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For Martin Holladay

charliechan | Posted in Energy Efficiency and Durability on

Your article in FHB Nov 2015 contains a drawing of an airtight window installation. The drawing for the windowsill shows a bead of expanding foam sealant to the outside of the back dam. Would the sealant not prevent any water reaching the sill from draining to the outside as encouraged by the back dam?
Perhaps such sealant is pervious to water?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, my article in that issue of Fine Homebuilding was about furnaces, not window installation. Maybe you are thinking of the article by Brian Knight -- the one titled "A Weatherproof Window Installation"?

    It would be better for Brian Knight to answer any questions about that article. Here's my understanding of the recommended detail (reproduced below): any water drains onto the exterior side of the WRB (usually plastic housewrap) installed on the exterior side of the wall sheathing. While the siding is nailed to this WRB, the moisture is still able to dribble down and escape, usually by evaporation.


  2. charliechan | | #2

    Could you possibly pass my question along to Brian Knight ? My question is about the foam sealant shown on the outside of the back dam. Your drawing is different.

  3. GBA Editor
    Martin Holladay | | #3

    I have sent an e-mail to Brian Knight with a link to this Q&A thread, asking him to comment.

  4. GBA Editor
    Martin Holladay | | #4

    Installing expanding foam on the interior side of the window is a fairly standard air-sealing technique. It is unlikely that any liquid water will reach the area between the back dam and the air-sealing foam, but if it does, the moisture is likely to evaporate before it causes any problems. The flashing on the rough sill prevents any liquid water from entering the wall cavity.


  5. exeric | | #5

    Chan, perhaps you already know this. There is one important detail that needs to be observed. When dealing with a flanged window, which is mostly what is used in North America, then one needs to allow a gap in the caulking of the flange on the bottom portion of the window to the membrane flashing. In spite of every effort to make the water stay on the exterior, which is correctly shown on Martin's graphic, one still needs to allow any water that inadvertently eludes all that effort to drain harmless away. That caulking gap, usually a 3" gap on either end of the lower flange between the lower flange and the membrane, allows that water to escape.

  6. Brian Knight | | #6

    Hi Chan, the first drawing Martin posted is from the article. I understand the confusion as these 2D illustrations of 3D assemblies can be open to interpretation. I think what you are describing as a backdam is what I see as a spaced shim. As mentioned in the article, I am not a fan of backdams as I feel they needlessly complicate overall installation details.

    My preferred backdam method would use backer rod or flexible foam as shown in this Mike Guertin FHB window sill flashing video . Still, the details in that video and most residential backdam methods I see, don't meet the heights called for in ASTM 2012, which is a confusing detail as backdam heights are allowed to be reduced with proper airsealing. In my opinion, establishing slope on the rough sill and being thorough with airsealing to the interior, is more important that backdams for most common residential window installations.

    So in the first picture posted by Martin, I believe the dark brown piece of wood below the window is meant to be a spaced shim, not a continuous backdam. This creates an airspace between the window unit and sill flashing that allows drainage and drying. Most people install these window shims long, extending past the unit and framing to the interior and cut the shims back flush with the window unit and framing. The problem is, they interrupt the expanding foam bead or backer rod and caulk. That's why I like to make sure the shims are cut short prior, so they don't interrupt these redundant air sealing layers. For people that don't want to take the extra time to fuss with this extra shim work, taping the window unit to the framing becomes the most important airsealing layer because its the only layer not interrupted by shims.

    Here is the FHB window installation video that shows the field details discussed in the article.

    Ive been assuming your description of "expanding foam sealant" is what is drawn as an oval, to the interior of the spaced shim representing the backer rod for the caulk. Depending on the field details, I usually like to skip expanding foam at the bottom of the window because it could expand too much and fill the air cavity between unit and framing which is better left open for drainage and drying.

    It could be you are referring to the blue sealant that's better described there as caulked siding. Siding is not waterproof, and it's important to caulk those areas to keep bulk water from running behind the siding. Window units are rarely waterproof and leak more over time. That's why builders are including these sill flashing details to capture water leaks and redirect them to the exterior.

    Eric points to one of the most common blunders in current window installation practices, sealing the bottom flange. As mentioned in the article, some manufacturers call for a dashed bead of caulk on the bottom flange but this is asking for trouble. I prefer to skip the caulk at the bottom flange and go a step further, by using horseshoe shims or button caps to create a gap between the bottom flange and sill flashing tape to ensure water can drain from the sill pan. I drive by construction sites everyday where builders commit an obvious window weatherproofing sin: taping the bottom flange, don't do it!

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