How to Insulate Under Siding
LESLIE: Alright, well it looks like our friend Everett in Wisconsin is putting up some new siding and has a question about whether or not to insulate underneath.
Hey Everett, how’s your project going?
EVERETT: Oh, it’s going.
LESLIE: It’s going.
EVERETT: I’m checking in to finding out a good r rating, insulation-type thickness on it. When I was at the lumberyard, they had some different sizes, thicknesses, and r ratings on them.
TOM: Now is this the insulation that sort of backs the vinyl siding or are you talking about the insulation that goes over the exterior wall?
EVERETT: It would go over the exterior wall over the existing wood siding that’s on there now.
EVERETT: I guess what you do is you apply that and then you apply the siding over that. Is that correct?
TOM: Yeah, so you’re talking about like a Styrofoam panel or a Thermax insulation; you know, the insulation sheeting.
LESLIE: Like a sheet product.
TOM: Right. Mm-hmm.
EVERETT: Correct, correct. I see some of it’s got kind of almost like an aluminum foil type and then others pink and there’s some blue board.
TOM: (overlapping voices) Right, exactly. Yep.
EVERETT: Well, I’m wondering what’s a good r factor? I notice that some of this that I’ve looked at it, it has a 1/2-inch thickness and some of it goes to 2- and 4-inch thickness on the pink and blue board, the Styrofoam board.
TOM: Right. Well, the thing is if you put something on that’s super-thick it’s going to affect all of the windows and the door installations because you’re going to be pushing the wall out that much further; especially if you’re going to leave the old siding on.
Now if you pull the old siding off, you’re going to pick up about an inch of space there – especially if it’s like wood clapboard or something like that – and then you could use an insulating board. I resided my house – I have an old 1886 farmhouse – and I took off, actually, three layers of wood siding, believe it or not. There were two layers of clapboard and one layer of cedar shingles and when we got down to the studs in the old house, we actually had no sheathing. So what I did is I used 1-inch Thermax insulating board around the entire house and then of course had to put firring strips on to support the new siding. But by removing all of the old siding, I was able to preserve the window positioning because if you put too-thick insulation on the outside, you’re going to end up with these like recessed windows all around your house and it’s going to look very odd.
EVERETT: Kind of like a window well.
TOM: Yeah, exactly.
EVERETT: Right. Now my home is similar to yours. Mine was built in 1850. We recently – we’ve been doing the interior of the home and I’ve put all new insulation into the walls. And when I pull the existing interior walls down, I’m noticing that I have my siding boards there.
TOM: Right, so you have no sheathing at all.
EVERETT: Basically, no.
TOM: Right. Well, that’s common. Yeah.
EVERETT: I think maybe that might be the original boarding on it from back at least 50, 60, 70 years ago.
TOM: Sure. Well, that’s the way they did it back then. They didn’t have sheathing like we have today. In a balloon-frame construction, in an old house like that, the framing is self-sustaining. Today when we do a framed wall, we need sheathing to make it structurally sound. But in an old home it was framed so that the frame itself was structurally sound without the sheathing on it.
So you’re just going to have to decide how far you want to go here. If it was me, I would take the siding off, go right to the studs, put foam sheathing on it, and then fir it out and reside it from there.
EVERETT: OK. OK, that would help …
TOM: That would give you the best, tightest, warmest job that you could possibly have.
EVERETT: Alright. And then is there a certain r factor I want to look for or …?
TOM: Well, you certainly want probably – I would use – if I was going to take the siding off, I would use at least one inch.
EVERETT: One inch.
TOM: So the r factor’s going to vary based on product.
TOM: But one inch is terms of the thickness that you want to use.
TOM: Beyond that, if it’s within a couple of r from product to product, I would just find the one that’s available and is at the best price for your location.
EVERETT: OK, because I’d noticed like – I think it was a 1-inch board was coming in – on the pink board it was saying r5.
EVERETT: So I didn’t know if that was a good r or not.
TOM: Well, it’s usually, with insulation, it’s like 3 r per inch; with the sheathing it’s a little bit more than that.
LESLIE: Because it’s so dense.
TOM: But it’s also going to help seal out drafts. Remember that as well.
EVERETT: Right, I’m finding – yeah, I’m finding that as I did the interior.
TOM: Yeah, you’re going to be really surprised with how much warmer the house is when you get it properly sealed up on the outside.
EVERETT: Good, good. I appreciate your help.
TOM: You’re very welcome. Thanks so much for calling us at 1-888-MONEY-PIT, 888-666-3974.