Eric Nash is renovating a kitchen with “old school” 2×4 walls. He’s gutting the room, but not touching the outside of the house. He’d like to present the homeowners with some options for increasing the R-value of the exterior walls while making a better air seal.
Here’s his plan: fill the stud bays with cellulose, add 1 1/2 in. of rigid foam insulation on the inside, then a layer of 1/2-in. plywood on which to hang cabinets, and, finally, 1/2-in. drywall.
How about that? he asks in a Q&A post at Green Building Advisor.
Is plywood necessary to hang cabinets?
Anyone who’s installed kitchen cabinets knows how reassuring it is to have solid material in the walls to catch mounting screws. That’s what the layer of plywood beneath the drywall would accomplish.
But is 1/2-in. plywood enough?
Robert Riversong doesn’t think so. “If it’s only the cabinets you’re concerned about, I would forgo the plywood, install appropriate blocking and use 3-in. cabinet screws,” he writes. “I would rather have cabinets secured to framing than to 1/2-in. plywood.
“You’ll have to move the electrical boxes inward or use deep box extensions,” he adds. “For that, I’ve used plywood pieces attached to sides of studs to attach the boxes.”
David Meiland wouldn’t bother with continuous layer of plywood, either. “But,” he adds, “I might run a 2×4 across the wall at the top of the uppers, interrupting the foam. Tape the 2×4 to the foam.”
Andy Ault, however, favors the plywood route. In fact, it’s standard operating procedure for his crew when they build wine cellars.
“The only difference being that we step up to 3/4-in. ply to increase our holding power,” Ault writes. “Granted, those racking systems have many more points of attachment, but it’s still super helpful to know that no matter where the layout lands and no matter where we may need to shim that we will have a solid material to bite into.”
The problem with blocking, he adds, is that it introduces more voids in the insulation and adds to thermal bridging. In addition, trying to sink 3-in. screws through a layer of insulation and catch blocking perfectly may be harder than it sounds.
What about moisture issues in the walls?
The existing wall may not be at the top of the class from an insulation standpoint, but it’s also unlikely to trap much moisture, either, as Meiland points out. But, he says, “Once you add the foam you reduce the amount of air and heat going out and it may not dry as well.”
Moisture working its way in from the outside and collecting in the cellulose insulation is another concern, Ault says.
“Given the possibility of reduced drying potential, I would probably go with something like Roxul vs. cellulose,” Ault says, referring to a type of rock wool insulation. “That way it’s just one less food source if moisture does end up in the cavity for longer periods.”
He also suggests adding some asphalt felt to the back side of the exterior sheathing while the wall is opened up. In the event that some water is pushed through the wall, it would be a hedge against water penetration while still allowing the wall to be vapor-permeable.
“And depending on what region you’re in and what direction the wall faces, this may also help to give you a little resistance to inward solar vapor drive,” he adds.
What type of foam is best?
Riversong likes the original plan, minus the plywood, and recommends that Nash use XPS (extruded polystyrene) rather than foil-faced polyisocyanurate, to allow for some vapor permeance.
“I would not install any felt or other membrane inside of the sheathing, as that will do little to prevent water intrusion, which will still soak into sheathing and framing members, making them more vulnerable,” he says. “And I would not hesitate to use dense-pack cellulose, since it can tolerate cyclical wetting and redistributes moisture so well as to reduce the vulnerability of the framing in the event of leakage.”
Nash wonders about the choice of XPS over the less permeable polyiso: “If a goal is to reduce the moist air moving into the wall in winter, why not go with the foil-faced? Is it to have some potential to dry to the inside when needed?”
XPS and foil-faced foam board both have the potential to be perfect air barriers, Riversong replies. But 1 1/2-in. XPS has a permeance of 0.75 perm while the polyiso has a permeance rating of close to zero.
“So XPS will allow some inward drying potential, while foil will trap moisture,” he says.
An expert’s opinion:
We asked GBA Technical Director Peter Yost for his take on the question. Here’s his reply:
1. On securing the cabinets:
While I like the 2xs as less material-intensive for securing the cabinets, I have to go with Andy Ault on this one. Eliminating the thermal bridging and being able to secure anywhere on the plywood wins out with me. When I built my SIP kitchen addition, I got some really superior screws from Winter Panel with a thread count and coarseness of thread that gave excellent holding power even in 1/2-inch OSB.
2. Moisture management in 2x walls with interior rigid insulation:
If you are reducing the drying potential of the framed wall, make sure that you have checked or even improved the bulk water protection of the wall. Overhangs, flashings, etc. should be checked to make sure that the wall is not already challenged by moisture.
Because this kitchen remodel is in the Northeast, I agree with Robert Riversong in terms of the XPS and its 0.75 perms being a better bet than the 0 perm foil-faced polyiso rigid foam. We want to maintain whatever drying potential we can even as we seek to keep wintertime moisture out of the wall assembly.
If the rigid insulation is taped and installed as an interior air barrier, it would be nice to choose a cavity insulation that restricts air flow in the framing cavity. While neither dense-packed cellulose nor Roxul batts are considered air barriers by a long shot, they are less air permeable than some other batt or blown products. Having just used Roxul batts in my attic, I really like how they perform.
Finally, match high performance walls in the kitchen with a high performance exhaust system. Keep the kitchen drier, more efficiently, with an Energy Star exhaust fan vented to the exterior.