Zane Bridgers is building a single-story house in northern New Mexico and has nearly completed the framed exterior walls. As his mind turns to air sealing, he’s considering whether to install an interior vapor barrier — and whether his uncle, a builder of 40 years, is giving him good advice on how to proceed.
Walls on the slab-on-grade structure will include R-19 fiberglass cavity insulation, 5/8-inch OSB sheathing, a 2-inch layer of polyiso rigid insulation, a drainage layer and, finally, three-coat stucco. In the roof, Bridgers plans 6-mil poly under the drywall, followed by 2 inches of polyiso, R-38 fiberglass batts, a 2-inch ventilation channel, OSB sheathing and metal roofing.
“This is a predominately cold and dry heating climate with big temperature swings,” Bridgers writes in a Q&A post of the Climate Zone 5 locale. “I was talking with my uncle who has been a builder here for 40 years. He was explaining the importance of dry heat for optimal performance of fiberglass insulation, hence his recommendation to put the 6 mil plastic on the ceiling … I was planning to tape the foil faced polyiso for this effect, but he thinks it’s a waste of time and effort vs. the 6 mil poly.”
Bridgers is equally concerned about another bit of advice he’s been given: Add a layer of poly to the walls.
“This caused a bit more concern as it seems it could potentially trap moisture in the wall cavity, especially since I had planned foil-faced polyiso under the stucco, also taped,” Bridgers adds. “He suggested leaving somewhere for the moisture to go.”
Bridgers has two other questions. First, is it a waste of time to seal the OSB and framing when the exterior foam and drywall seem like much easier ways of controlling air leaks? And second, with a continuous layer of rigid foam over the wall sheathing, where is the point in the assembly where condensation is likely to occur?
Those concerns will get us started on this Q&A Spotlight.
Water doesn’t need to escape
GBA editor Martin Holladay, referring Bridgers to an article he had written on the topic previously, notes that interior moisture doesn’t really need to go anywhere.
“Water doesn’t need to escape from your house,” Holladay writes. “Although it’s true that indoor air is warm and humid during the winter, while outdoor air is cold and dry, that doesn’t mean that indoor moisture needs to ‘escape’ from your house. It’s perfectly OK if the indoor moisture stays where it is without ‘escaping.’ ”
Holladay explains that walls with exterior rigid foam should never have interior polyethylene, since foam-sheathed walls need to be able to dry to the interior. Either polyiso or expanded polystyrene (EPS) insulation would make a good choice, and taping the seams would be time well spent.
“Air sealing efforts are almost never a waste of time,” he says. “Whether or not you need redundancy (basically, multiple air barriers) depends on your airtightness target and your budget.”
What about Bridgers’ concerns about condensation inside the walls?
“The idea is to specify enough rigid foam on the exterior of your wall sheathing so that condensation does not occur,” Holladay says. “In your climate zone, your rigid foam needs a minimum R-value of R-7.5 if your walls are framed with 2x6s.
“Skip the interior polyethylene,” he adds, “pay attention to airtightness, and everything will be fine.”
What about venting the roof?
If the walls are better off without the poly, should it still be used in the ceiling? Bridgers asks.
There’s no code requirement for including a vapor barrier on the interior side of a vented roof assembly, Holladay replies. The most important thing is airtightness, not preventing vapor diffusion. Drywall can be an air barrier, providing Bridgers pays close attention to sealing any penetrations, particularly the electrical boxes.
Bridgers sees a problem with adding foam or plastic ventilation baffles above the fiberglass in the roof because either would prevent moisture from being wicked out of the insulation.
“The purpose of the vent channel is to help keep the roof sheathing dry,” Holladay says. “You are not trying to wick moisture out of the fiberglass … You aren’t trying to help indoor moisture escape. It’s OK if indoor moisture stays where it is, all winter long.”
One air barrier is enough
Peter Engle writes that Bridgers has three potential air barriers in the ceiling: the foil facing on the foam insulation, the poly, and the drywall.
“You only need one, well detailed and airtight,” Engle says. “If you can make the drywall airtight, you can skip the poly and use any polyiso foam you want, or EPS foam. If you are worried about the drywall being airtight, you can tape the seams in the foil polyiso.”
Jon R, however, suggests there’s nothing wrong with multiple air barriers.
“More air barriers will generally outperform one,” he says. “If you can only have one, the best (for your climate) is the interior side.”
Addressing the risk of condensation
Jon R adds that the choice of exterior foam may make a difference to how well the wall performs: “With external foam, a wall that can dry a little to the exterior (say EPS) will outperform a similar wall that can’t (say same R-value of foil-faced foam).”
He takes issue with the idea that a wall must be designed so that there is a zero chance of condensation taking place.
“The idea is to reduce the amount of condensation to the point where it sometimes occurs but isn’t enough to cause a problem,” he writes. “Going beyond that, all the way to ‘no condensation’ is unnecessary expense. Some in-wall condensation *will occur* at the minimum recommended foam R values.”
Holladay replies: “When sheathing is cold in the winter, and in contact with warm humid air, what happens isn’t really condensation. It’s sorption. The moisture content of the cold OSB or plywood sheathing increases when the warm humid air is in contact with it.”
That said, the building science in this case is clear. When the exterior foam is thick enough, the sheathing stays dry. If the foam is too thin, the siding may get damp, which is risky.
With that in mind, Bridgers asks, would a 2-inch layer of polyiso with an R-value of 13 be a better bet than a 2-inch layer of EPS, with an R-value of 8?
“That’s exactly the right type of question,” Jon R replies. “WUFI might provide a useful answer. Forced to weigh various factors and guess in your case (never a good way to do design), I’d say the R-13. Better than either if it were unfaced/higher perms.”
Agreed, says Holladay: R-13 is preferable to R-8.
Our expert’s opinion
GBA technical director Peter Yost made the following points:
Cavity or interstitial condensation: There are two primary drivers of this phenomenon: the difference in temperature between the interior and exterior, and the interior relative humidity. Yes, it’s plenty dry in New Mexico during the winter, but occupants can generate quite a bit of moisture (for more, see this). So, make sure you manage interior sources of moisture and have humidity-sensing devices so that occupants know what the interior relative humidity is.
Location and nature of air control layer/barrier: If I only get to choose one location for an air barrier, I choose the exterior, for two reasons. One, it’s much easier to get continuity on the exterior (no intersecting interior partitions or floor assemblies to worry about). Two, exterior air barriers deal better with wind-washing at the corners of buildings. Continuity is key. It’s easy to designate elements of the air control layer, but more difficult to get them all connected. Pick one plane for the air barrier, and then make it continuous. If you can get more than one air barrier, great. But one continuous barrier is way more beneficial than two or more discontinuous ones.
Building assembly drying potential: It’s ideal to select every individual layer of an assembly (based on vapor permeance) so that there is drying in both directions, but our assemblies are complex enough these days that settling for drying potential in one direction is reasonable. Avoid selecting Class I — and if you can, Class II — vapor retarding materials on one side of your assembly or the other to get that single-direction drying potential.
Bottom line: There’s absolutely no need for polyethylene. Don’t put in a Class I vapor retarder/barrier unless you have to.