Does window film damage double glazing, and how do you get the right film?

A thin polyester film fitted to the inside or outside of your glass, most often to cut heat and glare from the sun. The Glass and Glazing Federation (GGF), the UK trade body for glazing, describes it like this:

The GGF's window film FAQs say it's applied to the inside or outside surface of windows and doors, to reduce heat and glare from the sun, increase comfort and security, and cut energy use. Whatever the film is for, it sits on your glass, so it has to suit that glass. Here's why.

Through thermal stress: one part of a pane expanding more than another. When sun hits a window, the exposed middle of the glass warms up. The edges, tucked under the frame and in shadow, stay cooler. The warm middle tries to expand, the cool edge holds it back, and the edge ends up under tension. If that tension gets too high, the glass cracks.

The GGF's consumer guide to glass breakages lists this among the reasons any glass can break: the glass's thermal stress capability is exceeded by a large variation in temperature within the same pane.

Film matters because it can change how much of the sun's energy the glass absorbs. The International Window Film Association puts it plainly:

A film that absorbs more heat makes your glass run hotter in the sun, which widens the gap between the middle of the pane and its edges. Which windows are most at risk.

Windows of ordinary (annealed) glass, the standard type that hasn't been heat-treated, in full sun, where part of the pane runs hotter than the rest. The GGF's guide gives these examples of what causes a large temperature difference within a pane:

• Curtains, blinds or other structures close to the glass that cover part of it
• A poster, graphic, sign or paint covering part or all of the pane

Toughened glass (glass heat-treated to be stronger, which breaks into small fragments rather than shards) copes better. Glass maker Pilkington lists increased resistance to thermal stress compared to annealed glass among its properties.

Double glazing adds another layer to the question. A sealed unit has two panes, which can be different types, and one may have a Low-E coating. Film can go on the inside or outside surface, and the right film depends on what's in that particular unit. How to tell what's in yours.

Because the film has to suit the glass, and windows in the same house are often different. A newer back door may be toughened, an older bay may be ordinary glass, and a replacement unit may have a Low-E coating the others don't.

Ordinary (annealed) glass

The standard type, which hasn't been heat-treated. It's the type most affected by thermal stress.

Toughened glass

Heat-treated to be stronger. It breaks into small fragments rather than shards, and copes better with thermal stress.

Laminated glass

Two panes bonded with a clear plastic layer, so the glass holds together if it breaks. From most angles it looks like ordinary glass, so it needs checking rather than assuming.

Low-E coating

An invisible microscopic coating that reflects heat back into your room, as the Glass and Glazing Federation describes it. Because it's invisible, you can't tell by looking whether your glass has one.

On Low-E windows, the International Window Film Association says whether film should be used depends on the type of Low-E surface, where it sits in the window, and what you want the film to do. None of those can be answered by looking at the window. How to find out.

The one that suits your glass and does the job you need, whether that's cutting heat and glare, adding security or saving energy. There's no single best film for every window, because the same film can suit one pane and not the next.

Film manufacturers publish tables that match their films to glass types, and the IWFA suggests asking for a copy if you're ever in doubt. The final choice stays with your supplier or installer, and it's a far easier choice once they know exactly what glass they're fitting to. What to check before you buy.

Check what glass you have, what the film maker says about it, and whether your windows carry a warranty. Then you can compare quotes on the same footing:

• Which type of glass is in each window you want to film
• Which film the installer is proposing, and the maker's guidance for that film on that glass
• Whether your window or double glazing warranty says anything about applied film
• Whether the room already has condensation or mould around the windows

The wrong film on the wrong glass can cost you more than no film at all. Fitting the wrong film to the wrong glass can cause cracking, failed double-glazing seals and voided warranties.

If there's already condensation or mould around your windows, deal with that alongside the film choice. Why mould keeps coming back explains how condensation works.

Have each window measured before you choose a film. On a window film survey I measure every window you ask about and record:

• The glass thickness
• Whether each pane is toughened or not toughened, tested with a Merlin Lazer toughened glass indicator (a handheld tester that shows whether a pane has been toughened, without damaging it)
• Laminated glass, where it can be identified
• The cavity gap (the space between the two panes) in double-glazed units
• Whether a Low-E coating is already present

You get a written report of the measured findings for each window. Take it to your own film supplier or installer, or I can pass it to local film suppliers who can advise and quote. Either way, the film advice you get is based on measurements, not assumptions. If you're a film supplier or installer, the same report gives you measured glass data before you quote. I've completed 3M window film training.

The survey is a diagnostic assessment and does not include installation or film supply. Toughened glass is reported as "toughened" or "not toughened"; the survey does not certify glass as safety glass or as meeting any British Standard. Final film selection and warranty stay with your supplier or installer. See what a window film survey includes and survey prices.