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Community and Q&A

Housewrap over exterior foam – flashing detail

CTSNicholas | Posted in Green Building Techniques on

I read the two articles, one being ‘where to place the housewrap’ and the other was

The method I will use is an outie window. Not 100% sure, but I will either a.) install house wrap over the foam or b.) have it under and over the foam.

Either way, I want to make sure I get the flashing detail 100% correct. Reading through the 8 steps everything made sense until step 7. “7. Plastic housewrap is installed over the foam. The housewrap laps over the peel-and-stick flashing. Under the windows, peel-and-stick flashing laps over the housewrap.”

So the window is framed out with 3/4″ plywood to form a box – Then a 1×3 or 1×4 or custom 3/4″ thick strip piece of wood is used to frame around the window over the foam and butt up against the window box – to bring the window to the desired depth of siding. That makes sense. If I only use 2″ of foam, the window box is not necessary, correct? A flexible flashing is stuck to the bottom of the window, and ran up the sides a bit to waterproof the sill and joints at the two bottom corners. Then I install the window, and fasten it either with masonry straps or via the pre-drilled holes on the window frame depending on the window. Should caulking be used on the two sides and top as the window is inserted?

Next, I go ahead and use a flashing to cover the window flanges on top, and both sides, leaving bottom open, I assume.

This then is where my housewrap layer comes in. I leave excess and cover the flashing with house wrap…something is left to be interpreted here, and my best guess is to use housewrap tape and tape the ends of the wrap against the window or flashing used. The bottom however is apparently left loose to be covered with flashing that overlaps the bottom window flange & house wrap. This is foggy to me, and seems like moisture would easily get in behind or around the tape corners and be trapped and work down the foam near the bottom of the window flange.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "If I only use 2" of foam, the window box is not necessary, correct?"

    A. There is no hard-and-fast rule about when to use a window buck. The choice depends in part on how heavy the window is and how the window is being supported and attached.

    Q. "A flexible flashing is stuck to the bottom of the window, and ran up the sides a bit to waterproof the sill and joints at the two bottom corners. Then I install the window..."

    A. This description is unclear to me. You want to be flashing the rough opening, not the window. So you need to flash the sill and jambs of the rough opening before the window is installed.

    Q. "Should caulking be used on the two sides and top as the window is inserted?"

    A. You should follow the manufacturer's instructions, but in general, you shouldn't need any caulk for water management. Sometimes, caulk is used for air sealing -- but other methods (usually either canned spray foam, a gasket, or a high-quality European tape) are usually better than caulk for air sealing.

    Q. "My best guess is to use housewrap tape and tape the ends of the wrap against the window or flashing used..."

    A. I'm not sure whether I understand your confusion here. If you are using the housewrap as a WRB, you need to think like a raindrop and lap your layers to allow water to flow properly at horizontal laps. If you are installing a flanged window, the WRB usually laps over the window flange or the head flashing at the window head. At the jambs, the housewrap is usually folded into the rough opening, but there are other ways to do it -- and many ways will work, as long as your rough opening has been flashed before the window was installed.

    At the rough sill, the sill pan flashing should direct any water to the exterior -- so that your sill pan flashing overlaps the housewrap at the sill area.

  2. CTSNicholas | | #2

    I did make a typo on the part about flexible flashing stuck on the bottom of the window. I meant of the rough opening as you say. I envision using a piece to cover the bottom and go up each side about 10". I may go up the sides and the top opening if I find a more affordable flashing.

    The housewrap will have to be my WRB as foam and moisture are not something I want to mix. I'm a bit hesitant about the struggle of fastening my house wrap with long enough staples to penetrate thru the foam into the OSB. I don't see a better method to install it, though.

    I will have flashing over the window flange and adhered to the foam, then the house wrap will cover that and tape to the flashing to cover the seam the flashing has with the foam.

