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Energy-Smart Details

Integrating Windows to Ensure Building Performance

A detail for wrapping the sill into a wall assembly’s four control layers

If buildings consisted of only foundations, walls, and roofs, then adding water, air, vapor, and thermal control layers would be straightforward. Doors and windows, of course, complicate matters. They significantly interrupt the continuity of those layers. The question is, how do we handle the interruptions to ensure a building’s successful performance?

Windows and their components make up a self-compartmentalized system of control layers, which are installed in a wall assembly’s rough openings. To maintain overall continuity of the assembly, the window unit’s control layers need to be stitched together with those of the wall. The detail shown is of a flanged double-hung window in a 2×6 16 in. o.c. wood-framed wall sheathed in 1-7/16-in. Zip R-6. In detailing the windowsill, the four control layers are taken into account. 

With respect to water management, the objective is two-fold. The primary goal is to keep water out of the system with a barrier. Secondly, water needs to be managed should it breach that barrier. We use a “down and out” strategy, meaning if water does find its way into the assembly, it has a managed pathway out. In this case, the strategy is a rainscreen made of 1×3 vertical wood furring strips spaced 16 in. o.c. to align with the framing studs. The rainscreen provides free drainage and ventilation for the wall cladding and the window unit. Connecting the rough opening to the rainscreen system maximizes the opening’s drying potential.

Note the piece of beveled lap siding placed on top of the rough sill to create a slope to direct water toward the exterior. The siding is accompanied by a 1×3 back dam, the top of which is slightly higher than the high point of the beveled siding. The raised edge of the back dam contains any water at…

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19 Comments

  1. Russell Chapman | | #1

    Great article and details Alexandra! This was perfect as I went onto GBA today looking for this exact back dam detail. Thank you!

    1. Alexandra_Baczek | | #5

      Thank you Russell!

  2. Jim Pierce | | #2

    Thanks for the drawing. What are the unlabeled wood pieces on the left and right of the foam sealant just above the sill seal? Are they part of the window unit? In the picture, it looks like they perfectly conform to backdam and sloped sill, and even a vertical section wrapping around the backdam aligning with the drywall face. Is it typical to find windows with this type of bottom contour?

    1. John Clark | | #4

      Air gap?

    2. Alexandra_Baczek | | #6

      They were both mistakenly colored in. Under the sill we would typically have some intermittent blocking for the finish sill to attach too. The space to the right of the foam should be open air.

  3. Jim Pierce | | #3

    This diagram has an interesting spray foam detail that I've not seen described anywhere else. Is the author suggesting adding a bead of spray foam around the entire perimeter of the rough opening or just on the sill? I think this detail could be particularly advantageous on the sill for air sealing. Window sides and top have double air sealing barriers - the tape and caulk around the window flange on the outside and the foam, backer rod, and/or caulk on the inside. But normally along the bottom sill there is only one air barrier, the inside one, since neither taping nor caulking is recommended on the outside at the bottom. This bead of foam along the bottom added just prior to window insertion provides a 2nd air barrier.

    Has anyone else had experience with this?

    1. Alexandra_Baczek | | #7

      We typically use a minimal expanding foam air seal around the perimeter of the window on all four sides.

  4. Expert Member
    Malcolm Taylor | | #8

    Another useful article. One th0ught though:

    I'm beginning to have doubts about the utility of sloping window sills - especially if there is no provision (beyond not sealing the flange at the sill) to allow water to escape.

    My doubts are the same as those I have about making provision for water to drain out of the bottom of rain-screen cavities. Bulk water may make it's way into both areas, and may saturate any unprotected framing, but if there is sufficient water to actually drain, something has gone very wrong and needs to rectified.

    To me the important part of detailing the sill is to protect the framing with a waterproof pan, and allow the area some drying potential to the outside. Whether the pan actually slopes is secondary. If we wanted to make the detail more effective, either shimming out the flange slightly to maintain a drainage plane, or drilling holes in the bottom flange would be more effective.

    I've seen and repaired a lot of rotten framing around the bottom of windows. Even in the worst cases of water intrusion all I've ever found is large areas of saturation. I'm not sure moisture accumulation sufficient to actually drain isn't an apocryphal event.

