Window back dam
AKZach
| Posted in General Questions on
Going to be installing windows in the near future and wondering about back dam detailing. Wondering if people have an efficient method of trimming out the inside window returns to accommodate the backdam sticking up, other than padding out the entire RO to match the back dam. I have never trimmed out window returns featuring backdams. Planning on using 3/4 local spruce for the trim material. Seems like using backer rod and high performance caulk or tape on the gaps around the window would prevent water from moving inside anyway. Thanks
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Replies
I wouldn't sweat it. If you've done a careful job with the rest of the window installation, and it sounds like you have, then a back dam isn't really even necessary. A backer rod and caulk flush with the inner face of the frame should suffice. When properly installed, very little water will end up on your rough sill. If it can evaporate away it will do no harm.
BSC's detail for a backdam are shown in the page below. A reasonable go-to is a rip of 1/2" plywood that also acts to lift the stool/sill upward a bit. Then an interior air seal around all four sides of the window, from the "wrap" to the window unit.
I strongly recommend having *either* a backdam or a sloped sill. I have done forensic cases where they did a very nice "wrap" of the rough opening... but the windows were leakers. Half the time, the windows leaked outward, and nobody had a problem. The other half, the water leaked inward, and the builder got a callback.
Info-302: Pan Flashing for Exterior Wall Openings
https://www.buildingscience.com/documents/information-sheets/pan-flashing-for-exterior-wall-openings
Kohta, that was a very informative link. Thank you. I have a question. It shows a recessed area in a concrete slab forming a back dam for door sills. Would a recess such as this still get a metal pan flashing or something similar? Or is the recessed concrete enough to provide the necessary protection?
AKZack,
To add to what Antonio and Kohta have said, a back-dam can also be as simple as turning up the membrane sill-pan 3/8" onto the interior of the window frame. It then gets sandwiched between that and the interior casing.
Once installed, the window usually sits so that there is a bigger gap at the top than at the jambs and sill, so you can also use 3/8" plywood run from the interior of the window frame on the sill, and still end up with a consistent reveal around the window.
Ok, thanks for the advice. I didn’t plan for sloped sills but sounds like making a back dam with 3/8 ply is a good idea that won’t slow me down too much when it comes time to trim out the windows.