Writing from Long Island, New York, Joe C describes repairs that he’s been making to his house to correct water and insect damage. Two of the four walls of the room are exterior walls, and Joe is concerned that thermal bridging through the new framing he’s adding significantly degrade energy performance.
“With all the extra studs, I’m concerned there will be a lot of thermal bridging,” he says in a Q&A post. “To counteract this I’m considering installing rigid foam [insulation] on the interior side of the wall framing.”
For reference, on the outside of the studs is a layer of exterior gypsum board, followed by a small air gap, and then brick veneer.
His plan is to insulate the stud cavities with fiberglass, then add 1/2-inch thick extruded polystyrene (XPS) on the interior side of the wall before installing drywall over the rigid foam. The only 1/2-inch foam available from Home Depot is Owens Corning Foamular, which he says has an R-value of 3.
“Is this a reasonable plan of attack?” Joe C asks. “There shouldn’t be any issues drying to the outside. Would condensation on the outside face of the foam board in the summer (AC on in the house) be an issue?”
That’s where we start this Q&A Spotlight.
The foam isn’t thick enough
Martin Holladay doesn’t try to steer Joe C away from his interior insulation approach, but he does suggest the layer of foam Joe wants to use isn’t thick enough. For starters, a realistic R-value for 1/2-inch rigid foam is R2.5, not R-3.
“I don’t think it’s worth doing with less than 1 inch of rigid foam,” Holladay says. “Of course, 1 1/2-inch foam would be even better.” For more details, Holladay refers him to an article he wrote for GBA several years ago (see the first entry in the “Related Articles” sidebar).
Joe C says he understands that by bumping up the foam layer to 1 inch, thermal bridging becomes less of a problem. But it might not be worth the trouble.
“I’ll give this some thought, as it does increase installation hassles (I’ve got a window and baseboard heat on one of the walls),” Joe C says. “Also, this is only one room in an old house, so the overall difference between 1/2-inch and 1-inch rigid foam will be very small, though it may make this room more comfortable in summer and winter.”
Joe’s basic assumptions are off
Joe C has been assuming the R-value of the 2×4 stud will be R-1.2. But it’s more realistic to assume the R-value of the wood is R-1.2 per inch, writes Dana Dorsett. So the R-value of the 3 1/2-inch studs is closer to R-4.2. By adding 1/2-inch polyiso insulation (R-3), Joe C could reduce the framing fraction heat flow by more than a third. If he were to use 3/4-inch polyiso, that could be cut more than half.
Dorsett suggests a couple of other alternatives. One is to make the stud cavities 1 inch deeper with strips of rigid foam attached to the edges of the studs. That would reduce thermal bridging. Then Joe C could compress R-19 batts into the stud bay. The compressed bat would perform at about R-15, he says, and the insulation would have minimal voids.
“Alternatively, installing cut’n’ cobbled polyiso up against the gypsum sheathing to restore the cavity depth to 3 1/2 inches, installing R-13s would bring the center-cavity R up to R-19, almost meeting current code-min on R-value basis,” he adds. “And with the R-6 thermal break on the framing it would meet code on a U-factor basis, even with the higher framing fraction of the sistered studs.”
Putting vapor-retardant foam board against the very vapor-open gypsum sheathing would mitigate summertime moisture accumulation in the wall, Dorsett says. [He also adds this reference to a Fine Homebuilding video on cutting rigid insulation precisely.]
While the stud bays are accessible, Dorsett says, Joe C should take the time to caulk the framing where it meets the sheathing, while also caulking any doubled-up framing members. Horizontal seams in the sheathing should be sealed with house wrap tape.
Caulk a better sealant that canned foam
Joe C was planning on using insulating foam in a can to seal the framing to the sheathing, but Dorsett has recommended instead that he use caulk. Is caulk more effective than Great Stuff canned foam in this application?
“In general, yes, caulk is better than expanding foam for sealing those seams,” Dorsett replies. “Expanding foam doesn’t always adhere smoothly, and won’t force it’s way into the tiny gaps between sheathing and stud edge as easily as [polyurethane] caulk will. The expanding foam is also a bit bumpy and uneven, making it hard/impossible to get the batt to snug perfectly into the edges & corners without voids.”
