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Musings of an Energy Nerd

Installing Windows In a Foam-Sheathed Wall

Advice on fastening and flashing details for both innie windows and outie windows

This innie window has a durable copper sill. The photo was taken before the rigid foam layer was installed on the exterior side of the Zip sheathing.
Image Credit: Image #1: Martin Holladay

UPDATED on April 3, 2016.

Every now and then, a GBA reader posts the question, “How do you install windows in a wall with exterior rigid foam?”

The answer to the question is surprisingly complicated. The best method will depend on several factors, including the answers to these questions:

No matter which installation method you choose, you have to address two main challenges: fastening the window securely in place, and flashing the window to limit water entry.

Step one: Decide whether you want innies or outies

If you aren’t familiar with the distinction between “innie” windows and “outie” windows, you should read this article: ‘Innie’ Windows or ‘Outie’ Windows?

Each approach has advantages. Outie windows provide deep interior stools that many homeowners appreciate, and outie windows are (arguably) easier to flash and easier to trim on the exterior. On the other hand, the innie approach does a better job of protecting window sash from the weather, and innie windows perform better from an energy perspective. If you’re planning to install mineral wool insulation on the exterior side of your wall sheathing, innie windows make the most sense. (For detailed instructions on installing windows in a wall with exterior mineral wool insulation, see Roxul ComfortBoard IS Installation Guide.)

Step two: Determine what material to use as your WRB

Once you’ve decided between innies and outies, you’ll discover that this decision will influence the location of your WRB. If your building has innie windows, you probably won’t be using the rigid foam as your WRB. Instead, you should use Zip sheathing, asphalt felt, or housewrap as your WRB. If you choose asphalt felt or housewrap, it should be installed between the wall sheathing and the rigid foam.

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25 Comments

  1. Charlie Sullivan | | #1

    Can window manufacturers make it easier?
    This is a food for thought question stemming from Martin's observation that "The answer to the question is surprisingly complicated."

    Could a window manufacturer make a window with an integrated mounting/ flashing/sill/etc. system that would mount inside of the foam, but integrate with the WRB over the top of the foam? Essentially integrate the equivalent of a Dudley box or one of the other options into the window unit, either integral to the window unit or as an accessory that comes with it.

    It seems like such a system would add a little cost to the window, but save a little more cost of labor in installation, making it a minor net win on those direct costs. But the real benefit would be avoiding the time spent in figuring out how to do this on each job ... and avoiding the cost of re-doing jobs that get done wrong.

    It seems like there are window manufacturers catering to the high-performance market--aren't enough of their windows going in buildings with thick foam on the walls that they would engineer something for this?

    One hazard might be that the NFRC ratings U values, which include the frame, might get worse when they include the mounting system. But the real thermal performance could be better than the system we install now which include some amount of thermal bridging through the including the site-built mounting. It should be a system is engineered for thermal performance as well as structural and moisture control purposes, and easy foolproof installation.

  2. GBA Editor
    Martin Holladay | | #2

    Response to Charlie Sullivan
    Charlie,
    I'm sure that some clever engineers can rise to your challenge and come up with a system that will address your needs. It may not be cheap, however, and it would probably complicate window ordering.

    Remember, variations in foam thickness and siding type will require different types of mounting hardware and flashing. Stucco is different from board & batten -- and 6 inch thick foam is different from 2 inch foam.

    Designers, sharpen your pencils... see if you can make Charlie happy.

  3. Michaela Riley | | #3

    Protecting the foam on innies
    Perfect timing on this article, Martin! I have been scratching my head the last couple days trying to figure out my window flashing. :-) I have European, finless windows, and my wall setup will be Zip sheathing with 1" polyiso attached to the outside. I plan to use the innie instructions you've outlined above. For protecting the edges of the foam around the window, should I attach the peel-and-stick flashing just around the foam edge, or seal from the foam to the edge of the window unit? And then should I caulk the return trim?

