Flashing and sealing a window against wind-driven rain

Stand outside during a hard Austin storm and watch where the rain actually travels. It does not fall straight down. It is driven sideways against the wall by the wind, and replacement windows austin gagewood photo covers it in detail, because that sideways water finds the one gap in a window opening that straight rain never would. A replacement window in Austin, TX keeps water out through layers lapped like shingles, and the sealant is only the last of them.

The sill comes first

Before the new unit goes in, the bottom of the rough opening gets a sill pan, either a formed metal or plastic tray or a taped membrane that turns up at both ends. Any water that gets past the window lands in the pan and runs out to the face of the wall. Without it, water sits on the framing, and the wood below a window is where rot starts on most houses a decade after an install.

Flashing tape in the right order

Tape goes on in a sequence. The bottom piece goes on first, then the sides, and the head piece last, each one overlapping the one below it so water can only run over a lap and never under one. Corners are folded and pressed flat. Tape is smoothed with a roller until it has no fish-mouths. A head flashing that lies on the wrong side of the wall layer is a leak waiting for a thunderstorm.

Backer rod and sealant

Where the window frame meets the siding or the stucco, the joint is filled with foam backer rod and capped with sealant. The rod sets the depth, so the bead is wide and shallow and can stretch as the gap changes. Austin summers have long runs of high heat, and a vinyl window or aluminum window grows and shrinks against its surround every day, so a bead without backer rod splits along the middle in a couple of seasons.

Weeps and the glass itself

Many frames have small weep holes at the bottom of the track. They drain water from the sill and must stay open, so sealant never goes across them. Insulated glass units are sealed at the factory, and a double pane or triple pane unit with a low e coating and a warm edge spacer will not fog unless the edge seal fails. Seal failure is a manufacturing or handling fault, not something caulk can fix on site.

The test

The last step is water. A hose, run from the bottom up, with a second person watching the inside for dampness, shows whether the layers work before the trim goes back on.