  3. GBA Editor
    Martin Holladay | | #3

    The best way to fasten your housewrap is with cap nails. You can get cap nails in almost any length; if you have 2 inches of rigid foam, use 2 1/2" or 3" cap nails.

  4. CTSNicholas | | #4

    Okay, cool. Any spacing suggestions or just whatever the manufacture provides?

  5. CTSNicholas | | #5

    Just had a thought occur. If I use 1x3 or 1x4 to build a 'window frame' for the window flange to fasten to, how am I to properly flash that window flange to the foam if there is 3/4" depth difference, and an additional dimension to step-flash down to because of the 1x3 or 1x4 material? I was for some reason envisioning the window directly against the foam, but it will be built out 3/4" to account for the siding's vertical firing strips. The local peel and stick flashing available comes in 6" thickness, and I would not have much additional width of flashing to stick to the rigid foam after covering 2 to 3" wide window flanges, 1-2" of wood frame, 3/4" of wood frame side, then down to the rigid foam's plane.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "How am I to properly flash that window flange to the foam?"

    A. Your main task is not to flash the window flange to the foam. Your main task is to flash the window rough opening. The window rough opening flashing needs to be integrated with your WRB -- that is, your housewrap -- not the foam.

    If you want a secondary flashing that covers your window flange, you can install one. But remember -- your window flange is an aid that helps install the window. It is a support system that holds nails or screws. A window flange is not a piece of flashing.

  7. CTSNicholas | | #7

    I guess I did not think of the flange as not needing flashed. I just did not want to leave something exposed that could be additionally protected. All the wrap companies demonstrate to use a peel and stick flashing to cover that flange and make a seal to the sheathing the window flange is attached to.

  8. CTSNicholas | | #8

    Reviewing my notes from the past question I asked and this one here. To clarify, if using 2" of EPS (or XPS) rigid foam, it will not matter if I locate the house wrap below the foam, or over the foam. Correct? The article on 'where to locate the house wrap' made it sound like it boils down to the type of window install method (innie/outie).

    I understand the house wrap has to tie into my openings and weatherproof them.

    Locally I do not have much choice on house wrap, I believe the brand is green guard. If I install the house wrap prior to 2" of foam, should I try to find a wrap that is textured, commonly labeled as 'drain wrap'? I assume if I use wrap on the exterior side of the foam, either product (drain wrap or standard house wrap) will suffice...?

  9. GBA Editor
    Martin Holladay | | #9

    The answer to your question depends on the details of your wall assembly.

    A wall with a layer of rigid foam on the exterior side of the wall sheathing is designed to dry inward.

    This type of wall is more forgiving (less risky) if it is sheathed with plywood than if it is sheathed with OSB.

    Moreover, this type of wall is less risky if it includes vapor-permeable insulation between the studs than if it includes closed-cell spray foam between the studs.

    The worst-case scenario is a wall that is sheathed with OSB and insulated on the interior with closed-cell spray foam. In that case, it is mandatory to install a bumpy or wrinkled housewrap between the exterior face of the OSB and the rigid foam.

    If your wall is sheathed with plywood, and you have specified a vapor-permeable insulation between the studs, then the wrinkled housewrap is optional.

    For more information on this issue, see Mind the Gap, Eh!

  10. CTSNicholas | | #10

    My wall assembly will include 2x6 studs with either dense packed fiberglass or mineral wool insulation. No spray foam for me. Does this mean I should not have a house wrap under the foam, and instead over the foam, so everything below the wrap can dry to the inside? Or does this mean I should locate a *crinkled* wrap between the OSB (plywood may be too expensive) and Foam, so that there is no 'hydro pressure' moisture can get to the OSB (or plywood) to harm the structure? I don't want to offer the chance for OSB to get wet, where in the article it is discussed if it gets wet it can dry inwards, but I prefer to prevent it from getting wet in the first place. This makes me think a drain wrap would be necessary before I install foam. I will have 1x3 firing strips on the outside of the foam to install my siding choice.

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