Don’t forget about the drying potential of the wallAs the discussion has evolved, Colleen says the nod is now going to a 2×6 wall, but she still has concerns about the plane where the OSB sheathing and the exterior foam meet.“As I understood, there needs to be some kind of air space so, in case of water, it could be directed away — and that it just needs to breathe,” she says. “That’s why I thought of [Tyvek] DrainWrap in that location.”A gap between the sheathing and exterior foam is only a concern when the builder uses cavity insulation that does not allow drying toward the interior, Holladay says. While exterior rigid foam is an excellent choice, make sure to use vapor-permeable insulation between the studs.“I like dense-packed cellulose between the studs, but other materials — blown-in fiberglass, mineral wool batts, or even fiberglass batts — can work if they are installed conscientiously,” Holladay says. “This approach also requires that there be no interior polyethylene. Once you follow this advice, you don’t need any crinkly housewrap between the OSB and the rigid foam. The OSB will stay remarkably dry all winter long — dryer than if the wall had no rigid foam.“By the way,” he adds, “a wall doesn’t need to breathe. It just needs to be designed well.” Exterior insulation is the right ideaColleen is on the right track with exterior insulation, writes GBA senior editor Martin Holladay, even if some of the details in her wall assembly are less than ideal.“Installing a layer of rigid foam on the exterior side of a 2×6 wall is a good idea,” he says. “This will increase the R-value of the wall, reduce thermal bridging, and reduce air leakage.”But, he adds, installing spray polyurethane foam in the stud cavities is not a great idea.“The spray foam is expensive, not particularly green (because most types of closed-cell spray foam are manufactured with a blowing agent that has a high global warming potential), and the spray foam won’t perform much better than dense-packed cellulose.”Further, Holladay says, in Colleen’s climate zone, a 2×6 framed wall with an adequate thickness of exterior rigid foam makes a lot more sense than a 2×4 wall.Extruded polystyrene (XPS) also has a high global warming potential, says Reid Baldwin. Expanded polystyrene (EPS) and polyiso both are more environmentally friendly, Baldwin says.“In the stud cavities, cellulose, fiberglass, or open-cell foam are more environmentally friendly and cheaper,” he adds. “Although they have lower R-values per inch than closed-cell foam, the impact on whole-wall equivalent R-value is pretty small because of the thermal bridging of the studs.” Wall thickness and the prevailing view of local buildersIn some ways, the industry seems to be going backwards, Colleen says. A few friends who have built in the area recently all have gotten the same advice from their builders: build a 2×4 wall and insulate it with spray foam.“We were shocked, because when we built this house 18 years ago, 2×6 was the way to go and I’ve always thought that,” Colleen writes. “I like the interior look of a house with 2×6 better with the thicker window sills.”Baldwin, who also is building a high-performance house in Michigan, agrees that it’s difficult to find builders who are eager to adopt building techniques for better-than-code houses.“Custom builders may be willing to go beyond code if you are willing to pay the extra cost,” he says. “The bids will probably be padded a bit due to the uncertainty of doing things differently. We were fortunate to find a builder that was willing to be a partner on designing the house. He was open to doing things differently and brought a lot of knowledge to the table that I wouldn’t have gained from reading GBA. I think he learned a lot from my house and I hope he applies it to future projects.”With exterior insulation added to a 2×6 wall, however, total wall thickness starts to approach 8 inches, and Colleen wonders whether that complicates the installation of windows and doors.On that issue, Steve Vigoren says his choice of 2×6 walls with an added 1 1/2 inches of exterior foam, plus 3/4-inch strapping, has worked out just fine.“I ordered Marvin windows and just gave them the wall width which adds up to 9 3/4 inches, and they built the windows and sliding glass door jambs to match,” Vigoren says. “I ended up with a 6 1/4-inch jamb from the inside of the window to the face of the Sheetrock. I was very pleased with this approach.” Colleen’s questions are the topic for this Q&A Spotlight. Choose foam insulation carefullyIt’s not necessarily wrong to choose spray foam for cavity insulation, Dana Dorsett adds, but make sure it’s the right kind of spray foam. One key consideration is the thermal bridging — the loss of heat through the wood framing — inherent in 2×4 construction.“The thermal bridging discount is huge,” Dorsett writes. “A 2×4 wall with 3 inches (R-20) of two-pound closed-cell foam has almost exactly the same thermal performance of a 2×4 wall with 3 1/2 inches (R-13) half-pound open-cell foam, despite the higher center-cavity R, due to the thermal bridging issue. And the 3 inches of closed-cell costs more than twice as much as 3 1/2 inches of open-cell foam. Save the foam budget for the exterior.”The air-sealing qualities of 3 1/2 inches of open-cell foam are as good as or better than 3 inches of closed-cell foam, he says, while open-cell foam uses only half the polymer per R as closed-cell foam. Plus, it uses water for a blowing agent instead of a hydrofluorocarbon (HFC) called HFC245fa, a compound with much more climate-damaging potential.