A quick guide to air sealing a window

Installing the window and connecting it to the water control layer only gets you partially through the steps needed for a proper installation. Making sure a window is continuously tied to the air control layer is also important. 

When I first started in the construction industry back in the mid 1990s, we insulated the space between the window’s rough opening and the window frame using fiberglass insulation. We packed the fiberglass thinking this was an effective way to stop heat transfer. At that time, we weren’t thinking about air leakage. Fast forward 30 years, we’ve figured out that air sealing this space is more important than insulating it. 

Over the past couple decades, most of us have been using canned foam for this purpose. That approach can be effective, but we also have other methods that work just as well, if not better. In this blog post, I’ll discuss different methods of air sealing a window.

Before we get into air sealing, let’s go over two different window installation methods that can be used. Using one or the other may slightly alter the air sealing detail chosen.

1. Drainage method: In this installation, the exterior rough opening sill is left unsealed to allow any incidental water that may enter between the window frame and rough opening to drain out the bottom. We use sill pan flashing with sloped sills or back dams to aid drainage. The drawback with this system is that in very high windstorms, the water can be pushed into the drainage gap. Those building in areas with strong, wind-driven rains might want to consider an alternate water sealing approach. 
2. Barrier or fully-sealed method: In this installation, all sides of the window are sealed on the exterior so that no water can enter (or leave) the system. This method is less forgiving, should there ever be an issue with the window — such as an installation misstep or an interior condensation problem caused by high indoor humidity. A fully sealed window can trap water between the window and rough framing. If water were to find a pathway into the system, it could become stuck, potentially damaging the structure.

The taped air sealing method for windows is what I have been using over the past couple of years. Though slow to install, and a higher material cost, I feel taped systems provide the best end result for this assembly.

Window air sealing tapes are tough to source. I have only been able to purchase the products online and an online search for “window air sealing tapes” has yet to identify the correct product. Two tapes designed for this purpose that I am aware of are Siga’s Fentrim 20 and Pro Clima Tescon Profect.  Both are from Europe.

This option has both the highest labor cost and highest material cost. Because of this, we typically only see this window air sealing method used on higher-end projects where stringent air tightness is required.

Tips for air sealing a window with tapes

• I start the process by installing a snug-fitting backer rod between the rough opening and window frame; this provides some thermal resistance. If a water leak occurs and you’ve used the drainage method of window installation, the backer rod would help to keep most of the water from contacting the tape, which is important since it might affect the tape’s adhesive bond.   
• The tape itself has a small fold with exposed adhesive. The rest of the tape’s adhesive is covered with a peel-away release liner. The air sealing process starts by adhering the folded edge to the face of the window frame, then peel the release liner and extend the remaining portion of tape to the window rough opening. The portion of the tape that is applied to the window will be concealed by an extension jamb or window casing.
• The tape manufacturers also have an installation method of adhering the fold to the side of the window frame before setting the unit in the rough opening. After setting the unit, the remaining portion of tape is sealed to the window's rough opening. This method can be a little tricky in that you have to make sure the tape remains accessible after the window is set. It’s easy to unintentionally fold the tape into the wrong place while setting the window in the rough opening, which then necessitates digging it out of the rough opening.

Windows can be a challenge to effectively air seal. The only way to know if the sealing method was done successfully is by testing. While blower door testing isn’t a required installation step, it’s a step that’s increasingly common as more high-performance homes are built.

My suggestion is having a mid-build blower door test performed. This blower door test happens while the structure is under construction. Usually, this test will take place after windows and doors have been installed, and all mechanicals and insulating are complete, but before interior finishing begins. This test can help find window installation deficiencies before they get covered with interior finishes that would be costly to remove in order to diagnose and repair the deficiencies. Additionally, it can tell you if the project is on track to meet your overall airtightness metric.   

Blower door testing is the most common method of finding holes in the air control layer, but there is an up-and-coming method that uses sound to test continuity of the air barrier. Stay tuned for more information on finding air leaks using ultrasonic sound.