A $3 Sheet of Plastic Can Halve a Window's Heat Loss

Bare glass versus the same window with a taped film: about half the heat loss.

Hold your hand near a bare window on a cold night and you can feel the heat leaving the room. The fix is cheaper than almost anything else in home energy: a clear plastic film kit, taped to the window frame and shrunk tight with a hair dryer.

On a single-pane window it cuts the heat flowing out through the glass by about half. On a double-pane window, by about a third.

Glass is not the insulator. The air is.

This is the part that surprises people. A pane of glass barely slows heat at all. Almost all of a bare window's insulating value comes from the thin layer of still air that clings to each side of the glass. Those two air layers do the work. The glass just holds them apart.

A film kit adds a second pocket of trapped air, about an inch (2.5 cm) deep, between the film and the glass. That pocket resists heat about as well as the two air layers combined. So the window ends up with roughly double the resistance, and half the heat flow.

Think of a windbreaker over a T-shirt. The windbreaker is paper thin, but the still air it traps against you is what keeps you warm. Same idea on the window.

In dollars: a common 3 by 5 foot (0.9 by 1.5 m) single-pane window in a gas-heated house, over a six-month winter, gives back about $16 a season once it is filmed. A kit covers five windows for $2 to $5 each, so the film pays for itself in the first month or two.

Double-pane windows already have one trapped air layer, so the film gains less, about a third off the heat flow through the glass. Still a good trade for a sheet of plastic, but not worth the bother on a window you open every day.

Hang it on the frame, not the glass

Run the double-sided tape on the frame or the trim, never on the glass. The air gap is the insulation, so you want the film about an inch out from the pane.
Wipe the frame and let it dry first. The tape holds poorly on dust or cold, damp wood, and a film that lets go at one corner leaks air all winter.
Shrink it with a hair dryer on low, working from the center out. Wrinkles are fine. Gaps at the edges are not.
Start with the windows you feel from the couch: the big one behind the sofa, north and east bedrooms, and any single-pane window in the house. Leave one window in each bedroom easy to open for an emergency exit, or use a kit you can peel back in seconds.

The mistake I see most often is film pressed flat against the glass with a squeegee, like a phone screen protector. It looks tidy and does almost nothing, because there is no air gap left to insulate. One homeowner I helped had done every window that way and wondered why the room by the big window still felt cold. We peeled it off, taped a new sheet to the trim an inch out, and the difference was obvious by hand within an hour.

People ask whether to film the south windows, since last week's post told them to open those blinds for free sunlight. My answer is yes, film them too. Clear kit film passes most of the sunlight, so you keep the daytime gain and lose far less at night. Bubble wrap is the one to keep off a south window, because it turns the view milky.

When film is not the answer

If a double-pane window has fog or water between the panes, its seal has failed and the trapped air that made it a double-pane window is already gone. No film fixes that. It is a window contractor's job, not a furnace tech's. The permanent version of the film trick is a low-e storm window, which ENERGY STAR credits with saving about 10 percent of heating and cooling costs over bare single-pane glass and paying back in about three years.

Go deeper: where a window's R-value actually comes from

A window's U-factor is the heat passing through one square foot per degree of temperature difference, in Btu per hour per square foot per degree F (W/m2K). Resistance, R, is the inverse. A bare single pane stacks up as: interior still-air film R 0.68, exterior film in a 15 mph (6.7 m/s) winter wind R 0.17, 1/8 inch glass R 0.03. Total R 0.88, U 1.14, call it 1.1. About 97 percent of that resistance is air.

Tape a film an inch out and you add an enclosed, unventilated air cavity, which the Seattle Energy Code tables rate at R 0.91. Total R 1.79, U 0.56, so half. A clear double-pane unit near U 0.5 (R 2.0) goes to R 2.9, U 0.34, a third less.

U-factors from the tables above. Lower means less heat through the glass.

Seasonal loss is U times area times temperature difference times hours. For 15 sq ft (1.4 m2), a 28 degree F (16 K) average difference and 180 days (4,320 hours): bare single pane 1.1 x 15 x 28 x 4,320 = 2.0 MMBtu (590 kWh). Film saves 1.0 MMBtu, 10 therms of heat, 11 therms of gas at 90 percent AFUE. EIA's 2025 residential average of $15.34 per thousand cubic feet is about $1.48 a therm, so $16 a window per winter. Double pane: 0.28 MMBtu, 3.2 therms, about $5.

A Pusan National University test (KS F 2278, Korean HVAC journal, 2015) measured bubble wrap, Korea's version of the trick: bare single glazing 3.59 W/m2K, 2.50 with the sheet over the frame, a 30 percent cut; frame mounting beat glass mounting by about 0.5 W/m2K. Bubble wrap trails a smooth film with a one inch gap: its bubbles are 4 mm deep and only a quarter to a third of the area is air.

Left out, both in the film's favor: a taped film also stops air leaking around a loose sash (ENERGY STAR credits storm windows with cutting whole-house leakage 10 percent or more), and a warmer inside glass surface makes the room feel warmer at the same thermostat setting.

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