Michael Sterner is ready to build a new house in northern Wisconsin: a simple, two-story box of about 2,000 square feet. After studying many possible wall assemblies, he has settled on the double-stud wall as proposed by the Building Science Corporation.
The assembly consists of an inner 2×4 structural wall sheathed (on the outside) with 1/2-inch plywood and an outer 2×3 stud wall. Between the two there is a 4 1/2-inch space. Both stud cavities and the space between the two walls are insulated with cellulose.
“We chose this wall for its high R-value to cost ratio,” Sterner writes in a recent Q&A post. “It seems to be the least expensive option for a high R-value wall. It is easy to build, finish on the exterior and seems very durable.”
But that’s not quite the end of it.
“As I’ve been receiving insulation quotes, some challenges have been brought up with the execution of this wall system,” he continues, “mainly that in order to dense pack properly we’ll have to divide up the bays so they can be dense-packed two stud bays at a time.”
Further, with all that cellulose going into the wall, there will have to be some way for the air to escape. Sterner sees two options:
- Install netting on the outside of the wall, insulate the wall, then add the fiberboard and water-resistive barrier.
- Finish the wall and use a hole saw to bore holes for both the insulation and the escaping air. Later, patch the holes.
“Does anyone have thoughts or experience on executing this wall system and best practice?” Sterner asks. “Does anyone want to try to talk me out of this wall system toward a different high R-value wall assembly?”
That’s where we start this Q&A Spotlight.
Another option to consider
An unnamed GBA reader — we’ll call him User 182 — suggests that Sterner consider another kind of wall assembly, one that doesn’t use dense-packed cellulose at all.
“I realize that blown-in cellulose in walls is on everybody’s pop favorite list today,” User 182 writes, “but there is a far more rational way to construct a double-stud wall.”
Here’s his plan: Frame the outer wall with 2x6s and insulate it with batts. On the inside face of the wall, attach 2 inches or more of polyisocyanurate foam, taping the seams. Then build an inner wall of 2×4, which becomes a service cavity.
If Sterner were to use 3 inches of polyiso insulation between the two walls, he’d have a whole-wall R-value of about 36, User 182 says, and a total wall thickness of about 12 inches.
The inner foil surface of the polyiso remains above the dew point for any temperature difference between inside and outside up to 88ºF. If Sterner wants higher R-values, he could increase the thickness of the polyiso, or fill the service cavity with batt or blown-in insulation.
User 182 cautions against using a 2×6 interior wall and more fibrous insulation because it would lower the temperature at the interior foil surface of the polyiso — potentially below the dew point. That would raise the possibility of mid-wall moisture condensation.
“Why fumble around with fragile vapor retarders, exterior foams vulnerable to insects and/or fire, awkward and labor-intensive siding installation over foam or, worse, insulation prone to settling over the year?” User 182 asks.
Does this assembly really work?
Both Malcolm Taylor and Zephyr7 challenge User 182 for more details, claiming that he has posted the same suggestion in the past. “If you want to promote this wall assembly,” Zephyr7 says, “why not write a real article about it with some pictures?”
User 182 replies that he has designed and built three double-stud wall houses and lived in them for a total of 26 years. He’s tried a variety of assemblies, but came across the one he suggests here late in the game.
“Unfortunately, I didn’t think this up until nearly finished with the last one,” he says, “or I would have built them all this way. If I still had pictures or plans I’d share them with you but it’s been more than 15 years since we sold the last one and retired to sunnier climes.”
One problem with cellulose, User 182 says, is that it can settle over time, a problem discussed in a video describing a net-zero home built in Alaska by Thorsten Chlupp.
There are some things that Taylor likes about User 182’s proposed wall, including ease of construction, its use of conventional framing, and the built-in protection for the air/vapor control layer. He also points to some potential problems:
- It doesn’t address the weakness of insulating and air sealing at the floor levels.
- It uses a lot of rigid foam.
- The foam doesn’t seem to offer any advantage over a mid-wall air and vapor barrier made of plywood or a membrane.
Try batts instead
Brendan Albano suggests that Sterner consider using two 2×4 walls separated by a 3½-inch gap. Instead of using cellulose insulation, Sterner might cut labor costs by using three layers of fiberglass batt insulation. In the center of the wall, the batts could be installed horizontally.
Divide the wall into sections with netting
Akos tells Sterner that the easiest solution to the cellulose installation problem is to subdivide the wall into sections with netting. “This way the wall can be packed in one shot,” he says. “Similar to how it is done with the Larsen trusses.”
In that case, netting installed on the trusses prevents the cellulose from migrating sideways as it’s blown into the wall.
Akos says it wouldn’t add much to labor costs since Sterner will already be installing netting on the interior of the wall.
The inner layer of plywood on the Building Science Corporation wall makes the assembly a little more complicated, Akos adds. “Maybe it is a decision you need to make whether this detail is worth the cost,” he adds. “There are a lot of simple double-stud walls out there.”
Other thoughts on the BSC wall
The Building Science Corporation wall has at least two things going for it, writes Rick Evans. First, it keeps the air barrier in the middle of the wall where it belongs; and second, it uses less lumber than a Larsen Truss wall.
However, he adds, there are simpler ways of getting there. He refers Sterner to articles describing two high-performance houses insulated with cellulose. One of them, a Passivhaus home in Rochester, New York, was insulated from the outside through the water-resistive barrier. The other is a home built by Ecocor’s Chris Corson, who insulated from the inside by cutting holes in the OSB air barrier, then patching the holes when he was finished.
“For my own double-stud wall house, our insulation crew had never dense-packed such a thick wall,” Evans adds. “They figured it out, however, despite no dividers between studs. I opted for dense-packed fiberglass because I feel it is less likely to settle if they didn’t get the density quite right.”
Our expert weighs in
GBA technical director Peter Yost wraps up the discussion with these thoughts:
“I’ve done a similar wall a few times and from a building science perspective, it’s a great wall assembly,” Steve said. “From a practical perspective, there’s a little pushback. There’s the challenge of getting the cellulose into each of the two cavities (and maintain the air barrier) and keeping the wall cavity dry thru becoming weathertight. Most builders I work with simply hate those 2x3s. We have enough trouble with the quality of 2x4s, much less 2x3s. And even if the quality is good, they just can’t take two end nails without splitting. If I were to stay with this wall, I’d keep them both 2×4 walls.”
OK, so what does Steve suggest?
“My go-to wall assembly looks like this. [See the detail below.] I call it the ‘two-by slider.’
I think of the four parts as the two-by frame, continuous exterior insulation, the air-impermeable cavity insulation, and the air-permeable cavity insulation. Each one of these can ‘slide’ based on climate and what level of performance I want to achieve.
“I might do 2×4, 1-inch rigid, and full-cavity fill with cellulose in a mild or hot climate and slide all the way up to 2×8, 2-inch rigid, 2-inch closed-cell flash and 5 ¼-inch chopped mineral wool in a cold climate (in each case, making sure that the first condensing surface is warm enough for the climate to avoid interstitial condensation). The buildability is straightforward and shared across all variations of the ‘slide’ and the cost of moving even to the 2×8 is really modest, especially going 24-inch on-center with the 2×6 and 2×8. There’s nothing out of the ordinary knowledge frame here.
“It’s not that I don’t use other wall assemblies, including versions of the double-stud. It’s just that this is my go-to system because so many details of installation, sequence, and materials stay the same, and most importantly are familiar.”