    1. hockipuck | | #9

      Malcolm, it's a good point. I do expect that over time the joint between the bottom sill of the window and the vertical will leak. With the sill pan flashed to shed water even a failure of a side or top flashing tape would divert water to the exterior with the caulk bead on the inside of the frame. Some things we can only address easily the first time, and the incremental cost of adding these pieces during a window install is negligible for a "belt and suspenders" approach.

    2. Alexandra_Baczek | | #15

      Malcolm, I just had a quick chat with Steven Baczek about your comment and he had said while he understands your thoughts completely he has done extensive testing on window leakage. A number of his tests included how well a sloped sill will drain through an unsealed nailing flange. His comment was it "drained surprisingly well". If your concern extends beyond his confidence he suggested a small finish nail or shim underneath the nailing flange to provide a more adequate drainage space.

      1. Expert Member
        Malcolm Taylor | | #18

        Alexandra,

        That's good to hear. I'm quite happy to rely on Steve's experience.

  5. Expert Member
    Malcolm Taylor | | #10

    hockipuck,

    One of the p0ints Christine Williamson made in recent BS & Beer episode was the desirability of keeping the cavity around the window at the same pressure as the exterior. She suggested that the air-seal is best placed closer to the interior of the frame. Especially with a rain-screen cavity, that speaks to me of the benefits of a gap or opening in the bottom flange to allow some air-movement and the accompanying drying it would provide. I think that's more important than trying to drain the water.

    Maybe there is some middle ground? Only sloping the last 1 1/2" or 2" of the sill, as a slope beyond the air-seal doesn't help anyway. My own preference is to put a 6 degree bevel cut on edge of the sill-plate while still maintaining enough flat surface to support the part of the frame inboard from the glazing.

  6. Doug McEvers | | #11

    Malcolm,

    I adopted your method a while back of sloping the outer edge of the sill plate. I use a reciprocating saw to bevel the outer edge along with the exterior sheathing, then install Flex Wrap. This provides positive drainage away from the framing and outside the WRB. This negates the need to make the rough opening larger to accommodate the bevel siding.

  7. Thomas Nedelsky | | #12

    I recommend the air barrier wrap into the window opening on the sides and head. The sticky back stretchy flashing material as shown in the drawing at the sill is ok. I prefer liquid applied flashing on the sill, jamb and head wrapping back into the opening almost as far as the interior edge of the window jamb. A continuous seal on all four sides should be made between the jamb and the air barrier. I recommend using a very stretchy sealant, not spray foam. Foam can crack over time. At the jambs and head, from the interior, push a backer rod into the shim cavity to create a space for the sealant that is then applied from the interior. At the sill, prior to setting the window I would run a heavy bead of the same sealant in the same plane as the head and jamb bead so they all connect. The shim cavities should be left open and they should be pressure equalize to the rain screen area via the cavity under the sill. No sealant should be used behind the bottom nail fin of the window. I recommend spacing out the bottom fin from the flashing behind it with a shim of about 1/8". I recommend horse shoe shims. This insures any water that enters the shim cavity or the under sill cavity, has an easy access out. The sill to jamb joint in just about all domestically made window should be assumed to fail at some point. They are sometimes damaged in shipment and no one notices it. The point here is that you move the plane of the air barrier to near the interior of the window, that the shim cavity is pressure equalized to the rain screen cavity(no pressure drive of moisture to the shim cavity), and long lasting sealant is used in place of spray foam. Any slight gain in R value by the foam is not worth giving up a robust air seal that will be lost when the foam cracks. I like the Henry products for the liquid applied flashing and sealant but Prosoco makes something similar.

    1. Alexandra_Baczek | | #16

      Thomas, your points are dually noted but we have homes that have relied on foam air sealing around the perimeter of the windows that have been retested numerous, and there is no degradation in the air tightness numbers.

  8. Richard Langley | | #13

    What is the unlabeled upside down L in the diagram just ender the window sill? Is that a metal flashing? How is it attached?

    1. Alexandra_Baczek | | #17

      Richard, it is the 10" adhered stretch tape noted.

      1. Richard Langley | | #19

        I think the dashed line is the stretch tape according to the arrow associated with the note. I am talking about what actually looks like an upside down L above the stretch tape and below the sill. Just read the other comments and reread part of the article...this is a flanged window. I should have realized that. Thanks for your response.

  9. Jibu J | | #14

    Nice detail - is there one for retrofitting new windows onto existing 2*4 framing?

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