Installing Windows In a Foam-Sheathed Wall

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Installing Windows In a Foam-Sheathed Wall

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

Posted on Jul 10 2015 by Martin Holladay
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UPDATED on April 3, 2016.

Every now and then, a GBAGreenBuildingAdvisor.com 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:

  • Are the windows innies or outies?
  • What type of water-resistive barrierSometimes also called the weather-resistive barrier, this layer of any wall assembly is the material interior to the wall cladding that forms a secondary drainage plane for liquid water that makes it past the cladding. This layer can be building paper, housewrap, or even a fluid-applied material. (WRB) does the wall have: Zip sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , housewrap, or rigid foam?
  • How thick is the rigid foam? (For more information on this question, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

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.

If your building has outie windows, you should use the rigid foam or housewrap installed on the exterior side of the rigid foam as your WRB. To learn why the innie/outie decision affects the location of your WRB, see Where Does the Housewrap Go?

Building scientist Joe Lstiburek advises that rigid foam makes a good WRB. I remain somewhat skeptical of this approach, preferring to use housewrap rather than rigid foam as a WRB. For more information on the topic, see Using Rigid Foam As a Water-Resistive Barrier.

In all cases, the WRB has to be integrated with the window flashing.

Step three: Determine whether you need a window buck or picture frame

Will you need a plywood window buck or a “picture frame” of lumber to help support your window? Maybe.

There are five possible approaches:

  • Install the windows as outies, with the window flanges on the exterior side of the rigid foam, and simply nail or screw the window flanges to the sheathing and framing using long fasteners through the foam. This approach requires neither a window buck nor a “picture frame,” but the approach only works if you are installing 1 1/2 inch or less of rigid foam and if the siding — for example, vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). siding — does not require vertical furring strips. (If you are installing 2 inches of rigid foam, you can still get away without the plywood buck — but you’ll need to fasten the windows with masonry clips, not the window flanges.) For more information on this approach, see Nailing Window Flanges Through Foam.
  • Install a plywood 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). This approach is typically used for outie windows or in-betweenie windows when the rigid foam layer is between 2 inches and 6 inches thick. According to Lstiburek, this approach works well for windows that are less than 3 feet wide. “Wider windows get a 2-by underneath the plywood at the sill, to give the center of the window some additional support,” says Lstiburek. Remember that you will need oversized rough openings if you take this approach. (Instead of using a site-built plywood window buck, it's also possible to use manufactured foam bucks like the ThermalBuck.)
  • Install a Dudley box — that is, a plywood window buck with attached flanges. The Dudley box approach is especially useful for retrofit work. For more information on Dudley boxes, see Window Installation Tips for a Deep Energy Retrofit.
  • 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.
  • 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. This approach is the one to use if your foam is more than 6 inches thick, or if you are installing outie windows on a house with mineral wool insulation on the exterior side of the wall sheathing.

Step four: Decide how you will be fastening your windows in place (and whether to order windows with or without flanges)

There are three common ways to fasten a window in place: with fasteners installed through the window flanges; with masonry clips; or with trim screws installed through the window jambs.

Masonry clips are rectangles of galvanized steel that extend from the window jambs toward the interior of the house. These clips allow the window installer to secure a flangeless window to the rough jambs.

If you are installing your windows as innies, you’ll probably order windows with flanges. The flanges are nailed to the sheathing and framing.

If you are installing your windows as outies, and the rigid foam layer is only 1 1/2 inch thick or less, or you are installing outie windows to a picture frame, it is probable that you will also be ordering windows with flanges.

If you are using plywood window bucks, your windows will probably need to be fastened in place with masonry clips.

Step five: Choose your pan flashing material

Before you install a window, the rough opening (or the plywood window buck) needs to be flashed. In most cases, flashing installation follows a well-known sequence. (If you're unfamiliar with methods used to flash a window rough opening, you might want to watch an 8-part GBA video series on the topic. Here is a link to the first episode in the series: Window Sills That Won’t Rot.)