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Michaela Riley
    Michaela,
    Q. "For protecting the edges of the foam around the window, should I attach the peel-and-stick flashing just around the foam edge, or seal from the foam to the edge of the window unit?"

    A. It wouldn't hurt to try to attach the peel-and-stick flashing to the edge of the window frame, as long as your exterior jamb extensions will hide the peel-and-stick. But remember, with an innie window, everything is already waterproof before the exterior jamb extensions (or the peel-and-stick under them) are installed, so you don't really have to intercept 100% of the wind-driven rain that hits the jamb extensions and sill.

    Q. "Should I caulk the return trim?"

    A. The caulk is optional.

  5. Russ Hellem | | #5

    Liquid Applied Flashing
    One thing you may consider in lieu of peel and stick flashing is utilizing a liquid applied product. All of our windows are flangeless so waterproofing the rough opening is really the only way to go and we have found it is much easier to achieve a continuous water proofing layer using liquid applied systems. The caveat to using liquid applied is there are typically temperature ranges that product can be installed in.

    Regards,
    Russ Hellem

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to Russ Hellem
    Russ,
    You are right that liquid-applied flashing systems (which are generally used in conjunction with a liquid-applied WRB) work well. For more information on this approach, see Housewrap in a Can: Liquid-Applied WRBs.

    I appreciate the reminder, and I will edit the blog to include a mention of liquid-applied flashing.

  7. Christopher Vlcek, Littlewolf Architecture | | #7

    Frame & Jamb Extensions
    I agree with Charlie that this could be an option offered by window makers. Most provide jamb extensions on the interior, and some will provide frame extensions on the exterior, usually painted aluminum to match the outside frame. A great option, but usually limited to 2". These extensions can be snap-in-place or set in receiving grooves, well-known and tested details, they just need to be deeper. Any manufacturers out there listening?

  8. Kohta Ueno | | #8

    Mass Save Deep Energy Retrofit Builder Guide
    For anyone's interest--this BSC document has a few of the "picture frame" type details (lumber outboard of foam) for window support. I know that Joe's article mostly emphasized the "innie" and "boxed opening" approaches. http://www.buildingscience.com/documents/guides-and-manuals/gm-mass-save-der-builder-guide/view

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Kohta Ueno
    Kohta,
    Thanks for the link. I've added the link to the Mass Save document at the end of my article.

  10. Michael Bluejay | | #10

    Confused about the process
    For outie windows with no plywood buck:

    (1) Step #2 is to install housewrap, but step #3 is that sill pan flashing should cover the foam, directing any water to the surface of the foam. Why are we not flashing over the housewrap, and directing water to the outside of the wrap?

    (2) Step #3 is to "flash the rough opening". Does that include the sides too? It's not clear, especially as the steps for an outie with a buck explicitly say to flash the bottom but make no mention of the sides, indicating that at least for that installation, the sides aren't flashed.

    For outie windows with a plywood buck:

    (3) Are the sides of the buck indeed not flashed?

    (4) "The housewrap should lap over the peel-and-stick flashing." What do you mean, it should "lap" over the peel-and-stick flashing? In the previous sentence, you said to install the housewrap over the foam, so the housewrap is already on top of the foam, so what further effort is needed to "lap" it? Or is the lapping directive just a redundancy?

    (5) "Under the window, the peel-and-stick flashing should lap over the housewrap." What peel-and-stick flashing? We already flashed the rough sill long ago. Do you mean that after installing the housewrap, I should put a new piece of peel-and-stick over the bottom window flanges (if any) and the housewrap?

    (6) If I read you correctly the installation order for outies is:

    No buck or frame: install housewrap, flash the opening, install the window, flash the window (sides & top).

    With buck: flash the opening, install the window, flash the sides & top of the window, install the housewrap, flash the bottom of the window over the wrap.

    (a) Is that right?

    (b) Is it bad to follow the "no buck or frame" order when installing windows with a buck? If so, what's the potential negative consequence?

