Construction Design-Air Sealing Windows

I often hear during energy audits, complaints about windows.  Many homeowners feel the windows are cheaply made and replacement will result in substantial energy savings and an increase in comfort for the home.  Sometimes the windows are a major cause of comfort problems, more often, the window installation is the issue.  This blog post is about three different ways we air seal windows today.

Before we get into the techniques used to air seal windows, we should go over how windows are installed.  Starting with the rough opening, most window openings are built to be one half to two inches larger than the window itself.  This allows room for any leveling of the window when they are installed in the opening.  Personally, I like a slightly larger rough opening, in the past, I’ve had to tear apart framing because the window was too tight and could not be correctly leveled.  I usually plan on one-inch larger rough opening than the window size, half inch of an inch all the way around.

The sill rough framing should also have a pitch towards the outside.  This pitch will allow any water that finds its way to the sill a way to drain out.  This pitch can be created by cutting the framing supporting the sill at a slight angle or by placing a piece of clapboard siding on top of the windowsill frame.  Another option is called a back dam, a little beyond the intent of this blog post, something I’ll cover in a future post.

Before the window gets installed, we need to protect the rough opening at the sill from water.  This protection can be provided in a few different ways.  The most common in my market is with some sort of tape, but a fluid applied membranes and plastic, or metal pans can also be used.

The windowsill above was protected using a flexible tape designed to “bend” around a corner.  There are a few manufacturers of this type of tape, the one in the photo is Henry’s Butyl Flash.  Tyvek (FlexWrap) and Zip (Stretch Tape) are other options.

This windowsill was protected using Zip’s Liquid Flash.  There are other manufacturer’s of fluid applied membranes, Prosoco is another I’ve used in the past, both work well.

I prefer to air seal all sides of the window rough opening using tapes or caulks.  Any gaps or cracks in either the framing or where the sheathing is fastened to the framing can allow air to move through the window cavity and into or out of the wall.  Sealing these connection points can help reduce these potential air leaks.

Once the window is installed, (the majority of window installations in North America are flanged windows, there are other types as well which require different install methods), the window is taped on the exterior to the water resistive barrier (WRB) to prevent water from leaking into the window rough opening.  We do not tape the bottom of the window; this remains open to allow any water that does find its way into the rough opening an escape route out.  (Nearly all windows will have some sort of water leak sometime in their lifespan.) The photo below shows the top and sides taped, but the bottom is not taped.

Something else I prefer, when ordering a window, I do not want an extension jamb to be factory applied.  An extension jamb is a finish part of a window that extends the window to the interior window trim.  The extension jamb can be wood, some sort of PVC or other man-made product, or simply drywall.  I feel when the jamb is present during installation, it complicates the air sealing of the window.  I typically build the jamb and window trim as one piece and install at the finishing stage of construction.

Now that we have an installed window, we can discuss ways to air seal on the interior.  Back when I first got into the construction industry, the space between the window and window framing was simply filled with scrap pieces of fiberglass insulation and then the window trim installed.   The issue with fiberglass is air can easily move through the fibrous material.  We weren’t thinking about air sealing back then.  Times have changed, now know we want to achieve a better level of air tightness in new homes, this requires use to use something other than fiberglass.

Canned Spray Foam

The mid-1990’s was the first time I recall seeing someone use canned foam to air seal anything.  Thinking back to those times, we had a misunderstanding of how the product performed.  I can remember trying to use canned foam to seal water out of an assembly or as a way to glue two products together.

For windows, the product can work as an air seal.  There are a few rules to follow and things to be aware of.  First, choose the right type of canned foam.  There are formulas specifically designed for low expansion use in window and door applications.  The high expansion variety can cause the window or door frame to bow and prevent them from operating correctly.  Next, be sure to fill the entire space side to side, the application needs to be continuous.  This can be challenging when the space between the window and window framing is tight or there is a part of the window installation that is in the way, such as a shim.

The downside to using canned foam, you cannot see how well the foam is sealing.  It may look continuous, but gaps may be present, these gaps are usually detected during a blower door testing.  The spray foam should not fill the entire cavity front to back.  I know, sounds counter intuitive, but there needs to be a space near the exterior side of the window that will allow any water that should leak in a way to freely drain out.

Another concern is the longevity of the installation.  Wood products tend to move, especially some of the wood we are using for framing today.  Wood can expand and contract depending on moisture content, and I’ve seen some framing twist horribly as it dries out.  If this movement happens after the spray foam has been installed around a window or door opening, chances are, the seal is no longer intact.

