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Community and Q&A

Flashing details

homedesign | Posted in Energy Efficiency and Durability on

I understand the advantages of using “outsulation” (rigid insulation on the outside)
I am still having trouble finding satisfactory details.
Especially concerning Window head details used with siding.
I went for the pro account here at GBA and started looking at the details.
I still wonder if anyone has actually constructed these details to see what they will look like and how they fit together.
It looks to me like the head details show the water draining behind the window trim(casing) and somehow magically weeping out between the bottom of the casing and the flashing.
If you imagine the intersection of the Head detail with the jamb detail… what you get is certainly not the traditional “look” of a cased window….here is a photo of what I assume the GBA detail would look like..
(photo taken at a Peter Pfeiffer project in Austin)
Not only does this look bad…But I think the flashing is in the wrong place.
Any weather/water that gets past the siding and hits the weather barrier should be diverted to the outside ABOVE(not below) the casing that caps the window. That’s how they did it in the old days.That’s why the photo does not “look right”.
Is it just me?

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  1. homedesign | | #1

    I have a similar concern about this Fine Homebuilding published detail...see window head on page 57
    In this case I think that the flashing above the window casing should be integrated all the way back to the weather barrier.
    It looks to me like most of the water/weather that gets past the siding will get trapped above the window instead of being diverted out and over the window casing.

  2. Riversong | | #2

    In addition to the challenges of properly integrating WRB, windows, flashings and trim, I hope you also understand the other disadvantages of "outsulation".

    In a heating climate, it can become a wrong-side vapor barrier and if there is also an interior vapor barrier, the wall assembly cannot dry in either direction. If punctured by thousands of siding nails, it's thermal benefit can be severely diminished and the nail points penetrating the sheathing can become condensation points. If there's not sufficient "outsulation" to keep the sheathing above dew point temperatures and there is no interior air barrier, then the sheathing can be a condensation plane in cold weather. And, if there IS sufficient "outsulation" to keep the sheathing and wall cavity above 40° and the cavity cannot dry to the exterior, then the framing and sheathing might be in the mold-growth zone for long periods.

  3. homedesign | | #3

    Thanks Robert,
    I am from a hot/mixed humid climate.I always learn from your comments.

  4. Daniel Morrison | | #4

    I've got a great video clip showing Mike Guertin installing head flashing including a counter-flashing inset to a reglet cut into the foam. We'll get it posted for you as soon as we can. Mike does it differently than these details.

    But about our details, Mike, Steve, and Pete poured over each one with the eyes of a builder/remodeler, architect, and building-science-wonk. The [head flashing detail here](node/3184) actually shows two pieces of flashing. Some of the others have one piece of flashing, as you note.

    Not sure that the second piece really buys you much though as there's a vented air space behind the siding, so most water getting back there will go right past the upper flashing.

    I agree that the photo doesn't look right. The old timers did, indeed, put the flashing above the trim, which is where I've always put it, but the old timers didn't have vented rain screens behind their siding either.

    On Betsy's house in the FHB article, there's a second flashing above the window and below the trim; it's labeled metal flashing tucked under housewrap. The upper flashing directs water above the trim, but because the trim is furred out with 1x3s, most of the water that gets behind the siding will shoot right past it to the second flashing above the window which will direct it out of the wall.

    As for inside and outside vapor barriers, yes, Robert is right, but you shouldn't be putting vapor barriers inside the house in Texas.

    Hope that helps,

  5. homedesign | | #5

    Dan, Please see my other comment
    Just looking at the details as looks like there is a path for the water to drain...But if you draw them to scale(not exploded) and push the layers together...the drainage path disapears. The only way the water can go is sideways toward the jambs.

  6. homedesign | | #6

    Dan, concerning Betsy's house
    "but because the trim is furred out with 1x3s, most of the water that gets behind the siding will shoot right past it"
    EXACTLY..That is what I see as the problem with the detail (and the GBA details)..
    MOST of the water is directed to a Flat Horizontal "gutter" that is not perfectly watertight. The wider the window the greater the chance of a "puddle" .
    Why not divert MOST of the water over top of the head casing (the shortest distance to the outside)
    The way the OLD GUYS did it.

  7. Daniel Morrison | | #7

    I guess you're calling the second flashing above the window a "flat horizontal 'gutter' that is not perfectly water-tight."

    I don't understand what you mean by this. It's flashing. If you install it poorly, it's a gutter that is not water tight. If you install it well, it sheds water quickly. It should slope away from the wall (don't be a dope, slope, right?). If the flashing slopes the wrong way, water will puddle at the sides, no matter where the flashing is -- above the trim or below the trim.

    In Betsy's detail there's flashing to protect the window trim and secondary flashing to protect the window and opening.

    Personally, I agree with you, I'd run it above the trim and behind the furring strips with a counter flashing let in to the foam. But Betsy's detail works and looks good.


  8. homedesign | | #8

    Dan, Try this....start a pencil line at the top of the 3/4" furring strip/drainage channel...draw a continuous line down along the drainage channel until it finds its way to the outside.(if you can)

  9. GBA Editor
    Steve Baczek | | #9

    Agreed, some builders do choose to drip cap the casing rather than the window. We chose to drip cap the window. The problem with the photo is that the drip cap should be cut to the width of the window, not the casing. In either case the window should be installed into a water-managed rainscreen system, making the location of the drip cap rather moot (personal preference).

  10. Expert Member
    Peter Yost | | #10

    Great to have folks poring over our details, particularly the window flashing ones. Two quick comments:

    1. We tried to vary flashing details for the same or similar claddings for this very reason (see 3-00003 instead of 3-00006). And the Overview is written to recognize, even emphasize, how complex the installation can be, and how many variables there are. What we need to add to the overview is, "mock it up and water test it." The proof is in that pudding.

    2. I prefer to install the window unit first and then run spacer mesh or only vertical furring to and around windows to get free and clear drainage behind the trim. Then you have to deal with reveal and trim return issues, depending on the thickness of furring strips or even whether or not you install the window before or after the rigid insulation. Just more ways to skin the cat.

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