  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Michael Lutkenhouse
    Michael,
    Thanks for your questions. You have pointed out some ambiguities in my instructions, and I'm grateful for you observations.

    Q. "For outie windows with no plywood buck, step #2 is to install housewrap, but step #3 is that sill pan flashing should cover the foam, directing any water to the surface of the foam. Why are we not flashing over the housewrap, and directing water to the outside of the wrap?"

    A. The instructions cover two cases: the case where rigid foam is the WRB, and the case where housewrap is the WRB. If you are using housewrap as your WRB, then you are absolutely right -- the sill pan flashing should extend to the exterior of the housewrap. I will edit the article to remove the ambiguity.

    Q. "Step #3 is to 'flash the rough opening.' Does that include the sides too?"

    A. Yes. When flashing a rough opening, it is always necessary to flash the rough jambs. The flashing on the rough jambs should always be lapped over the sill pan flashing. This is explained in the section of my article that follows the heading, "Step five: Choose your pan flashing material."

    Q. "For outie windows with a plywood buck, are the sides of the buck indeed not flashed?"

    A. Although my instructions failed to mention it, it's a good idea to flash the sides (jambs) of the window buck. I will edit the article to include that step.

    Q. "For outie windows with a plywood buck, the instructions note, 'The housewrap should lap over the peel-and-stick flashing.' What do you mean, it should 'lap' over the peel-and-stick flashing? In the previous sentence, you said to install the housewrap over the foam, so the housewrap is already on top of the foam, so what further effort is needed to 'lap' it?"

    A. I never said that there was "further effort" to this step, although you need to plan ahead at the sill. At the window head and on the sides of the window, the housewrap laps over the peel-and-stick flashing installed over the window flanges. At the sill, you need to leave some of the release paper on the part of the peel-and-stick flashing that extends out from the site-built sill pan, so that this sill flashing can be lapped over the housewrap.

    Q. "The instructions say, 'Under the window, the peel-and-stick flashing should lap over the housewrap.' What peel-and-stick flashing? We already flashed the rough sill long ago."

    A. The peel-and-stick flashing at the sill is the same peel-and-stick flashing used to create a site-built sill pan. When this site-built sill pan is created, you need to leave some release paper on the tongue of the flashing, so that the peel-and-stick flashing can be lapped over the housewrap later in the installation process. I will edit my instructions to make this step less confusing.

    Your summary in (6) sounds correct to me, but since it written in telegraphic style, it's hard to know whether your summary includes all the necessary steps.

    Q. "Is it bad to follow the 'no buck or frame' order when installing windows with a buck? If so, what's the potential negative consequence?"

    A. You question is confusing. I suggest that you follow my instructions as written (although I admit that there are other ways to install a window that deviate from these instructions). If you choose a method that differs from my advice, that's fine, as long as you have thought through the water-management details and as long as you feel confident in your approach.

  12. Whitney Scurlock | | #12

    Thank you for the very
    Thank you for the very helpful article. I do have a couple questions still regarding innie windows. The "Deep energy retrofit" details demonstrate 2x2 blocking around the window including below the sill, but this isn't present on the JLC detail. Is this blocking needed, and if not, how is the sill typically attached over thicker foam (we will have 4" foam on our home)? Also, any suggestions on the best way to make the angled cut in the rigid foam?

    Thank you!
    Whitney

  13. User avater GBA Editor
    Martin Holladay | | #13

    Response to Whitney Scurlock
    Whitney,
    To make an angled cut in the rigid foam under a window sill, use an ordinary handsaw. You can trim it up to make it neater with a kitchen bread knife.

    Installing picture-frame blocking to support the sill extension and jamb extensions of innie windows is useful but not strictly necessary. In some cases, it may be possible to secure these extensions (with small-diameter stainless-steel screws) to the rainscreen furring strips. In some cases it may be possible to support the extension at the head with the jamb extensions below. Another approach is to screw all of these extensions together (to each other) before installing them on the house, and using foam-compatible glue or canned spray foam to secure the extensions.

