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Musings of an Energy Nerd

Nailing Window Flanges Through Foam

Builders have been doing it for years, but some window manufacturers aren’t sure it’s a good idea

A technical manual published by the U.S. Department of Energy shows details for installing a flanged window in a foam-sheathed wall.
Image Credit: Building American Program

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.”


  1. Nicholaus Baxter | | #1

    Here in Washington State
    Here in Washington State new energy codes require exterior foam (prescriptive requirements) on all buildings in Eastern Washington and all non-residential buildings in Western Washington. With that kind of force promoting energy efficient buildings and the use of continuous exterior insulation as a prescriptive method, window manufacturers should no longer have an excuse to ignore foam installations now that many buildings in Washington State require this type of installation or similar.

    A year ago I called up Milgard Windows to get some updated waterproofing details from them and they told me they no longer provide those details because they do not want to be liable for water intrusion issues. If manufacturers can simply ignore water intrusion details, I guess window manufacturers will continue to ignore alternate installations as well and try and pawn off the liability on someone else.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to J Chesnut
    Thosten Chlupp is a great believer in innie windows -- he asserts that the better thermal performance comes from protecting them from the wind -- pulling them closer to the interior keeps their frames warmer. Of course this must be balanced against possible shading; larger windows are less affected by shading than smaller windows.

    I think that Joe believes that medieval windows were inset to protect the glass. If the peasants are throwing pitchforks at your cathedral or castle, the glass in an innie window will not be as vulnerable to pitchforks as an outie window. Of course, the more popular the Pope, the less he has to fear pitchforks.

  3. J Chesnut | | #3

    Minnesota unhelpful
    Marvin and Anderson sell too many windows to need to be helpful at this point. That is why I advocate for the superior imports.

    When did flanges come about? As I'm working on my 1907 abode its fun to compare the old sashes that were simply pinced between 1/2" thick trim with current day standards - and no header to boot. Frames weren't even fastened to the studs, held in place by the lathe. My 1907 home is advanced stick framing before the term was coined ; ).

    I don't quite follow Lstiburek's reasoning for preferring innie windows.

    - “For optimal thermal performance, innie windows rock."
    The passivhaus literature I think is convincing that the glazing should be set towards the middle of the thermal gradient. Also sometimes innie windows compromise solar heat gains when wanted by shading the opening.

    - "Glass windows were expensive — only the Pope and rich people could afford them — and it was important to protect the glass."
    Protect the glass from what? Water or reformationists? I assume Joe is talking about the old wood sashes here (which in cold climates suffer more from interior condensation than from rain). Pazen's windows are interesting because the glass covers even the sash on the exterior. Glass itself is very durable for exterior use.

  4. Douglas Horgan | | #4

    Foam is plenty strong enough
    A lot of carpenters seem to be very worried that foam isn't strong enough to hold the window flange. I've installed Andersen and Pella flanged units over foam, the wiggly soft vinyl Andersen will suck in a bit if you overdrive the fastener but they still work fine...the Pella aluminum flange is even easier since it's somewhat rigid to begin with. And the windows feel and act the same as when nailed to wood sheathing.

  5. User avater
    James Morgan | | #5

    Conservative engineers
    “Structural engineers are conservative, because when they make a mistake, people die,” he said. “That’s why they learn to be conservative.”
    I agree that structural engineers, with rare exceptions like the Ove Arup practice, are extremely conservative, but it's not because theor mistakes cause fatalities. Deaths caused by structural engineering failures are so rare as to be almost non-existent compared to those caused by medical mistakes. Structural engineers are conservative because relatively minor failures (think cracks, not collapses) can lead to very expensive lawsuits and huge increases in their insurance premiums. Even spectacular failures like the Tacoma Narrows Bridge (1940) resulted in no human death (a small dog lost its life). The Hyatt Regency atrium bridge collapse (Kansans City 1980, 115 deaths) is the notorious exception that proves the rule. The Hyatt failure, caused by a conceptual flaw in the suspension system, attracted a $3 billion settlement.

