Over 30 years ago, when builders first began installing rigid foam wall sheathing, they had to figure out their own methods of fastening flanged windows. In 1982, when I sheathed my house with 1-inch-thick EPS, I installed “picture frames” of 1-inch lumber around each window rough opening. As it turned out, Joe Lstiburek was also building a foam-sheathed house in 1982, but he used a different approach.
“Back then, we weren’t using OSB or plywood sheathing — just XPS foam sheathing, usually Dow blue board, with diagonal metal strapping for bracing,” Lstiburek told me recently. “At first, we would run a horizontal 1-inch thick board under the bottom flange of the window, to help support the weight of the window. Under the other three flanges, there was nothing but foam. We attached the windows to the studs with screws through the foam. We built thousands of houses this way, using foam up to 1½ inch thick. We never experienced any problems.”
Eventually, Lstiburek realized he didn’t need to install any wood under the lower flange. “In the late 1980s, I took that practice with me to the U.S.,” said Lstiburek. “Then we stopped installing the horizontal board underneath — except for wide windows. If the window was wider than 4 feet, we’d still install a board under the bottom flange. But most of the windows we installed were basically hung from the flange fasteners.”
What about thicker foam?
I asked Lstiburek how he attaches flanged windows on walls with very thick foam. “I don’t have enough of a track record with 2-inch foam, so when we go above 1½-inch-thick foam, I recommend using side straps,” Lstiburek told me. (Side straps are also called masonry clips.) “You attach the windows at the sides of the rough opening with the straps. The side straps work with up to 4 inches of foam. If the foam is 2 inches or more thick, I like to line the window opening with projecting plywood — ½-inch plywood is fine. If you use a plywood buck, you still need to attach the windows with masonry clips.”
I asked him whether a plywood buck can support the weight of a window when the window is hanging entirely beyond the wall framing. In response, Lstiburek pointed out that plywood bucks are stronger than most builders realize. “Let’s say you have a wall with 4 or 6 or even 8 inches of exterior foam, and the window rough opening is lined with ½-inch plywood,” said Lstiburek. “The vertical plywood at the jambs is acting like a beam. If we strap the bottom piece of plywood around the corners and up the sides — if we extend metal strapping up a few inches at the corners, and fasten the strapping to the vertical plywood — we’ve made an unbelievably strong box. It may not be very strong in the middle, especially if the window is wide, but it’s unbelievably strong at the corners. As long as the window isn’t any wider than 3 feet, that’s all you need. Wider windows get a 2-by underneath the plywood at the sill, to give the center of the window some additional support.”
The Lsiburek method of fastening window flanges through rigid foam is illustrated in “Installing Windows with Foam Sheathing on a Wood-Frame Wall,” a technical manual published by the Building America program.
What about retrofit jobs?
For a deep-energy retrofit job that includes new windows and new exterior foam sheathing, it’s worth considering the use of “Dudley boxes.” For more on the Dudley box approach, see Window Installation Tips for a Deep Energy Retrofit.
Window manufacturers didn’t used to care
In the 1980s, builders who were sheathing with rigid foam were off the radar screens of most window manufacturers. “Back then, the window industry didn’t care whether we were fastening window flanges through foam,” said Lstiburek. “The only people using foam were the hippies and the engineers, and we were basically ignored. Now things are happening on a larger scale, and all of a sudden some of the window manufacturers are making things ridiculously complicated.”
According to Lstiburek, a representative from Pella Windows recently told him that the flanges of Pella windows cannot be fastened through foam unless the window is also supported by screws through the window jambs or masonry clips.
What are the window manufacturers recommending?
Lstiburek’s version of Pella’s requirements was confirmed by Cordell Burton, an installation engineer at Pella Windows. “You can’t screw through foam sheathing — the foam will compress,” Burton told me. “You have to have something solid to attach the window to. That can be done with jamb anchoring — screwing through the window jambs with finish screws — or by using masonry straps. If you attach the window by anchoring the jambs with screws or masonry straps, then the window flange is just a flashing component. You’re not attaching the window through the flange anymore. If the foam is so thick that the window ends up beyond the framing, your best approach is to install a window buck to support the weight of the window.”
