Where to Install Vapor Barrier
LESLIE: Dennis in Alaska is on the line with some questions about building a new home. What can we help you with?
DENNIS: Hey, that is some jazzy music you’ve got on hold.
TOM: Alright. We’re glad you enjoy it, Dennis. What’s going on?
DENNIS: I just finished building a ranch-style home and I did the regular stick frame, 2×6 fiberglass insulation, T1-11 outside. But my question is – I’m going to put some cedar-log siding on it and I thought I might fur the outside, get a few more Rs out of it. And I’m kind of concerned about possibly trapping moisture between the inside vapor barrier. And if I use blue board or some type of foam insulation on the outside, I didn’t want to trap water inside the wall. And I hope that made sense.
TOM: Well, so you basically used T1-11 as the sheathing but that’s a finished siding type of a product. That’s not designed to be a sheathing. Are you sure you meant T1-11 when you said that? Because it has sort of vertical grooves in it.
DENNIS: Yeah, it was just a temporary sheathing is all.
TOM: Yeah, because for many buildings, that is the siding. But OK. So you want to know how to cover that? Well, I would put tar paper on it or I would put vapor barrier on it, like Tyvek. And then I would put the blue board over that.
DENNIS: Vapor barrier and then Tyvek and then the blue board. OK. Yeah, I was just concerned about trapping moisture inside the wall.
TOM: Yeah, I wouldn’t put the foam right against it because that’s going to be like holding sponges against it. No, you want to put the moisture barrier against the wood.
DENNIS: So, do I need to put any type of a – some type of drainage on the bottom of the wall then or …?
TOM: Well, not for blue board, no. What you would do is you put the vapor barrier up and then you put the foam on it and you attach it with long nails that are flat-headed. They look like super-long roofing nails.
DENNIS: Right.
TOM: And they have other special fasteners for it, alright? And then you could put your furring strips over that and your siding on top of it.
DENNIS: Oh, OK. So you don’t actually butt it up to the furring strips; you put the furring strips right over it.
TOM: Right. Exactly. I mean that’s the way I would do it. I sided a house that way and that was 30 years ago. Hasn’t leaked yet.
DENNIS: Good enough then. That’s my project for this coming summer.
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