Step-by-Step Approach to Properly Air-Seal Windows

Originally published by: Journal of Light ConstructionMarch 30, 2018

The following article was produced and published by the source linked to above, who is solely responsible for its content. SBC Magazine is publishing this story to raise awareness of information publicly available online and does not verify the accuracy of the author’s claims. As a consequence, SBC cannot vouch for the validity of any facts, claims or opinions made in the article.

Editor’s Note: As the first sentence in this article states, “windows are complicated.” While this article addresses one approach to effective air sealing, a number of effective approaches to air- and water-sealing windows installed over exterior foam sheathing (very complicated by today’s standards) are outlined in step-by-step guides on continuousinsulation.org/best-practices/window-installation.

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 SummyHarrison CampbellCarl 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.

Air-sealing a window is distinctly different from flashing  a window. Flashing is done to keep water out. It can’t  completely stop air because the bottom needs to be left open  to allow water to drain out of the assembly. A window’s air  seal, on the other hand, is typically applied from the interior.  The bottom of the window is sealed along the backdam, as  shown in the illustration above. This bead of caulk should  be applied when the window is installed. As a backup to the  inner seal shown above, a fillet of caulk can also be applied in  the corner between the blocking used as a backdam and the  window unit, as shown in the photos above.

Air-sealing a window is distinctly different from flashing a window. Flashing is done to keep water out. It can’t completely stop air because the bottom needs to be left open to allow water to drain out of the assembly. A window’s air seal, on the other hand, is typically applied from the interior. The bottom of the window is sealed along the backdam, as shown in the illustration above. This bead of caulk should be applied when the window is installed. As a backup to the inner seal shown above, a fillet of caulk can also be applied in the corner between the blocking used as a backdam and the window unit, as shown in the photos above.

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.

SEQUENCE OF STEPS

Window air-sealing typically takes place after the window is installed, but there is one exception: If a backdam is used, it’s best to apply a bead of caulk along the outer face of the backdam while the window is being installed (see “Window Sill Detail,” above).

Instead of a backdam, some builders may use a piece of beveled siding on the rough sill. The bevel slopes to the outside and helps drain away water that might leak through the window. In either case, there is not enough room to seal the inside with foam; the bottom must be caulked tight instead.

Click here to view the full step-by-step approach to installation the author takes.