How to Insulate Foundation Walls
LESLIE: Now we’ve got Jim in North Dakota on the line who is looking to insulate the outside of his home. Tell us what’s going on.
JIM: I’m up there in the cold country and I want to – I’m looking to – this past summer, I put siding on my house and new windows and doors and did all that stuff myself. And then I decided I’d better do my basement foundation, especially the portion that’s exposed. And I was – what I’m thinking of doing is – Menards sells those – I don’t know if I’m supposed to say brand names – but sells those 3-foot by 5-foot sheets of Styrofoam with rocks already glued, or however they do that, on one side.
And that’s what I was going to use but I don’t know how to attach that to the wall. And then I had a neighbor come over and told me that even if I do use that, because I’ve got about 2 foot of foundation exposed – so I’d only be going a foot-and-a-half or so under the ground with the rest of that. But my neighbor tried to tell me that if I don’t go all the way to the bottom of my foundation with foam, that I’ll create a hot/cold where I stop. And he says I’ll break my walls doing that and I’m like, “Well, really?”
TOM: Well, Jim, first of all, are these foundation walls exposed on the basement side?
JIM: Yeah, there’s – I’ve got, well, one, two, three windows in there and that’s another thing. I don’t know how to trim around them with that foam stuff.
TOM: Well, here’s what I would do. I would recommend that you insulate the inside of the foundation walls, not the outside of the foundation walls. Typically, when you build a house and you excavate around the house, you do put a foam insulation around the outside walls. But short of you digging down to the footings, I don’t think it’s worth doing at this point.
I would insulate the interior of the walls. There’s a wide variety of different types of insulation products. Yes, you can get the kind that attach to the wall. There’s also a fiberglass batt that’s sort of covered with kind of like a foil vapor barrier, that’s specifically made for foundation walls. And any of those are good options to insulate the interior of the walls.
But just as important, if not more important, make sure you have plenty of insulation up in the box-beam space, which is the beginning of the floor structure. A lot of folks will insulate walls and leave that uninsulated and that’s actually more at risk for drafts. So make sure that the box-beam area, the sill plate, all that area is sealed and insulated. And insulate – add the additional insulation at the inside of the foundation walls, not the outside. A lot easier to attach that way.
If it’s the foam, you could attach it with a construction adhesive. If it’s the batts, there’s different types of a clip system that comes, typically, with those. You’d buy it at the same place you got the insulation, where it would be clipped into the wall. But I do think it’s a good idea for you to insulate those walls.
JIM: OK. He told me they do that because that’s why – he’s trying to tell me that’s why they insulate the outside of their walls up.
TOM: No, your foundation would only crack if your soil got really wet and it expanded and it cracked the walls. If you have good drainage, then your soil should not crack based on having insulation on one side and not the other. That doesn’t make any sense to me at all.