Window back dam – GreenBuildingAdvisor

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