Installing windows for air control

Windows challenge the air barrier. The window assembly fills an intentional hole in the building, which is designed to open and close. Window manufacturers design their products to meet specific airtightness standards; Andersen lists the air infiltration rate on the product label. The engineering and manufacturing processes help ensure the window meets the tested performance requirements. Per building code, the WRB is required to be connected to the window’s full perimeter and should be done as close as possible to the exterior.

As builders, our job is to make sure the window or door is installed so that it is continuous with the home’s air control layer. How we make that connection will depend on which one of the two methods of window installation we choose — the drainage method or barrier method. For a more detailed explanation of the installation methods, check out the first story in this series covering installation details for connecting the water control layer.

The barrier method  

This method is simple. The products used to connect the window to the water control layer typically will also provide the air barrier connection. These products are tapes, sealants, and liquid-applied membranes. 

The drainage method  

The drainage method is more challenging. A drainage gap is left at the bottom of the window to allow any incidental water that finds its way into the assembly, a way out. This gap creates an air leak that requires a different approach to sealing than methods used for the barrier installation method.  

Air sealing will happen either on the interior face of the window, or between the window and framed rough opening. For this to be effective, the primary air barrier assembly needs to extend into the window rough opening. Solid wood makes a good air barrier material, but the connection points, the joints of the opening, need to be sealed to provide continuity.  

Once the window’s rough opening is prepped, the window can be installed per the window manufacturer’s instructions.

When I first started in the trades, we stuffed fiberglass in the gap between the window and rough opening and called it good. We’ve since learned that fiberglass is a terrible air barrier because it’s highly permeable. Today, there are common — and some less common — approaches for connecting a window to the air control layer:

1. Canned low-expanding spray foam
2. Backer rod and sealant
3. Tape
4. Expanding foam gaskets 
   
   

Method 1: Canned low-expanding spray foam 

Canned low-expanding spray foam is a common method for air sealing between the window frame and rough opening. Low-expansion foams designed for windows and doors can be effective air seals and provide a level of insulation, but there are caveats:

• Choose the right product: High-expansion foam can bow frames and prevent windows from operating correctly.
• Continuity is tricky: It’s hard to achieve full side-to-side coverage if shims or tight clearances get in the way.
• Drainage must be considered: Foam should not fill the cavity front-to-back. Leave space near the exterior so any water that finds its way into the assembly can still drain.
• Durability is suspect: Wood framing moves as it dries and twists. Foam doesn’t always move with it, and seals can fail.

My recommendation: Because of these risks, I no longer rely on canned foam alone for air sealing windows. If you do use foam, my suggestion is to place a backer rod first to make sure the foam doesn’t completely fill the cavity. This will help maintain the gap for water to drain out of the system.

Method 2: Backer rod and sealant

This method is straightforward: Insert a round foam backer rod in the space between the window and rough opening. Then, apply sealant to seal the joint. The purpose of the backer rod is two-fold: The backer rod creates some R-value increasing the resistance to heat flow. It also gives the sealant a backstop, so you use less.

Other advantages, besides ease of application, include the fact that it’s inexpensive, flexible to accommodate wood movement, and allows you to see if the seal is continuous. Its disadvantages are that it’s slower to apply than foam and it can easily get messy. Finally, it’s not an ideal way to fill wide gaps. If the space between the window and framing is more than ½ inch, spanning with sealant becomes more difficult. 

My recommendation: For modest gaps, this method is reliable and durable.

Method 3: Tape (my preferred method)

Using tape as a window air sealing method requires the tape to span from the window frame, across the rough opening, to the rough opening framing. 

My process typically starts with a backer rod to fill and insulate the gap before applying the tape. While the exact method will depend on the tape product. For example, Siga® Fentrim IS 20 has a narrow strip that bonds to the window frame and a wider strip that attaches to the rough opening. There is also a method where the tape is applied around the outer edge of the window frame before the window is installed.

One great advantage of tape is that you can see if the seal is continuous. The tape accommodates framing movement. It spans larger gaps better than caulk. The disadvantages of tape include the fact that it’s slower to apply and more expensive than spray foam. On a recent project, each window took nearly 30 minutes to detail.

My recommendation: Specialized interior window air-sealing tapes have become my go-to. I use Siga Fentrim IS 20 most often. Other manufacturers with similar products include Pro Clima (Contega Solido SL) and Rothoblaas (Plaster Band In). As far as I know, no North American manufacturer has produced a tape specifically designed for this purpose. 

Method 4: Expanding foam gaskets 

More common in Europe, this method uses a pre-compressed foam strip installed in the rough opening or around the window frame before setting the window. The foam slowly expands to fill the gap. When using expanding foam gaskets, the gaskets must be placed around the window before the window is installed. 

The advantage of this method is once the window is installed and the foam has expanded, air sealing is complete. You won’t have an additional process to complete later. Another advantage is that the interior extension jamb can be in place before the window is installed. 

There are a few disadvantages to be aware of. You have to work quickly when installing the window. If the gasket expands before the window is fully installed, you may have to start over. Cost is another consideration. I am not aware of any North American manufacturers making this window air sealing product. You may be ordering materials from Europe. 

My recommendation: This method can be effective. I’ve only used similar tapes in steel roofing, where they worked well to seal corrugations. My concerns for window use are the short working times — especially in hot weather when expansion speeds up — and the need for hands-on training and/or practice to use them properly.