Do Window Insulation Film Kits Work? An Installer's Answer

A kit is a roll of thin, clear heat-shrink plastic and a roll of double-sided tape. You run the tape around the inside face of the window casing, stretch the film across the whole window, press it to the tape, then warm it with a hair dryer until it shrinks smooth and taut. The result is a pocket of air a few inches deep between the film and the glass, sealed on all four sides by the tape.

Still air is a good insulator as long as it stays still. Trapping a pocket of it over the window does two things. It puts a layer of air between the room and the cold glass, so the room side of the pocket is warmer than the glass would be. And it blocks the drafts at the sash edges, the meeting rail and the sash-to-stool joint, because the room air can no longer reach them.

The second effect is what most people notice. On an old double-hung with worn pile seals and a loose meeting rail, the film stops the moving air, and moving air is what feels cold. The insulation gain is real but small.

The film is very thin, thinner than the polyethylene sheeting sold for vapor barriers, so its own insulating value is next to nothing. The gain is all in the trapped air, and published estimates put that at roughly an added R-1 to R-1.5 depending on the depth of the pocket. For scale, a standard double-pane window is about R-2. A film kit over a double-pane window is a small improvement on a modest number; over single-pane glass it is a meaningful one.

The film does not touch the frame. Air that leaks between the window frame and the rough opening, behind the casing where there is no foam or the foam was never done, comes into the room around the casing and out the gap between casing and drywall. The tape sits on the casing face, inside that path. If your cold air is coming out at the edge of the trim, film will not stop it.

It does not change the glass. A window with intact seals and a Low-E argon glass unit loses heat by conduction through the frame and glass, and by radiation off the glass surface. The film slows the first a little and the second not at all. It also blocks any hope of solar gain through a south window being felt directly, since the film absorbs and re-radiates some of it into the pocket.

And it is not a vapor solution. The pocket is sealed from the room but not from the window, so moisture that leaks in from the room, or from a leaking sash, condenses on the cold glass inside the pocket where you cannot wipe it. On a leaky window this can leave the stool wetter than it was without the film.

Film earns its place on windows where the glass is the problem, the sash leaks, and you are not going to open the window until spring. That covers a lot of old houses in the metro.

• Single-pane windows with no storm, or with a storm that is missing its glass. This is the best case; the pocket roughly doubles the insulating value.
• Fixed or rarely opened windows: basement windows, a stairwell window, the window behind the couch.
• Windows on the north and northwest walls where wind hits and no sun ever comes in.
• Rentals where you cannot change the window and the landlord will not, provided you deal with the tape carefully.
• Rooms you have already given up ventilating for the winter: a spare bedroom, a three-season porch you are heating a little.
• A window with a failed double-pane seal that you plan to replace next year. Film is a one-season bridge.

The tape is the problem, not the film. It is a strong double-sided acrylic tape meant to hold a taut sheet for five months. When it comes off in April, it takes the weakest layer with it. On latex paint that was applied over old oil or over dust, that is the paint. On a stained and varnished wood casing, the finish can lift in a patch. On wallpaper, the paper. On MDF casing with a thin factory primer, the tape can pull the primer and fuzz the surface.

Vinyl and painted metal casings usually survive, but the tape can leave adhesive residue that collects dust and is hard to remove without solvents that also soften the vinyl. Freshly painted trim, less than a month old, is the worst case; the paint has not fully cured and the tape bonds to it.

To limit the damage: put the tape on the jamb face or the stop rather than the face of the casing, where a lifted patch is hidden by the sash or the film next year. Test a short piece in a hidden spot for a week and pull it. Remove the tape warm, with a hair dryer on it, pulling slowly back on itself rather than out. Dissolve residue with a citrus adhesive remover, not a solvent.

If a casing is stained wood you care about, do not tape it. Use one of the stopgaps below instead.

Most of what the film is doing, stopping drafts at the sash, can be done at the source for less effort and with no tape on the trim. Work through these in order before you reach for the kit.

It moves it. Condensation forms where room air meets a surface colder than its dew point. With no film, that surface is the glass, and you see the water. With film, the film itself stays close to room temperature, so it stays dry, but any room air that gets into the pocket, through a poor tape seal or over months of diffusion, finds the glass just as cold and condenses there, out of reach.

On a tight double-pane window that rarely matters. On a leaky single-pane window with a wet stool every morning, it can mean the stool keeps getting wet and you do not see it until the film comes down and the finish is stained. If your window sweats heavily, the film is not the fix. Lower the indoor humidity, run the bath fan, and deal with the leaking sash. Our post on winter window condensation covers what is normal and what is a warning.

Film is a one-season bridge, and there are windows where every season it goes up is a season the real problem gets worse. A window with soft wood at the stool or the bottom of the jamb has water getting in from somewhere; film hides it. A double-pane window with fog between the panes has a failed seal; film does not change the U-factor of a dead unit, and it is not something to live behind for years. A window where cold air pours out from behind the casing has an unsealed frame, which is a carpentry fix, not a plastic one.

The other wrong case is a good window. A modern double- or triple-pane unit with Low-E coatings, argon fill and a warm-edge spacer, foamed and sealed into the wall, already has the still-air layer built in and sealed properly. A film pocket over it adds little, costs a coat of paint on the casing, and blocks the sun the Low-E was chosen to let in. Spend the same evening on the doors instead: a worn sweep under an entry door leaks more than any window.

Our guide on drafty windows, fix or replace, walks through how to tell a fixable sash from a window whose time is up.

JMB Builders is a licensed Minnesota residential building contractor (BC809005) based in Zimmerman, working across the Twin Cities metro. We do the permanent versions of what the film is trying to do: weatherstripping, foaming and sealing the frame behind the casing, storm windows, and replacement when a window is done. Call 763-204-2099 or send photos through the site.

• Fall weatherproofing checklist: weatherstripping, sealing and storms before snow — jmbbuildersmn.com/services/insulation/fall-weatherproofing-package
• Door sweeps and exterior door weatherstripping — jmbbuildersmn.com/services/doors/door-sweep-and-weatherstripping
• Cracked or stained drywall returns around a window, repaired and air sealed — jmbbuildersmn.com/services/window-trim/drywall-window-returns-repair
• Labor-only installation of windows you bought at a store or online — jmbbuildersmn.com/services/labor-only
• Send three photos per window, written quote within one business day.