basement insulation

November 12, 2008 7:11 PM

Need opinions on insulating a basement...

I'm finishing my basement. About 2/3 of the walls are below grade, framed with 2x6 lumber set an inch from the wall. I've read competing opinions on optimal insulation strategies, with the primary concern being moisture. A contractor friend has basically stated he would never put batt/fiberglass insulation in a space like this (even with moisture barrier) that might get condensation and is recommending foam board.

I'm OK with foam board, but not sure how the hell to put it in so it's nice and airtight, imagining tons of minor air gaps where I didn't slice the panels correctly, and not sure I'll get much of an R value unless I lay the panels in 4 inches deep or more, so I need to double or triple lay them, I guess.

Another option we are investigating is sprayed foam, so anyone with firsthand knowledge of this techniques, please spill it.

posted by docpops to home & garden (15 comments total)

I don't know how to make a link.

Go to: askthebuilder.com

do a search for insulate basement walls.

Here is the long link.
http://www.askthebuilder.com/Search_Results.shtml?cx=partner-pub-6304043743499945:gac4qn-wxfh&cof=FORID:10&ie=UTF-8&q=insulate+basement+walls
posted by JayRwv at 7:39 PM on November 12, 2008

JayRwv - Thanks. I've been all over it. I am, possibly delusionally, hoping some sort of insight will come from this board that I have yet to glean elsewhere.
posted by docpops at 7:44 PM on November 12, 2008

Got a hundred or so pairs of old jeans?
posted by turgid dahlia at 8:33 PM on November 12, 2008

I think the more likely risk in a basement isn't condensation, but the moisture from the surrounding ground migrating through the walls. Here's a good discussion on basement insulation. You shouldn't have too much trouble with foam board, just be sure to tape the seams. Also, Owens-Corning makes a product specifically for finishing basement walls.

Have you already studded it out? If you're really concerned about moisture, you could use metal channels and foamboard. Sprayfoam could be an option as well, but it's pretty expensive.
posted by electroboy at 8:47 PM on November 12, 2008

Closed cell foam board insulation is the best bet, not something that will absorb moisture, grow mold etc. Seal the wall with a proper sealant paint, place the foam and seal the joints with an expanded foam or whatever the manufacturer recommends. You will also want and likely require a fire barrier over the foam.
posted by caddis at 8:54 PM on November 12, 2008

electroboy, that article is incredible. I just skimmed it so far, but it's exactly what I needed and couldnt find. My engineer's mind needed real explanation and not dogma. Now I have it. Thank-you.
posted by docpops at 9:06 PM on November 12, 2008

yes, that was great electroboy
posted by caddis at 9:31 PM on November 12, 2008

Also consider putting down a thin layer of rigid pink insulation on the floor with a layer of plywood over it, screwed down into the concrete.
posted by bonobothegreat at 9:57 PM on November 12, 2008

I agree with the responses here- foam is the way to go.

This isn't from experience, but (my own variety of) common sense. I'd also make sure that when you fasten the foam to the walls, that you don't create pockets or dead zones where moisture can collect. Meaning, when you put the adhesive on the walls, use vertical stripes of it. If moisture gets in, it will be able to weep up or down and out.

If it was me, I'd also do this- for the flooring, use something completely water resistant. I'd lay down ceramic or composite tile. And then put the insulation up like above, but leave a small gap at the bottom so water that might get in can leak down onto the floor. If you use the options here, you should be good.
posted by gjc at 4:50 AM on November 13, 2008

If you're looking for personal experience, I may as well say that we insulated our basement with fibreglass batts and a vapour barrier. I'm happy with it after 4 years but we tend to keep it closed up on muggy days in the summer with the de-humidifier running. I did use rigid foam on one section of wall and secured it with the sort of plastic washers they use for attaching styrofoam for stucco jobs. Then I sealed the joints with red tuck-tape. I could have done the whole thing with the styrofoam but it seemed really expensive to get the r value recommended for Toronto. Also, we had waterproofed the exterior with sealer, a dimpled membrane and weeping tile.

gjcs concern about water is valid and I think it's important to allow for a path for water to get down to your interior weeping tile or the floor drain. It might not come from where you'd expect it, either. It's possible that some spring day your eaves might overflow and the water will run down the brick and be drawn down inside the wall. I'd want to be sure that there's some kind of poly sheet that's at least tacked to the back side of the stud wall and runs under the baseplate and whatever kind of sub-floor you put down. I used the Dri-core panels for my floor but as I said above, I'd lay a sheets of foam down and plywood over it if I did it again. For living spaces, don't tile directly on the concrete because it gets uncomfortably cold in winter and will tend to get slick with condensation if it stays really muggy for a long stretch in the summer.
posted by bonobothegreat at 6:35 AM on November 13, 2008

...and just to be sure, you know you'll have to take down your stud wall to insulate with the rigid foam, right? The foam is available in different thicknesses as well, so you won't need to put up several layers.
posted by bonobothegreat at 6:39 AM on November 13, 2008

Excuse the partial hijack, but if I have painted concrete walls and want to hang a whiteboard on a portion of one wall, I don't need to do all this, do I? Can I just run 2x4s down from the joists and across the floor and back up, and then hang my whiteboard from those?

(It may not even be a whiteboard: I already have a lot of hard drive magnets, some blackboard paint, and a pile of steel PC case panels: I am thinking of assembling them into a huge, magnetic chalkboard made mostly of salvaged computer pieces.)
posted by wenestvedt at 6:41 AM on November 13, 2008

Regarding the above comment about the flooring, we put down 6 mil plastic and then screwed 3/4 ply into it, which was a huge bitch, as the concrete was incredibly hard, but it's worked well.

I had the closed-cell foam contractors out today for a bid, fwiw. Will post back once the bid is in if anyone cares.
posted by docpops at 8:32 PM on November 13, 2008

Bid was $4400 bucks for the whole space, which is roughly 900 sq. feet, with 5 inches of foam applied. I've decided to cut and insert R13 foil faced EPS board with a rim of canned foam applied around each one to keep it tight.
posted by docpops at 7:11 PM on November 14, 2008

There's DIY kits, which would run you about $3k just for the materials.
posted by electroboy at 12:56 PM on November 16, 2008