My mantra is, “Flash the rough opening, not the window.” Here is a brief description of the usual sequence used to flash a window rough opening:

  • Start by installing an interior dam or a beveled shim on the rough sill.
  • Then install your sill pan flashing — either a manufactured sill pan, a site-built sill pan made with peel-and-stick, or a liquid-applied flashing. (Liquid-applied flashings are often used in conjunction with liquid-applied WRBs.) The sill pan flashing directs water to the WRB below the rough opening.
  • Then install your jamb flashing — usually a peel-and-stick product — which wraps around the rough jambs and laps over the sill pan flashing.
  • Then install your head flashing, lapping the head flashing over the jamb flashing. (If you are installing a flanged window, you'll install the head flashing after the window is installed.) If the WRB is asphalt felt or housewrap, the WRB should lap over the head flashing. If the WRB is the rigid foam, the peel-and-stick head flashing needs to be carefully sealed to the rigid foam, and the upper horizontal seam of the peel-and-stick should be sealed to the rigid foam with acrylic housewrap tape. (The purpose of the acrylic tape is to seal the top of any fish mouths in the peel-and-stick.)
  • If the window has exterior casing, install metal Z-flashing to protect the head casing. The top of the Z-flashing should either tuck under the WRB or be taped to the WRB.

This routine flashing sequence can’t be used if you are trying to install outie windows with a WRB (housewrap) installed near the sheathing plane. If there are compelling reasons why you need to have the WRB at the sheathing plane and outie windows, you’ll need to create custom pieces of head flashing to direct water from the WRB at the head of the rough opening to the head (upper frame) of the outie windows. This is possible but not recommended. In general, outie windows need an outie WRB, and innie windows need an innie WRB.

Putting it all together

With so many variables, it should come as no surprise that individual builders have come up with their own favorite methods of installing windows in foam-sheathed walls. There are many variations to these details.

I've provided instructions below for installing an innie window; an outie window with no plywood buck or window frame; and an outie window with a plywood buck. There are other ways to proceed, but if you follow these instructions, you should stay out of trouble.

Innie window

  • Install plastic housewrap over the OSB or plywood sheathing. Cut slits at the upper corners of the rough opening, and temporarily fold up a flap of the housewrap (with a horizontal fold) above the head of the rough opening.
  • Install an interior dam or beveled shim on the rough sill.
  • Flash the rough opening by installing sill pan flashing and jamb flashing.
  • Install the window conventionally, fastening the window flanges with nails or screws to the wall sheathing and framing.
  • Install peel-and-stick flashing over the side flanges and the head flange; lap the housewrap over the head flashing.
  • Install rigid foam over the housewrap.
  • Install vertical furring strips over the rigid foam to create a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. gap. There’s no reason to integrate the free-draining rainscreen gap with any of the other flashing details on the wall.
  • Make an angled cut at the exposed rigid foam at the window sill area so that the sill will drain toward the exterior.
  • Protect the exposed foam edges facing the window (including the foam at the sill area) with peel-and-stick flashing.
  • Trim out the exterior of the window with water-resistant jamb extensions and a water-resistant sloped sill made of cedar, cellular PVC, or copper flashing. These exterior jamb extension details don’t really have to be watertight — just resistant to weathering, durable, and attractive.

Outie window with no plywood buck or window frame

  • Install rigid foam on the exterior side of the OSB or plywood wall sheathing. (Remember: this installation method can only be used if the total thickness of the rigid foam is 1 1/2 inch or less, and if the siding — for example, vinyl siding — does not require vertical furring strips.)
  • If housewrap is being installed as a WRB, it should be installed on the exterior side of the rigid foam.
  • Flash the rough opening. Note that the sill pan flashing should cover the rigid foam, and should direct any water to the exterior surface of the rigid foam or the housewrap.
  • Install the window. The window should be secured by installing nails or screws through the flanges and through the rigid foam to the sheathing and framing below.
  • Install peel-and-stick flashing over the side flanges and top flanges of the window.
  • The window flashing needs to be integrated with the WRB. The details depend on whether housewrap or the rigid foam is used as the WRB.