    As you correctly note, the innie window detail from the Mass Save Deep Energy Retrofit Guide (reproduced below) labels the blocking under the exterior sill extension as "Blocking if needed."

    .

  14. Brian Hart | | #14

    Picture frame and flashing detail
    I am preparing to flash the window jambs on a retrofit of my house's addition and have a question about how far the jamb and sill flashing should extend. I am doing a lot of the work myself so I apologize for what may seem like a straightforward question.

    Here's the wall system: I am installing 1" foil faced polyiso over the zip sheathing on 2x4 framing and plan to use the polyiso as the WRB. I've taped all the joints and protected the exposed edges of the polyiso from bugs and moisture infiltration with screen and aluminum tape. I am beginning to attach the 1x4 furring strips, as we have decided to use fiber cement board for our cladding, and based on the recommendations from other articles, use 4.5 inch Headlok screws to attach. The windows are outies, with flanges, and because of the cladding choice, will therefore have picture framing of 1x4 around them.

    As I approach the time to flash the windows, I am a little confused about how far the flashing should extend beyond the foam. Should the flashing be installed prior to the picture frame with the picture frame installed over the flashing as it wraps around the jamb and sill? Or should the flashing be installed after the picture frame, therefore extending out and over and around the picture frame onto the WRB?

    The instructions in the article were great and clear for the other options, but I could not determine from those how to deal with picture frame outies.

    Thanks much!

  15. User avater GBA Editor
    Martin Holladay | | #15

    Response to Brian Hart
    Brian,
    There is more than one way to do this, but here's how I would do it.

    If the rigid foam is your WRB, you want to integrate your flashing with that layer. That means that you will flash your rough opening after the rigid foam is installed, but before the furring strips or picture frame is installed. The flashing should extend at least 4 inches over the rigid foam.

    As you install the picture frame, it's a good idea to install horizontal flashing at the head to direct water that may be dribbling down the rigid foam -- you want to direct this water out to the exterior of the horizontal part of the picture frame at the head. When you use rigid foam as your WRB, you can't lap the layers of the WRB -- that's one of the disadvantages of using rigid foam as your WRB -- so you will need to use peel-and-stick tape at the head to cover the head of the picture frame.

    I wouldn't bother to flash the vertical parts of the picture frame (the jambs) or the sill piece. They are beyond the WRB, and these pieces should be well ventilated and dry.

  16. Derrick Krienert | | #16

    Outie window w/ innie WRB
    The article states that it is not recommended to frame an outie window with an innie WRB (WRB at the sheathing). Why is this not recommended? Is it due to the fact that you create a potential spot for water to puddle at the head? Couldn't this be solved with a piece of sloped flashing?

  17. Dave De C | | #17

    Thanks for the detailed
    Thanks for the detailed information, Martin.
    Can you verify that I have this correct. I have 2.5" of foam outside of the sheathing. Windows will be attached through brick flange to a 3/4" plywood picture frame that is bolted back to the studs. Housewrap as the ultimate WRB.

    1) Bolt frame in place
    2) Flash sill with Vycor (or similar) and leave outer portion of release liner on. Paying attention to corner details per videos: https://www.greenbuildingadvisor.com/window-sills-won-t-rot-sloped-sill-or-backdam
    3) Flash sides of opening per videos
    4) Install window
    5) Flash sides and top of window
    6) Install housewrap
    Over-window: tape housewrap to previously applied flashing
    Sides: tape housewrap to previously applied flashing
    Bottom: remove release liner from sill flashing and install over housewrap

    Seems simple enough. Appreciate your "checking my work" though.

  18. User avater GBA Editor
    Martin Holladay | | #18

    Response to Dave De C
    Dave,
    Your description sounds OK to me.

