Writing from somewhere in the Pacific Northwest, Joe Norm is looking for advice on selecting a wall assembly that will perform well without breaking the bank.
“Building in Zone 4C Marine, and looking for a budget efficient wall assembly,” Norm writes in a Q&A post. “Climate here is not extreme, so I do not plan on having a 4-inch foam wrap or anything that colder zone people will do. But stopping thermal bridging in some fashion seems like a no-brainer.”
Norm has ruled out structural insulated panels (SIPs), and he’d like to avoid spray foam if possible. With those conditions in mind, let’s dive into this Q&A Spotlight.
What about a basic 2×6 wall?
Robert Opaluch suggests a straightforward assembly that starts with 2×6 framing and R-19 fiberglass batt insulation. Framing on 24-inch centers would be adequate structurally, but Norm may end up with 16-inch-on-center spacing if the siding he chooses requires more support than a 24-inch o.c. wall can provide.
Other elements recommended by Opaluch include plywood rather than oriented strand board (OSB) sheathing because it’s more water resistant, plus housewrap such as Tyvek or Typar.
“Especially in a rainy, humid marine climate during winter like the Pacific Northwest, a rainscreen assembly is a must to minimize water intrusion and enable better drying potential in your wall,” Opaluch says. “I’d suggest adding a layer of polyiso or EPS foam, then the rainscreen.”
Used foam is inexpensive and it would reduce thermal bridging through the framing. Norm’s rainscreen could be built with 1x material or with strips of plywood.
“Some may suggest a crinkled housewrap product, thereby skipping the rainscreen, but those products cost more than Tyvek/Typar and may not be as effective a rainscreen,” he adds. (For more information on rainscreens, see “All About Rainscreens.”)
Consider framing the wall with 2x8s
The International Residential Code gives Norm two options in order to meet the insulation requirement for his climate zone, says Michael Maines. Norm can opt for either R-20 cavity insulation, or R-13 cavity insulation plus R-5 of continuous insulation.
Standard 2×6 construction would yield an R-value of about 18.6 if studs were factored in, but would be lower when posts, headers, and other framing members are included.
“Framing with 2x8s yields R-24.5 for center of wall,” Maines says. “That’s more than 30% better than standard 2×6 construction, similar labor costs (though the wall will be heavier to lift) and almost certainly less than a 30% upcharge for materials. If I were you, I would lean toward this option.”
Maines suggests that Norm consider Huber’s Zip System sheathing over commodity-grade OSB because it provides an air barrier and a water-resistive barrier in a single step. Like Opaluch, Maines recommends a rainscreen.
“For extra insurance, you could add a variable-permeance membrane at the interior,” Maines says, “but if it strains your budget, painted drywall is enough as long as the sheathing is airtight and there is a robust rainscreen gap.”
Maines, however, is no fan of exterior foam. Even if Norm finds a source for used foam, it will complicate construction. “I don’t like any aspect of working with it,” Maine says, “or introducing foam dust into the environment (or my lungs).”
The Bonfiglioli option
Norm is assuming that a tight budget probably won’t allow the use of Rockwool Comfortboard, a rigid type of mineral wool insulation.
Probably not, replies Dana Dorsett, but the upcharge for R-23 mineral wool batts in a 2×6, 24-inch-on-center wall wouldn’t be overwhelming.
Dorsett also suggests that Norm look at a method of wall insulation developed by Massachusetts builder Stephen Bonfiglioli. He builds out the wall framing with narrow strips of rigid insulation and strapping to increase the depth of the wall cavity for more insulation and to reduce thermal bridging.
“Doing it with 2×4 16-inch o.c. and putting 1 inch polyiso and 1x strapping Bonfiglioli strips and installing R-23 rock wool (performing at R-22 at the 5.25-inch cavity depth) is comparable to 2×6 R-23 plus R-4 continuous insulation type performance,” Dorsett writes.
Cutting strips of foam insulation is quick and accurate when you use a taping knife with one side sharpened, as Chuck Miller explains in this Fine Homebuilding video.