  6. Philipp Gross | | #6

    That is how they do it in Germany
    you must have worked with a rough crew in Germany. Using the spray foam is not allowed nor common practice there. I am German and worked as a window installer and later planning a lot of window details following the "DIN NORM" which says that every window has to be secured on each corner and less than 2' 6" o.c..... . As far as the practical approach on site often goes: If the window is so small that your drill does not fit, the spray foam will do it, otherwise you secure it at leased with four screws and if it looks flimsy add a couple more :-).

    Innies vs. outies
    I think the main reason that it is either innie or outie here in the States, is because the windows are what they are and few people want to change that. I agree with J about the superior imports mounted in the thermal center of the wall (or at least close). It is really nice to keep the frames warm but rather than protecting them against the wind, we should cover as much of the frames with insulation as possible.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Philipp Gross
    Thanks for your perspective. I certainly appreciate knowing more about German methods of installing windows.

    The crew I was working with was installing windows and entry doors into walls made of a new prototype material, styrofoam concrete. The walls were made of precast panels of concrete mixed with styrofoam beads.

    In many ways, the construction method was experimental, although the experienced German builders I was working claimed that their methods have been approved by engineers. The entry doors and windows were foamed in place.

  8. User avater GBA Editor
    Mike Guertin | | #8

    Reengineered windows
    Building material manufacturers seem to lag behind better building practices unlike the technology sector that sets and leads new trends. Heck, even the construction tool makers are ahead of users at times; 10 years ago who thought we needed impact drivers, trade-specific gloves, upright orientation tool bags and so on. It's too bad window makers haven't stepped too far forward. It would be nice to see window designs that address the installation challenges associated with rigid foam covered buildings.

    I've heard through a reputable source that a couple of window manufacturers are modeling windows with deeper flange to frame face distance. Most clad or vinyl windows have a 1 in. to 1 1/8 in. distance that works fine for planing out with siding applied directly to the wall but leaves siding proud of the window when we install siding over a rainscreen space or foam (when the window is installed at the framing plane).
    The distances the window companies are looking into are in the 2 in. to 2 1/2 in. - essentially enough to accommodate about 1 in. of rigid foam and possibly a rainscreen space. I guess those companies are recognizing the trend in energy codes towards exterior layers of rigid foam insulation. Still, going to a 2 in. or 2 1/2 in. frame to flange distance won't address the array of possible installation details that innie / outie, various foam thicknesses and the combinations present.

  9. Gregory La Vardera | | #9

    the problem is the windows, not the exterior insulation
    FWIW, working with Inline Fiberglass windows which are configured more like an extruded aluminum commercial window, and less like a clad wood residential window, we found that Inline has exterior jamb and sill extensions that were deep enough for 2" of exterior insulation and allowed the window to be mounted with the nail flange against the sheathing.

    I think this is a much better solution than hanging the window away from the framing whether backed by plywood or just foam, particularly if you are using something like Zip sheathing which relies on its tape to lap over the nailing flange to establish a reliable water shedding assembly.

    But of course most commodity residential windows are not configured this way. It just points to the need for building component manufacturers to update their products to be able to work easier with high performance walls. Every commodity residential window now needs to accommodate a variable exterior stand-off from the sheathing to work with different thickness of exterior insulation. Andersen, Weathersheild, Pella, the lot of them, all need to revisit their product so we don't have to come up with jury rigged ad-hoc solutions such as outlined in articles like this one. The products need to adapt - not us. This is a product design problem, not a construction problem.

  10. Tyler Megehe | | #10

    New construction windows over foam
    I have been planning a energy retrofit of my home in Austin, Texas and here is how I plan on installing my new windows. I am adding 1 1/2 inches of foam with a radiant barrier over my existing T-11 siding. After flashing the window area I am installing 1 x 4 furring strips on top of the foam. 1 x 6 PT is installed around my window openings and will be flush with the furring strips. New construction windows will be attached with the flanges to the 1 x 6 PT. In addition they will be caulked and flashed with butyl tape. After installing the windows, I will install 1 x 4 hardi trim. This will leave a 1 1/2 to 2 reveal on the 1 x 6 PT to attach the hardi lap siding to the 1 x 6 PT.

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