Residential SW Washington

Tech tip: Protect window installations with more than caulk

High-efficiency windows are a popular home upgrade because they improve energy efficiency and comfort. However, improperly installed new windows can lead to moisture problems and structural damage. In the wet Pacific Northwest, proper installation is especially critical.

Often, the only thing separating a building cavity from a wet winter storm is a thin bead of caulking, and it is best not to rely on caulk as the sole method of preventing bulk moisture intrusion. Here are a couple of important strategies to properly seal windows.

Flanged retrofits: The Existing Homes 2015 Specifications Manual requires that you replace flanged (nail-fin) existing windows with flanged retrofit windows rather than insert (block, pocket, flangeless) windows. We discourage the practice of separating a flanged window from its flange, removing the frame and glazing, replacing it with an insert window and caulking it into place. Replacing flanged windows with insert windows weakens a home’s weather barrier because caulking provides the only defense against moisture, and existing trim wood and caulking will degrade. Flanged windows, on the other hand, are ideal for retrofit applications since they usually can be re-integrated into the weather-resistant barrier. If you have a situation where an insert replacement is necessary, contact Existing Homes for a waiver request.

Drip caps: Also known as rigid head flashing or z-flashing, drip caps are part of the Existing Homes program specifications and another important way to prevent moisture intrusion. Drip caps shield the top seam of the window from weather, which helps prevent seam failure and costly structural damage.

Drip caps must be installed properly to be effective. Don’t caulk the seam where the siding meets the drip cap. If water gets behind the siding, this seam opening allows drainage. It’s important that a drip cap slopes down and out away from the house. A flat drip cap, or reversed slope, will allow water to run backward and toward the corners of the window frame—weak spots where water can potentially leak into the building cavity. Drip caps are necessary only with flanged window installations, not insert windows.

Drip caps are unnecessary when a window is protected by an eave, cantilevered floor or other overhang, but not all overhangs provide protection from the elements. A window is considered protected only if the distance the overhang extends from the wall measures at least twice as long as the vertical distance between the topmost seam of the window/trim and the bottom of the overhang. If you’re not sure if an overhang meets this rule, be cautious and install a drip cap.

These are just a few of the strategies that can help protect against moisture damage. However, they won’t apply to all installations due to the variety of windows and other variables. Contact the Existing Homes team for help ensuring effective, durable installations.