Air Sealing Windows - New and Existing

Key considerations:

Many recognize their windows to be a big source of winter drafts. Sash that are not far from the point of rattling in their tracks, which don't latch tightly, are warped, or which have worn out air seals are the subject of feverish seasonal air sealing for many. The more aware are also sealing windows on upper floors where they know that warm air will be leaving just as surely as cold air will be sucked in below. Yet air leaks often as large as those being sealed, and which could be sealed permanently go undetected and un-addressed.

All windows have a shim space between their frames and rough openings into which they're installed. This space is typically about 1/2" at top and sides. Under the sill, the space is usually wedge-shaped corresponding to the slope of the sill, and it's likely to be an inch or more wide at the inside. This shim space is typically covered by trim, "casings" and "aprons," inside and out. Despite such closures, these are direct airways between inside and outside. Air tightness here is beyond the reach of tight carpentry done to even the best standards of the trade. One needs sealants.

Traditional practice is to stuff shim spaces with conventional batting. Such work is typically assigned to laborers, such as the author, who did it as "apprentice-boy & nail banger" in the 1960's. Aside from the notorious limitations of batting as an air seal, and those shim spaces too narrow to stuff, quality of such detailing is marked by large standard deviations. Sealants work better, much better. Notice that I am writing about air sealing and not insulation. These spaces really are a minuscule part of a building's skin. An effective air seal, not high R-value, is what you're after.

It is equally important to understand that windows are the weak point in a home because they have low R-value. Even a modern 2 pane window has only a fraction of the R-value of a typical wall, somewhere between R2 and R3. Anything you can do to increase the R-value of a window, be it a storm window, window blinds or curtains will help the window perform better and increase comfort. Windows often feel cold not just because of air leaks but because they are the coldest objects in the room and we radiate our heat energy to cooler objects. See also Windows.

Possible downsides:

Expanding sealants may "push" window jambs: This was a problem with older generation sealing foams when used with lightly built windows. Today's formulations are slower rise. Some, including the better brands of "gun" foam are labeled for window applications. The goal is not to fill the shim space but to seal it. An inch deep bead of any foam will be sufficient and is unlikely to deform jambs. Try to apply it close to the outside if the exterior casing is installed, The casing will brace the jamb. If you really want more fill in the space, add cheap, correctly sized backer rod, not expensive foam preferably after foam.

You have to remove either interior or exterior trim: Yes, this is a "bullet to bite" with existing windows and finishes. Consider the alternative, extensive interior or exterior caulking with sticky caulk. You might not like the look. A thorough job caulks trim to walls, to jambs and to sills, all edges in other words. Exterior caulking especially will have to be renewed periodically. Caulking may unintentionally trap moisture that previously was able to dry to the inside or out. Finally, with old, weighted sash, if you take off casings, you have a chance to improve the sash weight pockets, in aggregate a BIG "hole."

Condensation: Especially with metal or metal clad jambs, one tries to keep the interior part of the jamb as warm as possible to retard condensation of interior humidity. Nothing will chill a jamb to below dew point faster than a steady stream of winter air through the shim space. Sealing the space close to the exterior side, gives the jambs a reasonable chance to stay dry.


Sealants either are, or must at least start out, sticky. These will be either air sealing foam or one of a variety of caulks. For those spaces and details that are too narrow for foam, use caulk. Understand that air can move under and around shims, spacers or mounting clips. This will be really important for those striving for state-of-the-art air tightness, so caulk here. The use of expensive caulks and compressible foam backer rod will be familiar to those experienced with heavy commercial and institutional construction.

Invest in a foam gun that allows actual control of delivery, So-called straw foams can be more like a grenade than a gun. A foam gun with screw on cans of foam is much easier to control and you will find yourself looking for things to seal because it is so easy to use.

Don't count on foam to expand and fill tight openings. If you can't get at least the tip of the foam gun barrel into the space, caulk it.

Use an "elastomeric" caulk made of urethane or silicone that will remain springy and will compensate for seasonal movement.

With surfaces other than wood, check with the manufacturer (read: ask for a spec data sheet or find on-line) about long term adhesion of sealants.

Single-part urethanes need a bit of moisture to cure properly. On hot days, in direct sun and especially with metal surfaces, it will be necessary to dampen surfaces before foaming. A hand held misting bottle works well for this.

Observe manufacturers' advice on temperature limits; sealants must set before they freeze.

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