Attaching Foil-Faced Polyiso to the Inside of a Finished Wall
Question: what is the best way to attach rigid foam (foil faced polyiso) board to the inside of a finished wall? Can I just use longer drywall screws to go through the 1″ foam and existing drywall? What if I’m going with more than that (say 2 or 3 inches of polyiso)? I’m worried that this method would have a greater tendency to pop the drywall screws out under small impacts like hanging a picture. I’m also worried that the walls will sound hollow or be bouncy when knocking on them. What are the best practices for each thickness of foam? At what foam thickness do I need to use furring strips over top of the foam to hang the new sheet rock?
I am retrofitting a 1941-built 1.5 story in Minneapolis (zone 6), and want to get the whole house insulation as close to IRC 2012 (R49 roof and R25 walls) as possible without going crazy to get there. My motivations are A) reducing carbon footprint and fossil fuel usage (home has no solar potential, so the 96% ef. gas furnace will remain), and B) increasing comfort. I will be replacing all windows at the same time, likely with triple pane. Siding must remain untouched.
After much deliberation, I’ve narrowed it down to two options for the walls:
1) tear out sheetrock, spray foam and hang a 1.0″ Rmax polyiso sheet to prevent stud bridging before sheetrocking.
or 2) leave existing sheetrock and old fiberglass, and hang two layers of 1.0″ polyiso directly over that before finishing with new sheetrock (I have also considered going with a total of 3.0″ of polyiso.
Separate question: There seems to be no good answer on how to get better insulation and air sealing in these walls. Because of a very low profile roof and attic living space, I have almost no choice but to spray foam the lid. I’m hesitant about the high GWP blowing agents. Are we seeing any good alternatives in the market yet?
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Q. "What is the best way to attach rigid foam (foil faced polyiso) board to the inside of a finished wall?"
A. Generally, the rigid foam is held in place with a few cap nails. Eventually the foam is further secured with furring strips or drywall.
Q. "Can I just use longer drywall screws to go through the 1-inch foam and existing drywall?"
Q. "What if I'm going with more than that (say 2 or 3 inches of polyiso)? I'm worried that this method would have a greater tendency to pop the drywall screws out under small impacts like hanging a picture. I'm also worried that the walls will sound hollow or be bouncy when knocking on them."
A. You're not alone in your worries. Most drywall contractors hate installing drywall directly over foam, for the reasons you mention. One way to improve the situation is to install horizontal or vertical furring strips, 16 inches o.c., between the rigid foam and the drywall. If the rigid foam is foil-faced, this air space will have an R-value of R-2 or R-3.
The furring strips are not necessary, however. I have installed drywall over 2-inch rigid foam without furring strips. You may get a few more screw pops than usual with this method.
Q. "Because of a very low profile roof and attic living space, I have almost no choice but to spray foam the lid. I'm hesitant about the high GWP blowing agents. Are we seeing any good alternatives in the market yet?"
A. The alternative would be to install a thick layer of EPS or polyiso on top of the existing roof sheathing. Of course, with this approach you will need a second layer of roof sheathing above the rigid foam, as well as new roofing.
Martin - Thank you very much for the detailled response. You are definitely correct that most contractors hate the idea of doing this. I have asked several about it, and so far no one has even been willing to bid the job. I'm wondering if attaching 2" horizontal furring strips directly to the studs and piecing in foam board between would provide a more solid nailing surface. This may defeat the purpose of the foam board though.
If you want to install rigid foam and furring strips, the best way to do it is to install the rigid foam in a continuous, uninterrupted layer. Then install the furring strips on the interior side of the rigid foam. I think that most drywall contractors will find this method acceptable.
Needless to say, your electrical boxes will have to be moved to accommodate the rigid foam and/or furring strips.
I'm not confident your recommendation allowing for an air space between the foam and the drywall via wood or steel lath is accurate, especially if this home caught fire. Here's why....word for word... no omissions and no additions to the language. This is a legal argument and not of my opinion.
Actual passage from a litigation case over SPFI... "One major company -v- another"
POLYURETHANE FOAM AND COMBUSTIBILITY
"At the present stage of development, all organic foams, whether they contain fire retardants or not, should be considered combustible and handled accordingly. Experience demonstrates that certain precautions must be taken to minimize the fire hazard in handling, storage and use."
"How polyurethane is used in a building system ultimately determines its fire safety. Exposed polyurethane foam must be protected from accidental ignition by completely covering it with a flame barrier as soon as possible after installation, preferably the same day. SPRINKLER protection may also be DESIRABLE."
"----" argues that its warnings adequately conveyed the need for the barrier to be in close contact with the foam. "----" relied on its warnings of the need to "cover" the foam with a barrier and to "always use a protective coating when applying sprayed urethane foam to the interior or exterior of a building."-----" claims this language "obviously" indicates a need for direct application."
1. A legal argument is not a legal ruling.
2. Foil-faced polyisocyanurate is not spray polyurethane foam.
3. There is widespread acceptance of the method I describe by code inspectors.
In this case the manufacturer stated;
"-----" claims this language "obviously" indicates a need for direct application."
SPFI and polyiso both burn just as fast with similar catastrophic results. Do you have an illustration published by the "polyisocyanurate manufacturer" accepting this practice when fire safety is challenged?
Code officials in most states are immune from liability.
this is a few years late but Polyurethane is not the same and Polyisocyanurate, or as PolyStyrene. Blue or pink XPS will melt quite rapidly if subjected to flame. Polyurethane is not used in construction today. Polyisocyanurate or polyiso will self extinguish when subjected to flame and will only char, not melt.
Best material for interior applications would be Polyisocyanurate!!
Polyurethane isn’t used in construction? What about SIPs?
Spray foam is a polyurethane foam, that's what "SPF" stands for, and it's pretty commonly used -- it's probably even used more than it really needs to be. We also have all kinds of polyurethane adhesives and sealants, many of which are commonly recommended by people on this site (including me).