Outie window with a plywood buck

  • Frame the rough opening so that it is oversized by 1 1/2 inches in both dimensions.
  • Install rigid foam on the exterior side of the OSB or plywood wall sheathing.
  • Line the rough opening with a frame (a buck) made of 3/4-inch plywood strips. On the interior, the frame is installed flush with the studs; on the exterior, the frame cantilevers out 3/4 inch beyond the outside face of the rigid foam.
  • Install a picture frame of 3/4-inch-thick strapping, fastened flat to the foam, around each plywood window buck. The outer face of the 3/4-inch strapping should be flush with the outer edge of the plywood buck.
  • Flash the bottom of the plywood box with a site-built sill pan (using peel-and-stick flashing). At the outer edge, where the sill pan terminates at the exterior, leave some of the release paper on the peel-and-stick, so that the peel-and-stick flashing at the sill can be lapped over the housewrap when the housewrap is installed. Flash the sides (jambs) of the plywood box, lapping the side flashing over the site-built sill pan, just as you would flash any rough opening.
  • Install the window in the plywood frame, and attach it to the jack studs (through the plywood buck) with masonry clips.
  • Install peel-and-stick flashing to the sides and top of the window; the flashing should overlap the window flanges (if any). The flashing should be wider than the picture frame; the flashing should cover the picture frame and extend back to the foam.
  • If the plan includes housewrap as the WRB, install housewrap over the foam. The housewrap should lap over the peel-and-stick flashing. Under the window, the peel-and-stick flashing used to make the site-built sill pan should lap over the housewrap at the sill.
  • Install vertical furring strips on top of the foam and the housewrap. Fasten siding to the furring strips.

For a well-illustrated article on various ways to install windows in a foam-sheathed wall — including some advice that differs somewhat from the guidelines I've provided here — see Joe Lstiburek's article, “Windows Can Be A Pain.”

Another useful resource on this topic from the Building Science Corporation is the Mass Save Deep Energy Retrofit Builder Guide.

Martin Holladay’s previous blog: “Is Weatherization Cost-Effective?”

Click here to follow Martin Holladay on Twitter.


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Image Credits:

  1. Image #1: Martin Holladay
  2. Image #2: Daniel Ernst
  3. Image #3: Journal of Light Construction
  4. Image #4: Jeff Nelson
  5. Image #5: Mass Save Deep Energy Retrofit Guide

1.
Jul 10, 2015 8:15 AM ET

Can window manufacturers make it easier?
by Charlie Sullivan

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.
Jul 10, 2015 8:20 AM ET

Response to Charlie Sullivan
by Martin Holladay

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.
Jul 11, 2015 2:28 PM ET

Protecting the foam on innies
by Michaela Riley

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.
Jul 12, 2015 5:44 AM ET

Response to Michaela Riley
by Martin Holladay

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.
Jul 12, 2015 8:54 AM ET

Liquid Applied Flashing
by Russ Hellem

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.
Jul 12, 2015 1:22 PM ET

Response to Russ Hellem
by Martin Holladay

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.
Jul 15, 2015 10:54 AM ET

Frame & Jamb Extensions
by Christopher Vlcek, Littlewolf Architecture

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.
Jul 17, 2015 5:43 PM ET

Mass Save Deep Energy Retrofit Builder Guide
by Kohta Ueno

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...


9.
Jul 17, 2015 6:32 PM ET

Response to Kohta Ueno
by Martin Holladay

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


10.
Apr 7, 2016 11:33 PM ET

Confused about the process
by Michael Bluejay

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.
Apr 8, 2016 3:24 AM ET

Edited Apr 8, 2016 3:35 AM ET.

Response to Michael Lutkenhouse
by Martin Holladay

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.
Jun 21, 2016 6:38 PM ET

Thank you for the very
by Whitney Scurlock

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.
Jun 21, 2016 7:36 PM ET

Edited Jun 21, 2016 7:37 PM ET.

Response to Whitney Scurlock
by Martin Holladay

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."

.

Innie window detail from Mass Save DER Guide.jpg


14.
Oct 18, 2016 8:38 PM ET

Picture frame and flashing detail
by Brian Hart

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.
Oct 19, 2016 3:05 AM ET

Response to Brian Hart
by Martin Holladay

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.
Jan 16, 2017 1:11 PM ET

Outie window w/ innie WRB
by Derrick Krienert

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.
Mar 7, 2017 12:55 PM ET

Thanks for the detailed
by Dave De C

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: http://www.greenbuildingadvisor.com/window-sills-won-t-rot-sloped-sill-o...
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.
Mar 7, 2017 1:42 PM ET

Response to Dave De C
by Martin Holladay

Dave,
Your description sounds OK to me.

-- Martin Holladay


19.
Sep 7, 2017 11:40 AM ET

Picture frame, no buck
by Dennis Aurand

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.
Sep 7, 2017 12:16 PM ET

Response to User-6909128
by Martin Holladay

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.
Sep 7, 2017 2:18 PM ET

User -69etc,
by Malcolm Taylor

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.
Sep 8, 2017 2:28 PM ET

Response to Martin & Malcom
by Dennis Aurand

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.


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