Michael Roland is designing a new house and trying to choose the right wall assembly. It’s down to a choice between a double-stud wall filled with fluffy insulation, or a single wall wrapped in a layer of rigid foam insulation.
“Using exterior rigid foam solves thermal bridging and prevents condensation within the batts in the wall cavity,” he writes in a Q&A post at Green Building Advisor. “Double-wall construction also solves thermal bridging, but what about the dew point within the batts? Won’t there be a condensation problem?”
That’s the topic for this Q&A Spotlight.
Moisture won’t condense in batts
Don’t worry about condensation on batts, writes Dana Dorsett, because it simply won’t occur.
“Condensation doesn’t happen in batts,” he says. “Because batts are extremely vapor-permeable, and low-mid density batts are so air-permeable, whenever the coldest surface of the cavity reaches the dew point of the entrained air in the cavity the moisture condenses on that surface, not in the fiber itself.”
While the condensing surface is picking up moisture, Dorsett adds, “at any other point within the batts the fiber temperature is above that temperature, and no condensation occurs.”
“If [the condensing surface] is a hygroscopic material such as OSB sheathing, [the moisture] doesn’t condense either, but instead adsorbs into the material, never achieving a true liquid state (unless there is so much moisture entering the cavity from air leaks that the OSB saturates),” he writes.
The real risk is that sheathing made from oriented strand board will rot. This problem is known as the “cold OSB” problem, according the GBA senior editor Martin Holladay. When OSB gets wet but never has the opportunity to dry out, it can, in Holladay’s words, “turn to oatmeal.”
According to Jesse Thompson, an architect in Portland, Maine, “A double-stud wall with any type of batt insulation is a high-risk wall system in a cold climate, due to the cold sheathing issue.”
Lots of houses built that way in the 1980s and 1990s are still standing, Thompson says, but that kind of wall assembly is “too risky for us to recommend.”
Reducing the risk of moisture damage
There are, however, ways of reducing the risk of moisture accumulation and decay in sheathing, as several GBA readers point out.
Dorsett says that one approach is to use a hygroscopic insulation — that is, a type of insulation that can absorb moisture — such as cotton batting or cellulose. “The insulation in contact with the cold surface will adsorb moisture too, which results in lower moisture accumulation in the sheathing,” Dorsett writes. “Cellulose can take on quite a bit of moisture before saturating and losing R-value, and can store then re-release the moisture as conditions change.”
This characteristic makes it a good choice in thick wall assemblies with high R-values, he says. Dense-packed cellulose also limits air permeability and increases the “moisture buffering capacity.”
Fiberglass batt insulation, on the other hand, is not hygroscopic.
Thompson seconds the advice. With double-stud wall construction, he suggests careful air sealing and blower-door results of less than 1.5 air changes per hour at 50 pascals of pressure (described as 1.5 ach50, a standard testing metric).
“A well-air-sealed double-stud wall (better than 1.5 ach50) with dense-packed blown-in cellulose (not loose fill) or perhaps blown-in fiberglass seems to be a low-risk wall system,” Thompson says. “The worse you air seal, the more the risk goes up with this wall.”
A layer of rigid foam insulation also lowers the risk of moisture damage, even with “mediocre air sealing,” because it keeps the sheathing warm. “That’s why Building Science [Corp.] is such a fan of exterior foam,” he adds. “You can be a pretty sloppy builder and still not get building failures with this system.”
Avoid OSB sheathing in the first place
Another strategy is to use plywood or diagonal board sheathing rather than OSB, along with a ventilated rainscreen gap between the siding and the sheathing, Holladay says.
“+1 on the plank sheathing and rain-screened siding recommendations!” Dorsett says. “OSB is mold-food, and needs to stay much drier than solid wood to avoid damage. Moisture gets into end grain much easier and wicks further than the side-grain of planking, and the processing heat that uses the wood’s own lignin as the ‘glue’ converts a good fraction of that lignin into simpler wood-sugars more readily fed upon by fungus and mold.”
Or, as Lucas Durand points out, in areas where building codes permit, it’s possible to build a wall with structural sheathing on the inside and only a weather-resistive barrier (WRB) such as housewrap on the outside. Durand says the plywood sheathing also is the air barrier and the vapor retarder. As for the details, he offers a link to photographs of his own house, currently under construction in Ontario.
And one more approach, as Patrick Walshe suggests, is to use a double wall with exterior insulation, “though of course it would be more expensive.” His approach for a house on Vancouver Island was a 9 1/4-inch-thick staggered-stud wall with 2 inches of rigid rock wool insulation on the outside.
Our expert’s opinion
Here’s what GBA technical director Peter Yost had to say:
There are lots of really great points in this exchange, representing good understanding of the hygrothermal performance of materials and assemblies.
Some key points:
- Interstitial condensation during cold months on the first condensing surface in the assembly is always a function of interior relative humidity, as well as the temperature profile of the assembly. Let’s not forget to manage wintertime interior relative humidity as we manage the assembly profile.
- A continuous air barrier is critical no matter which assembly is selected; interstitial condensation driven by air leakage trumps vapor drive by a long shot.
- Look at the wetting and the drying. The famous Canadian building scientist, Mark Bomberg, is known for, among many things, this quote: “Our buildings get wet during the winter but don’t rot till the summer.” He was speaking in the context of adding central air conditioning to homes in cold climates; without the AC they were getting wet in the winter and drying out in the summer. Add AC, and the reduced drying sends the assemblies over the moisture management cliff.
- Some builders using double-stud walls in cold climates are adding a smart vapor retarder, such as 475 Intello or CertainTeed’s MemBrain, to manage moisture drive into the assembly during the winter yet maintain drying to the interior during the summer.
Finally, I really like the double-stud wall system, with structural sheathing either on the interior or on the exterior wall. What is a shame is to use structural 2-bys for the non-structural wall. It’s just not resource-efficient. Really light-gauge, cold-formed steel framing, or non-structural 2x3s, or even something like the Site-Engineered Environmental (SEE) stud make a lot of sense to me for the non-load bearing wall.