Because of these deficiencies and risks in air sealing with canned foam, I have moved away from using this product to air seal around windows and doors.

Backer Rod and Caulk

I’ve used this technique a few times.  You simply fill the space between the window and window rough framing with a backer rod, then use a caulk to seal the window to that framing.  For those who do not know what backer rod is, it’s a round foam that comes in strips that can be tucked into tight spaces to act as a filler.

Simply tuck the backer rod between the window and rough framing using some sort of thin tool, like a puddy knife or thin prybar.  The backer rod should tightly contact both surfaces, window and rough opening, but not be so tight is causes the window frame to flex.

Once the rod is in place, a quality caulk is applied between the window and rough framing.  As you can see from the photo, this can get a little messy.

There are advantages to using the backer rod and caulk method of air sealing a window.  The biggest is you can see if the air seal is continuous.  It’s inexpensive, the times I’ve used this method, I was able to use up several open tubes of caulking that probably would have ended up in the garbage.  It’s also a fairly quick process.  I worry less about the movement of wood using this technique.  The caulking is flexible, some movement should not affect the air seal.

The disadvantages, it’s slower than the spray foam method.  It does take a little time to effective air seal using the backer rod and caulk method.  It can also get messy.  If you are worried about having a clean window, you may need to tape any surfaces before caulking to keep the caulk from being seen after the trim is installed.  It also doesn’t work well if the gap between the window and window rough framing is wide.  My preference of a one-inch difference in window to opening size can create an issue for this method, the bigger the space, the harder it is to span using a caulk or other fluid applied sealant.


The last method to air seal a window, and my preferred method is by using tapes.  There are a few tapes designed for this purpose, the manufacturer I’ve used the most is Siga and their Fentrim 20 tape.  This tape has a 15 mm adhesive area designed to contact the window and a 60 mm area with liner that needs to be removed that spans to the rough framing.

Before installing the tape, I prefer to use a backer rod as a filler and insulation between the window and window rough framing.  The photo below shows the installation of a backer rod at the Barndominium project.  The space between the window frame and rough opening was a little bigger than normal, a two-and-a-half-inch difference between window and rough frame.  The backer rod in the photo is 1.25 inch round.

Once the backer rod is in place, taping starts by applying the 15mm side of the Fentrim 20 to the window frame, then extending the 60 mm side to the rough framing.  This condensed video shows the process.

There are a few manufacturers of interior air sealing tapes, in addition to the previously mentioned Siga,  Rothoblaas, an Italian company, has a product called Plaster Band In, designed for interior use and Pro Clima has a few different versions of Contega Solido.  Unfortunately, I am not aware of any North American companies manufacturing a tape designed specifically to air seal the interior of windows.

Advantages to using tapes, again, you can see if it is not a continuous seal.  Tapes will also allow for movement in framing.  It is also easy to span larger gaps between the window and rough opening than with the backer rod and caulk method.

Disadvantages, they are slow to install.  The windows at the Barndominium project had about 12 lineal feet of gap that needed taping.  Between installing the backer rod and then taping, it took nearly 30 minutes per window.  The tapes are more expensive.  They also take some practice to become proficient at installing.

There is a fourth way to air seal a window, one that is used more in Europe.  I personally have not seen this method in person.  It’s the use of a foam expanding tape installing in the rough opening right before the window is installed.  This foam tape slowly expands to fill the gap.  I’ve seen videos of it used on flangeless windows.  I have used this type of expanding foam tape on steel roof projects where the corrugations in a steel panel installed in a roof valley were filled using this product.  My experience with the product is you have a fairly short working time, and the hotter the temperatures, the faster the product expands.  I like the idea but would want some on-hand training and practice before using that technique on a project.

There you have it, three different ways (plus a fourth I have yet to try) to air seal windows on the interior.  What are your experiences in window air sealing?  Leave a comment.

4 Replies to “Construction Design-Air Sealing Windows”

  1. Really glad I read this Randy as I am installing my windows in a few months with Siga Tapes. I only have 8 windows so nice to put a timeframe to shoot for on the details.

  2. Thanks for this helpful article. When I build my “someday” house, I will be sure that leaks around windows & doors are sealed properly during construction.

    I just discovered Siga a few weeks ago. Have you used any of their other products besides Fentrim 20? They make a self-draining building wrap that looks interesting.

    1. Hi Victor,
      I have used several of Siga’s products including Majvest, Majrex, Wigluv, Rissan and Fentrim 430. All very good products.

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