Expanding foam is great for filling gaps, he adds, but it’s slower, messier and less effective than caulk in this application. Unlike painter’s caulk, polyurethane says relatively flexible and will show good adhesion over the long haul. It’s also highly tolerant of the moisture and temperature swings it will experience in this location.
Let’s do the math
Akos has calculated the R-values of the different wall assemblies that have been discussed. (Presumably, he’s talking about the whole-wall R-value, not just the R-value at the center of the insulated stud cavity. This takes into account the lower R-values of the framing.) Here’s how they shape up:
- The existing wall with R-13 fiberglass batts actually works out to about R-12 when the brick veneer and air gap are figured in.
- By doubling up the studs in the wall, that drops to R-11.
- Adding 1/2-inch XPS insulation under the drywall brings the R-value up to R-14.
- Adding 1 1/2-inch polyiso boosts the R-value to R-22.
- With 1 1/2-inch thick strips applied to the face of the studs and R-23 batts in the stud cavities, the whole-wall R-value comes in at R-19.
“Overall, if not too much work, a layer of continuous foam (slightly more than 1/2 inch) is worth it,” Akos adds. “The last wall is the best $/R value but more labor.”
Will condensation be an issue?
There are still a few issues Joe C isn’t clear on.
- When a continuous layer of rigid foam is applied on the interior side of the wall, should the fiberglass batts in the stud cavities be faced or unfaced?
- Should the rigid foam board be unfaced, or does it even matter?
- If he applies a continuous layer of rigid foam to the inside of the wall, will there be a risk of condensation on the foam when the air conditioning is running in the summer?
Dorsett answers those questions this way:
If the interior of the wall is low-permeance foam, it doesn’t matter whether the fiberglass batts are faced or unfaced.
Because Joe C’s house is in Climate Zone 4, and assuming the wall can dry to the exterior, it doesn’t matter whether the foam board is faced or unfaced.
Condensation shouldn’t be an issue unless Joe C drops the interior temperature to 60° or so. The R-value of the foam (rather than its vapor permeance) is what keeps the average temperature on the exterior side high enough to avoid serious condensation, Dorsett says.
“Even if it’s impermeable foil faced foam it is superior to no-foam and a interior side polyethylene vapor barrier, since the exterior side facer of the foam will average several degrees warmer than the conditioned space temperature,” Dorsett continues. “With only fiber insulation and interior polyethylene the polyethylene sheet will be pretty much room temperature, and below the dew point of the outdoor air much of the summer in a Zone 4A climate.”
Our expert’s opinion
Peter Yost, GBA’s technical director, added this:
There are lots of great issues raised in this exchange:
Fully assessing the water damage: This is pretty quickly dealt with in the Q&A exchange. Although Joe C states that he addressed the original moisture problem, it’s critical to do this while the wall is open. I would take these steps:
- Carefully inspect the exterior sheathing and the bottom plate and look for patterns of damage and water staining. It could be evidence that there are clumps of mortar at the bottom of the wall behind the brick, which would provide a capillary connection with the sheathing and wood framing. That’s a common source of moisture problems in brick veneer wall assemblies. The tricky part is that to confirm and correct the situation you would have to strategically remove bricks from the bottom course to pull out the mortar.
- Is the brick veneer properly weeped?
- What direction does this wall face in terms of summer sun/storm-wetting?
- How many stories are there and what sort of roof overhangs are there?
Insulating with continuous rigid insulation on the interior: Yes, this makes the entire assembly exterior to the rigid insulation colder in the winter and warmer in the summer, the former less than ideal and the latter just fine. In Climate Zone 4A on Long Island, with roughly equal heating and cooling seasons, it’s a pretty good tradeoff but see number 2 below.
- Interior air barrier: It’s important in any climate to have a continuous air control layer, but if you are putting up interior continuous rigid insulation, it’s really important for this to be airtight (Airtightness is not mentioned in this Q&A spotlight summary, but it was one of the very first comments on this string).
- Caulk as air seal: I agree that high-performance liquid sealants can give a superior air seal, but be careful with “builder-grade” liquid sealants or caulks. I, too, really like the performance of polyurethane liquid sealant, but quite often the price premium means a cheaper, less effective sealant gets used. They just don’t have the sticking power of polyurethane or silicone (I checked at our local hardware store and polyurethane sealant was about 2.5 times more expensive than a builder-grade latex sealant). For more information on choosing liquid sealants read this.
-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.