Polyisocyanurate boards on the interior of a wall
Hi all, I’ve been doing a lot of research on this site and have emailed back and forth with Thermasheath to see if this is a wise thing to do.
I’m remodeling my ranch house which has brick veneer and 2×4 construction. The brick veneer really makes applying the foam to the exterior impractical.
The ceiling drywall is in acceptable condition, and I’ll leave that in place. I’ve removed the drywall down to the studs, and removed the cellulose that had settled into the bottom half of the stud bays. There’s what looks to be an asphalt paper between the studs and the sheathing. The house was build in 1956.
Currently, I plan to insulate the stud bays with non faced fiberglass bats (bearing in mind all I’ve read about how to properly install it on this site), and then cover that with 4×8 sheets of Thermasheath 3 that is 1″ thick, attaching the polyiso foam directly to the studs with a couple cap nails. The sheets will be taped over with foil tape, and I’ll use loctite foam adhesive where the sheets meet the drywall on the ceiling. I’ll install vertical 1×4 furring strips at the same spacing as the studs underneath, and the install 1/2″ drywall over that. I will install attic vent baffles near the soffit and then blow in 16″ of cellulose. I’ll use airtight electric boxes for any ceiling fixtures. Near the chimney, I’ll wrap the brick chimney in rockwool insulation so the cellulose isn’t touching the chimney.
Thermasheath says affixing it to the interior studs is an acceptable application. From what I’ve read here, 1″ is an acceptable thickness given I live in Midland (climate zone 3 and so very very dry) vis a vis dewpoint on the side of the foam that faces the exterior.
My questions are:
-Should I install horizontal furring strips at the top and bottom of the wall so the drywall is easy to tack down?
-Since this is a remodel, anywhere an interior wall touches an exterior wall, I won’t have complete coverage of polyiso boards on the wall, is this a big problem, or is there a relatively easy way to mitigate this?
-Is my method of sealing the polyiso boards to the ceiling drywall a good idea?
-1″ polyiso is the thickest the midland home depot sells. Is it worthwhile to do 2″ given I won’t be doing anything with the ceiling and the issue I’ll have with thermal bridging where interior walls intersect exterior walls? I’m getting new windows.
-Rmax says there is a side marked interior. Looking at their specs, there’s a reflective foil side and a non-reflective side, which would you face toward the interior? I’ll double check with them but I thought I’d get your opinion here.
Any help you can offer would be so appreciated!
Nate
Picture of tar paper behind sheathing:
https://www.dropbox.com/s/fls5pwroio867a1/20170618_161456.jpg?dl=0
Picture of interior wall meeting exterior wall:
https://www.dropbox.com/s/fvczr5b07zd7n2r/20170618_161421.jpg?dl=0
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Replies
Doh, I just found https://www.greenbuildingadvisor.com/articles/dept/musings/walls-interior-rigid-foam...
Some of the questions I asked were not totally addressed and I still have them.
Nate,
Q. "Should I install horizontal furring strips at the top and bottom of the wall so the drywall is easy to tack down?"
A. That's not necessary, but you can if you want.
Q. "Since this is a remodel, anywhere an interior wall touches an exterior wall, I won't have complete coverage of polyiso boards on the wall. Is this a big problem, or is there a relatively easy way to mitigate this?"
A. It's a disadvantage of interior rigid foam compared to exterior rigid foam. If you want your interior rigid foam to be continuous, you need to open up some of the drywall on your partition intersections. Since this is a hassle, most people probably don't do it.
Q. "Is my method of sealing the polyiso boards to the ceiling drywall a good idea?"
A. Any type of canned spray foam will work.
Q. "1-inch polyiso is the thickest the Midland home depot sells. Is it worthwhile to do 2 inches given I won't be doing anything with the ceiling and the issue I'll have with thermal bridging where interior walls intersect exterior walls? I'm getting new windows."
A. Only you can answer this question. The answer depends on your goals and your budget.
Q. "Rmax says there is a side marked interior. Looking at their specs, there's a reflective foil side and a non-reflective side. Which would you face toward the interior?"
A. The foil radiant barrier won't add any R-value to your wall assembly unless it faces an air space. Since you plan to install furring strips on the interior side of the polyiso, that means that the foil facing belongs on the interior side.
Mr. Holladay- Thanks for your fast reply. Excellent, it looks like I might not screw this up.
One other question:
A. It's a disadvantage of interior rigid foam compared to exterior rigid foam. If you want your interior rigid foam to be continuous, you need to open up some of the drywall on your partition intersections. Since this is a hassle, most people probably don't do it.
Let's say I was going to do this, would I simply put foam inside the partition wall? I.e. Imagine the exterior wall is the top of the letter T and the interior wall is the vertical line, you put a foam piece on the inside of the interior wall's first stud? Or are you suggesting that you'd have to move the first stud such that you could keep continuous foam along the inside of the exterior wall?
Nate,
Q. "Would I simply put foam inside the partition wall? I.e. Imagine the exterior wall is the top of the letter T and the interior wall is the vertical line, you put a foam piece on the inside of the interior wall's first stud? Or are you suggesting that you'd have to move the first stud such that you could keep continuous foam along the inside of the exterior wall?"
A. Either way is possible. As I said, most people aren't going to bother opening up the partition drywall to do this work. But if you are a fanatic for whom energy details really matter -- regardless of cost or hassle (and I understand the impulse if that is the case) -- you can remove the first stud of the partition wall, in order to slide the foam behind the stud.
This could turn into a nightmare, however, if there are electrical cables between the partition and the exterior wall. And if you are that fanatical, you'll be kept awake at night thinking about your rim joist details, and before you know it, you will be ripping out your ceilings. Beware.