How to Insulate Exterior Walls
LESLIE: Robert in Georgia is calling in with an insulation question. What can we do for you?
ROBERT: Well, need a little bit of information from you. I’ve got a 1970s-era house, so it’s 2×4 construction over old insulation.
ROBERT: Just the regular, pad-faced insulation. I’ve got a north wall and a south wall but the problem is is I’m up here about 1,000 foot and below Appalachians, so I have a half-heating, half-cooling situation. I am literally right on the line, if you look at the maps, of where they say put the vapor barrier inside and put it outside.
ROBERT: What I want to do is replace the cedar siding with new cedar; I really don’t like the plastic look of the HardiePlank and all of that.
ROBERT: So I’m going to have the wall open; I’m going to have all of the panels – the openings of the 2×4 – available to me.
TOM: Exposed from the outside, OK.
ROBERT: Right. So my question is: what’s the optimum way to reinsulate and vapor barrier – and do I just put the insulation pads into the 2×4 and then cover that with plywood as a seal and then put ½-inch foam board over top of that and then the cedar or what?
TOM: Well, here’s what I would do: I would insulate in between the stud bays with as much insulation as they’ll properly hold. So if they’re – if it’s 2x4s, then that’s going to be 3½ inches of insulation. And then I would resheath it, because you need the sheathing to hold the structure in place. Over the sheathing, I would put Tyvek and then over the Tyvek, you can put the foam.
The thing is, though, when you attach the cedar shingles – if you look at the installation instructions on cedar, you probably want to put those on furring strips; you want to have some air space under them.
ROBERT: Well, this would be a cedar board, so it would be …
TOM: Oh, oh. You mean like a clapboard?
LESLIE: Like a HardiePlank?
ROBERT: Right, right.
TOM: You mean like clapboard. Yeah, he means cedar clapboard.
ROBERT: Yes. That’s true.
TOM: OK. So then that, I believe, you can put right on top of the foam sheathing.
ROBERT: OK. So then the nailing would go through the cedar?
TOM: Right through – right. And make sure you use stainless nails or you’re going to get black spots all around the heads.
ROBERT: So the rules of the game – far north, far south – about vapor barrier, that’s picked up with Tyvek over top of the sheathing.
TOM: That’s correct.
ROBERT: So you use insulation and then I’ve got that really kind of in the middle.
TOM: Yeah. That’s right. And it’s vapor-permeable, too. So I mean it’s – you’re not going to trap moisture; it will move freely in between, the way it needs to.
ROBERT: Is that going to be good for the north side, as well, where I don’t get that sunshine in?
TOM: Yeah. I would do the whole – I would do all sides the same way.
ROBERT: Now, what happens where I have the brick – not the brick but the fieldstone – across the front that you just sort of …
TOM: Well, with the fieldstone across the front, you’re not going to do anything with that. If you want to insulate that space, you could do it from the inside. You might want to think about using blown-in in a place like that. A lot of the home centers will rent the equipment today where you could just drill one hole up high, one hole down low and use a blown-in fiberglass or a blown-in cellulose.
ROBERT: Which wouldn’t work real well if there is even a little bit of the old insulation panels.
TOM: Well, if you’ve got old insulation in there, then you’re correct: it will not work well. But you know what? The most important insulation, from an energy savings perspective, is what’s in the ceiling. So, the walls are important but don’t get crazy over them. Make sure you’ve got a good 15 or 20 inches in that ceiling. That’s going to do the best job possible at keeping your energy bills down.
ROBERT: Yeah, I’ve got 12 up there and then I’ve got reflective stuff below the shingles.
TOM: OK. Well, good luck with that project.
ROBERT: I do appreciate your help. Thanks, sir.