designed for what you're trying to do.
posted by suedehead at 8:57 AM on June 30, 2017 [1 favorite]

humboldt32, this is a great idea but it was prohibitively expensive 5 years ago.

suedehead, This Old House did an episode where they installed this type of insulation. The prices on their website seem competitive if it means no studwalls have to be built. I would look into this.
posted by LoveHam at 11:00 AM on June 30, 2017 [1 favorite]

If there is even the slightest chance of water intrusion, do NOT put in fiberglass or cellulose. Pulling out soggy, moldy insulation is the very worst thing.
posted by rockindata at 11:05 AM on June 30, 2017 [1 favorite]

I second the Building Science website. They also have articles that cover adding insulation to an existing basement.
posted by sepviva at 3:39 PM on June 30, 2017 [1 favorite]