Insulation Between Windows: The Right Way and The Wrong Way

When windows are installed, there is a gap all around the windows that allows the installer to center the window and make sure that the window is level. Before finishing the project, the gap between the windows is insulated to add R-value and air seal the edge of the window. This area is often neglected and is a major source for air infiltration. What is worse is that people fix this issue with the wrong types of products. Some of these products are designed for this application, but still do not work. In short, there is a right way to insulate a window, and a wrong way. There is nothing in between.

Can Foam Insulation

While this product has many great uses, it is not a good material to use for sealing windows and doors. The main reason for this is because window/door spray foam is open cell foam that expands significantly more than typical closed cell can foam that is used for filling large gaps. The reason for the open cell foam use in window door applications is because of the significant amount of expansion and contraction that the window will undergo throughout the years. Unfortunately, this expansion and contraction loosens the bond between the foam and the window and the framing allowing air to penetrate through the cavity and into the home. Because this product is so easy to use and quicker to install, it has become the primary insulation material for windows and doors.


While caulking works well for small gaps, it is not as effective with large gaps like those found in typical window installations. The rule of thumb is that any gap over ¼ inch wide should have another method of sealing besides caulking. The method of installing foam backer rods and caulking the edges around a window do work, but eventually the caulk seal breaks and air infiltration begins.

Fiberglass Insulation

While fiberglass insulation is not a very good insulation material, it is a great air sealer when stuffed tight into a cavity. For this reason, ripping off layers of fiberglass insulation and stuffing them between windows and doors and their framing, works very well at eliminating air infiltration. The key to properly insulating with this method is to push the fiberglass strips all the way to the back of the exterior sheathing. Continue to pack insulation into the cavity. Using a putty knife or a carpenter’s square works well for getting the insulation far enough into the cavity. One word of caution, make sure that you do not push too hard in the middle of the jambs of the windows or it may hinder the operation of the windows.

While stuffing fiberglass between the framing of the windows is a very old method, it is still the best way to insulate and air seal the perimeter of a window. It is, however, much more time consuming than newer can foam methods but will last as long as the house is standing.