    -- Martin Holladay

  19. Dennis Aurand | | #19

    Picture frame, no buck
    In Step three, the fourth choice is: Install a picture frame of 1x4 lumber, installed on the flat, on the exterior surface of the rigid foam. The 1x4 picture frame is screwed through the rigid foam to the sheathing and framing.

    How thick can the exterior rigid foam be and still use this method with 'outies'?
    What are the advantages/disadvantages to this method?

    I plan on 3" rigid foam outside OSB sheathed 2X4 studs. Most windows will be ~6 ft wide.

  20. User avater GBA Editor
    Martin Holladay | | #20

    Response to User-6909128
    For me, the issue isn't so much the fact that the rigid foam is 3 inches thick (although that affects my response), it's that the windows are 6 feet wide. You are talking about very heavy windows. In your case, it would be best to talk to an architect or engineer before proceeding.

    If you would rather save the money that would otherwise go for engineering fees, you could choose a different approach: "Install a picture frame of 2x lumber, with the lumber ripped from 2x6s or 2x8s. The width of the ripped lumber should equal the thickness of the rigid foam. This type of picture frame — the strongest from a structural perspective — is attached directly to the wall sheathing with long screws, with no intervening foam."

  21. Malcolm Taylor | | #21

    User -69etc,
    You might consider a hybrid approach. The nailing flanges are really only there to keep the window secured in the opening, the weight of the window (in typical construction) is taken up by the sill framing. If you secure a 3" sill extension to the sheathing, as Martin suggests, you can secure the jamb and head flanges in any other way you want.

  22. Dennis Aurand | | #22

    Response to Martin & Malcom
    Thanks to both of you for your responses. I forgot how heavy these large windows are.

    I think I'll go with a modified version 2: Install a (ripped 2x8) window buck that fits inside the window rough opening and cantilevers outward, with the exterior edge of the window buck flush with the exterior face of the rigid foam (or in some cases flush with the exterior face of the furring strips).

    The buck would act as (additional) jack studs and as a second sill plate. It would not require long screws to attach, either.

  23. Kim Dolce | | #23

    Outie windows w/ 1.5" foam
    We're a couple of weeks out from installing windows as outies along with 1.5" of rigid foam. I thought it smart to search here before proceeding. My question veers off a bit, but I notice (as did Derrick Kreinert in ques #16) that housewrap should be installed over the foam not the sheathing. I had been going on the info and image from the Feb 2016 post "Combining Exterior Rigid Foam With Fluffy Insulation" that shows housewrap over the sheathing. I did not see a response to ques #16. Can someone shed light on the difference between the two approaches and how to determine which is best for my particular situation? I'll be happy to supply more info if needed. https://www.greenbuildingadvisor.com/articles/dept/musings/combining-exterior-rigid-foam-fluffy-insulation

  24. User avater GBA Editor
    Martin Holladay | | #24

    Response to Kim Dolce
    Kim,
    Every wall needs a water-resistive barrier (WRB). It's a code requirement.

    Your housewrap doesn't have to be your WRB, but it usually is. Assuming you want to use the housewrap layer as your WRB, the easiest way to integrate the housewrap with the window flashing (assuming outie windows) is to install the housewrap on the exterior side of the rigid foam.

    For an in-depth discussion of this issue, see this article: "Where Does the Housewrap Go?"

    Among the other options:

    1. If you want, you can install two layers of housewrap (but of course, only one of these two layers of housewrap will be your WRB).

    2. You can used the rigid foam as your WRB. For more information on this approach, see "Using Rigid Foam As a Water-Resistive Barrier."

    3. You can create wide flashing, some of which needs to be sloped, to integrate the deeply buried housewrap with the window flashing of your outie windows. It's possible, but not particularly easy.

  25. Kim Dolce | | #25

    Martin,
    Many thanks. I've

    Martin,
    Many thanks. I've explored the rabbit hole of articles I always encounter on GBA and have passed along the pertinent details to my carpenter.

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