Instead of using rock wool in the stud cavities, Norm considers the Bonfiglioli strips in conjunction with dense-packed cellulose. That would have a lower center-cavity R-value than rock wool, Dorsett says, but the thermal mass of the cellulose would give that assembly a “modest” performance boost.
Weighing various types of cavity insulation
Opaluch would prefer installing rock wool batts over fiberglass batts in his own home, but he suggests the cost could be significantly higher.
“Dana notes that rock wool is a small upcharge, which in the greater scheme of things is true,” he says. “However, the actual cost difference for this is pretty large (perhaps triple the cost — fiberglass batts are dirt cheap).”
On the plus side, rock wool has a higher R-value per inch than fiberglass, so some of the cost difference pays off in the form of reducing the cost of exterior insulation, adding cross-strapping, or framing with 2x8s, Opaluch says. Rock wool also gets into tight spots better than flimsier fiberglass batts.
Opaluch adds that dense-packed cellulose would do a better job than batts in filling in around plumbing pipes and getting into oddly shaped cavities. But he questions whether it would be a good fit with the Bonfiglioli strips.
“Installing highly packed cellulose in one cavity may blow out those strips into an adjoining empty cavity, not to mention problems fastening the cellulose netting (or drywall) to and through those foam strips,” he says. “Doesn’t seem like a combination that works easily.”
Norm will be able to install batts himself, he adds, which will be cheaper than hiring an installer with the equipment necessary for dense-packing cellulose.
“Ultimately you have to decide how much you are willing to pay for these upgrades, as well as your personal construction preferences,” Opaluch says. “Lots of ways to build a code-compliant or high performance wall.”
The time factor
Norm’s original question had to do with budget, and that can have many meanings depending on circumstances, Walter Ahlgrim says.
“If you are flipping a house and the plan is to own the house for a few weeks, the budget is the lowest cost item that code enforcement will allow,” he writes. “If you plan to own this house for the next 60 years and believe the New Green Deal is a done deal in four years funded totally by fuel taxes, 10 times your current heating costs, [a] R-60 wall begins to sound budget-friendly.”
No, it’s no flip, Norm replies. He hopes to be there for as long as 30 years.
He adds that he had been assuming he would hire out the insulation because he had ruled out batts. However, if he could install batts at a lower cost, he’s open to the idea.
Our expert’s opinion
Peter Yost, GBA’s technical director, had this to add:
I have two sources of information that might be useful for finding an overall approach to an efficient, effective, and affordable exterior wall for the Pacific Northwest:
- The Best Practices Manual from Hammer and Hand, a regional design/build firm of high-performance houses.
- The Building America Solution Center for a marine climate, including the Pacific Northwest.
Textured (wrinkly) housewraps versus ventilated rainscreens: It’s pretty amazing how much drainage you get with just a 1 millimeter space. There is an ASTM standard — E2773 — for the minimum drainage efficiency of clad wall assemblies and it requires at least 90% free drainage. All of the textured housewraps pass this test.
What about that 10%? It’s probably liquid water held in tension that could wick. What sort of gap ensures that no water gets held in tension, that all the water drains out? I have heard 3/16 inch, but I am betting it depends on the surface tension of the backside of the cladding and the front of the water-resistive barrier. In Canada’s maritime provinces, no cladding can be installed without a minimum 10-millimeter gap (3/8 inch).
But what depth of the free space ensures complete drainage as well as air movement (ventilation)? And to get real air movement, does the space need to be open at the top and bottom? I am not sure there is agreement on this depth, but again, I have heard that a minimum of 3/16 inch is needed for real air flow. One thing for sure: Textured housewraps with gaps of 1 mm or less do not provide enough air flow.
Zip System in the Pacific Northwest: Even though the closest Huber plant to the PNW is in Oxbow, Oklahoma, there is a Zip distribution network based in Seattle.
Rockwool batts: I have installed a ton of batt insulation over the years, and I cannot say enough about mineral wool batts. They are robust, easy to cut, and easy to install well. They’re worth every penny, in my book.