Windows are complicated, big openings in a house that can leak both water and air. This article focuses only on air leaks around windows. Preventing water leaks gets even more complicated. (As an example, this article shows one method that works with ZipSystem sheathing. The methods vary when you use a housewrap and vary further depending on whether the window has a nailing flange or not. For a wide selection, see the articles by Gene Summy, Harrison Campbell, Carl Hagstrom, and Greg Burnet).
The goal in air-sealing windows is to seal the gap between the window rough opening and the window unit. Most JLC veterans know they can’t do this by stuffing window gaps with fiberglass insulation. That was a seriously flawed, old-school method. These days, you’re more likely to be handed a can of closed-cell foam, or a roll of backer rod and a caulk gun.
While window flashing is often performed by an advanced installer, air sealing is often a task that builders assign to novice carpenters. That should not be construed as this being a less important task. Both the air barrier and the water barrier are vital to building performance.
WHY WE AIR-SEAL
Like any task, air-sealing is best done when we understand why it’s important. Here are some principles to keep in mind:
- Air carries both heat and moisture. We air-seal buildings to keep conditioned air (warm or cool, depending on the season) inside the home, and to keep unconditioned air outside. Air-sealing also stops moisture-laden air from leaking through the building and condensing onto cold surfaces inside walls.
- Air is moved by pressure differences. Wind blowing against a house can push or pull air out through cracks. Fans inside the home can also build up pressure that pushes air through a building enclosure.
- No gap is too small. When you seal the gap around a window, you need to remember that any break in the foam or caulk can allow pressurized air to escape. Don’t rush the application.