“XPS is blown with a mixture of climate-damaging HFCs, which give it the higher labeled R/inch. But that performance boost over EPS of similar density is temporary, and does not last the full life cycle of the house,” Dorsett adds. “In 50 years, that R-7.5 for 1 1/2 inches decays logarithmically to about R-6.3. EPS is blown with comparatively low-impact pentane, a low environmental impact gas, and it is small molecule that escapes very rapidly (most of it escapes and is recaptured before it leaves the manufacturer.) Its R-value is stable over time.”Polyiso is blown with pentane and other low-impact gases, Dorsett says, but its labeled R-6 per inch is overstated when the mean temperature is less than 40°F. In Climate Zone 5, it should be derated to R-5 or so per inch when used on the exterior of an R-13 framed wall.“Bottom line: save the high-performance foam budget for the exterior,” Dorsett says. “Installing 2 inches of polyiso on the exterior and using 3 1/2 inches of open-cell foam is the same thickness wall as a standard 2×6 wall, and it outperforms the proposed 3 inches of closed-cell foam plus 1 1/2 inches of XPS. And it’s more resilient, since it gives the structural sheathing a good drying path, and it beats code minimum by about R-5 (whole-wall performance, all thermal bridging accounted for), a ~25-30% reduction in heat transfer compared to a code-min R-13 + 5 continuous insulation wall, compared to only a 10-15% improvement with the closed-cell plus XPS solution.” Calculating the Minimum Thickness of Rigid Foam SheathingHow to Design a WallChoosing Rigid FoamWhere Does the Housewrap Go?Installing Windows In a Foam-Sheathed WallGreen Basics: Insulation Choices RELATED ARTICLES Colleen A, planning a new house in Michigan’s Climate Zone 5, has discovered there’s a downside to the wealth of insulation products on the market: It’s hard to make a decision.“There are so many options on exterior insulation that my head is spinning,” she writes in a Q&A post at GBA. So far, her research has led her to an exterior wall assembly that includes a 2×4 framed structural wall filled with 3 inches of closed-cell polyurethane foam, oriented strand board sheathing, 1 1/2 inches of extruded polystyrene rigid foam, and Tyvek DrainWrap as a water-resistant barrier.She’s considered other possibilities, including a 2×6 framed wall, Roxul mineral wool insulation, and polyisocyanurate rigid insulation. There are drawbacks to all of them.And then there’s the issue of building industry inertia, the difficulty of finding local builders as interested in the topic as she is.“We will be our own contractors for the house,” Colleen writes. “I haven’t found any local builders that specialize in energy efficiency. From anyone I’ve talked to so far not many are interested in doing above and beyond code and seem to think it’s a waste of time and money. I don’t think so.” Our expert’s opinionGBA technical director Peter Yost added this:At first blush, I wondered just how hard could it be to find a high-performance builder. Turns out that for Port Huron, not so easy. But here is the path I took, one I would take for this sort of question regardless of location:1. Check with the Energy & Environmental Building Alliance to see if there are members within striking distance of your project.2. The website for the Building Performance Institute (BPI) has a contractor locator tool that uses zip codes to help you find a builder.3. Building professionals who have taken the Two-day Advanced Green Building: Building Science course offered by the National Association of Home Builders (NAHB) are another potential source. I don’t have a specific avenue for this, except to suggest that you submit your request for this to NAHB. Or, try your local home builders’ association. Building professionals who have completed this course should have the background needed for building science-savvy design, materials selection, and construction.4. Track down a verifier for the National Green Building Standard. This program is run by the Home Innovation Research Labs (formerly the NAHB Research Center) and you can use this website to submit an email request for a local verifier who should be able to identify high-performance contractors.Having said that (and having tried these four avenues for Port Huron), I have to admit I didn’t make much progress. With BPI, I came up with only three building industry firms within 50 miles of Port Huron.And after contacting NAHB, the local HBA, and the Home Innovations Research Labs, I came up with just two potential contacts that might help in Port Huron.One is Cobblestone Homes in Saginaw, Michigan. That’s not exactly next door to Port Huron, but the owners have good contacts for the Port Huron area. The other is Chris Schwarzkopf of Energy Diagnostics. Again, not exactly in Port Huron’s backyard, but this NGBS Verifier and NAHB-Certified Green Professional may have local contacts for Colleen to pursue.Note: To be fair, I got quick staff responses from NAHB and the Home Innovations Research Lab, but had difficulty connecting locally and that could be because I was just doing research as opposed to being a real customer for building services.Now, about the insulation options. I’ll start with a shameless plug for BuildingGreen’s special report on insulation. The report is currently offered as a freebie at the end of any of my recent GBA building science blogs.I recommend that anyone considering foam-in-place insulation should read this BuildingGreen blog:“Foam-in-Place Insulation: 7 Tips for Getting Injection and Spray Foam Right.”A high-performance wall for Climate Zone 5/6 (Port Huron is pretty much right on the border between these two climate zones): Per the 2015 Model Energy Code (and Martin’s article about calculating the minimum thickness of rigid foam sheathing), Colleen should think of the this wall as a starting place: 2×6 cavity insulation and R-11.25 exterior rigid foam.