Air sealing flanged window
stephenr
| Posted in General Questions on
Hi, what is best practice for air sealing a flanged window when the bottom flange is left untaped and un nailed to allow water to escape? Do we rely on spray foam and tape at the interior?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Stephen,
Yes. Typically the primary air-sealing is done on the interior face of flanged windows on all four faces. The tape on the exterior on the head and jambs does act as a secondary air-barrier but it is primary to stop water intrusion from the outside.
The foam which get used at the head and jambs can also act as a secondary air-barrier, but it's best to rely on caulking and backer-rod or tape at the interior face. The only difference at the sill is the omission of the insulation, and a gap in the exterior taping to allow water to escape.
If the air barrier layer is the exterior sheathing or WRB, you want to continue that layer into the window opening such that the inner plane of the window can be bridged to that on all four sides.
At the very least, this means taping from sheathing/WRB to rough framing, but also includes sealing framing intersections (corners). I prefer a fully taped window RO, where the tape extends past the interior plane of the window on all four sides.
Then, I like to use low expansion spray foam as a "backer rod" around all four sides, and then sealant to connect the window jamb to the framing.