Although Lstiburek is willing to comply with Pella’s requirements, he believes that the rule against fastening window flanges through foam makes little sense — at least when the foam is 1½ in. thick or less. Lstiburek would like to see window manufacturers develop common-sense, easy-to-implement recommendations for installing windows on foam-sheathed walls.
Other window manufacturers are still scratching their heads
Andersen Windows has no advice to builders who install their windows in foam-sheathed walls. “Your question about how to install a window when using Styrofoam on the exterior surface of the structure is not covered in the instructions we provide with our products,” wrote Stacy Einck-Eckberg, an Andersen Windows representative, in an e-mailed response to my question. “We are aware that this question is being asked but do not yet have an installation recommendation that ensures the long-term stability and performance of the window once installed.”
Marvin Windows was also unhelpful. “I had a chance to visit with some folks here at Marvin and I can tell you that rigid foam window installation is not part of our current recommended installation methods,” wrote John Kirchner, a Marvin Windows representative. “But we are aware that it is occurring in the field and will continue to explore that installation method.”
When it comes to fastening window flanges through foam, Weather Shield Windows also has no recommendations. Emory Budzinski, a senior product manager at Weather Shield, told me, “From Weather Shield’s perspective, we don’t care if you fasten the window flanges through foam.” However, Budzinski’s indifference was balanced by a healthy dose of common sense. “I’ve been in the window industry for 20 years, and I’ve yet to see a window that has failed for structural reasons,” he noted.
All you need is spray foam
The structural concern raised by Pella Windows reps is somewhat ironic, especially in light of the fact that the gap between window frames and window rough openings is usually filled with spray foam. Twenty years ago, I worked for six months on a German building crew. I was surprised to learn that when German workers install windows and entry doors, they hold them in their rough openings with temporary shims, and then secure the windows and doors permanently with one-component spray foam between the frames and the rough openings.
“Don’t you need to fasten the jambs to the wall?” I asked. “No,” they answered. “The foam is all you need to hold a window or door. That’s how we do it in Germany.” In other words, spray foam is structural.
When I told Lstiburek about the German method of installing windows, he agreed that it is true that spray foam helps hold a window in place, but also noted that engineers usually ignore the benefits of spray foam. “Structural engineers are conservative, because when they make a mistake, people die,” he said. “That’s why they learn to be conservative.”
Innies versus outies
Of course, questions about fastening window flanges through foam only apply to “outie” windows. (If you are unfamiliar with the “innie” versus “outie” distinction, see ‘Innie’ Windows or ‘Outie’ Windows?) If you aren’t comfortable fastening window flanges through foam, you can always install your windows as “innies.”
“I like innie windows,” said Lstiburek. “For optimal thermal performance, innie windows rock. If you look at old buildings, they always had innie windows. Glass windows were expensive — only the Pope and rich people could afford them — and it was important to protect the glass. But the water-management details are much more complicated for an innie window than an outie window. Some people like the look of outie windows. It’s not really a technical challenge either way, it’s just that the outie is easier to water-manage. I usually just ask the architect what he or she likes — sometimes they like outies, and sometimes they like innies.”
What do the window installation standards say?
During the 1990s, the U.S. window industry was called to respond to an epidemic of construction-defect lawsuits alleging that faulty windows and sloppy window installation methods were contributing to water entering walls. In response to this crisis, two standards were developed: ASTM E2112 (“Standard Practice for Installation of Exterior Windows, Doors and Skylights”) and AAMA 2400-02 (“Standard Practice for Installation of Windows with a Mounting Flange in Stud Frame Construction”).
These documents are long — my copy of ASTM E2112 is 89 pages long — and detailed. Yet neither document has any information at all about how to install a flanged window in a wall with foam sheathing. “What’s flabbergasting to me,” said Lstiburek, “is that the two groups that should be all over this are the people who sell and make windows and the people who sell and make insulating sheathing. But they have been absent from the conversation.”
Last week’s blog: “Building an Unvented Crawl Space.”