How to Insulate a Crawlspace Properly
LESLIE: Michaeline in Wisconsin is on the line needing some help insulating a crawlspace. Tell us what’s going on.
MICHAELINE: Well, I hung some plastic and insulation from the ceiling of the crawlspace and all the way around. And I’m still getting drafts and air coming in into the bedroom that faces the north, by the wall.
TOM: So you’re getting drafts up through the walls? Is that where you feel like it’s coming up?
MICHAELINE: Yeah. And up through the crawlspace. There’s half a basement, half crawlspace.
TOM: OK. Now, what kind of insulation did you use, Michaeline?
MICHAELINE: Well, I used the black plastic and I used the R-stuff with the …
TOM: The R-stuff. Let’s back up for a second, OK? The insulation that you put in, is it – was it unfaced insulation? Did you press it up into the floor joist, like nice and fluffy?
MICHAELINE: No, I didn’t press it into the floor joist.
TOM: How did you hang it?
MICHAELINE: I went with what the Reader’s Digest said, to hang it from the ceiling of the floor, down to the flooring of the crawlspace and let it …
TOM: So, where is the – the insulation that goes up in that floor should be unfaced: should have no paper face, no plastic face; it should be unfaced. And it should be big and fluffy and should be as thick as the crawlspace floor.
But here’s the steps. And if you had called me before you started this, here is what I would have told you to do. First of all, I would say the area on the outside of your house, where we have what’s called the “box joist” – that’s the beam that goes around the outside perimeter.
TOM: In that area, you want to seal the gaps with an expandable foam, like GREAT STUFF or a product like that, so you …
MICHAELINE: On the inside?
TOM: On the inside, right. You seal that, you spray it. Because you get little gaps that – where air can come in around that. Then once that dries, it gets nice and hard. Don’t try to scrape it away or cut it; it doesn’t matter. Just spray it, let it dry, stop right there, don’t cut away the excess. Then, add some insulation and the insulation would be unfaced fiberglass batts. If your floor joists were 2x10s, I would put 10-inch fiberglass batts there.
How do you support those? You use insulation hangers. They’re like pieces of wire that stick in between the joists. And let it hang there. And then, on the crawlspace floor – is it a dirt floor?
TOM: So if it’s a dirt floor, then you want to add the plastic right on the dirt floor. Now, that’s not for drafts; that’s to stop moisture from coming up.
LESLIE: That’s for moisture.
TOM: And those things – that’s the best you can do for that crawlspace.
LESLIE: And Michaeline, when you’re putting the plastic on the floor of the crawlspace, if you for some reason have to use more than one sheet, make sure you overlap by 2 or 3 feet so that you’re not getting any moisture releasing into it. Because, as Tom said, the moisture can really reduce the effect that the insulation is going to have.
MICHAELINE: Do you – do I tape it then if I’ve got to use more than one sheet?
LESLIE: If you overlap them by 2 or 3 feet, they’ll stay.
MICHAELINE: Oh, OK.
TOM: Yeah, they’ll stay. Gravity will hold it in place.
TOM: Alright? And that’s it. Alright, Michaeline? Good luck with that project. Thanks so much for calling us at 888-MONEY-PIT.