Can window film crack glass?

How film puts stress on glass

Glass grows slightly as it warms. In a sunny window, the middle of the pane heats up while the edges, tucked into the frame and in its shadow, stay cooler. The warm middle expands, the cool edge doesn’t keep pace, and the edge gets pulled. Glass handles being squeezed well and being pulled badly, and the edge is its weakest part, because cutting leaves tiny flaws there. When the pull is more than the edge can take, a crack starts at the edge.

That can happen to bare glass too. Film adds to it when it absorbs sunlight. A film that absorbs a lot of solar energy turns it into heat in the glass, so the sunny middle runs hotter than it would without film, and the gap between middle and edge grows. Clear and light films generally absorb less; darker films and some heat-control films absorb more. The more a film absorbs, the more the glass type matters.

A thermal stress crack usually starts at the edge of the pane, runs in at roughly a right angle to the edge and then wanders. A crack that spreads from a point in the middle of the glass usually has another cause, such as an impact.

Which glass needs a check

• Large annealed panes. Annealed glass is ordinary glass that hasn’t been heat-treated. It’s the most sensitive to heat stress, and bigger panes build bigger temperature differences. Older homes and fixed picture windows often have it.
• Dual-pane units. Film goes on the inside pane, which takes the heat the film absorbs, and the air space behind it holds that heat in. The inner pane runs hotter, and the unit’s edge seal sees more heat. Film makers often limit which of their films go on insulated glass.
• Low-E glass. A low-E coating changes how the panes absorb and pass heat, and which surface the coating is on matters. Some coating and film combinations push the inner pane hotter than others.
• Laminated glass. Two layers of glass with a plastic interlayer that absorbs some of the sun’s energy. The glass in it is often annealed, so it needs the same care.
• Tinted glass. Body-tinted glass already absorbs heat. An absorbing film on top adds more.
• Partly shaded glass. An eave, a lanai roof, a railing, a tree or a neighbor’s building can shade part of a pane while the rest sits in the sun. The shaded part stays cool and the stress builds along the shadow line. Things inside count too: heavy curtains close to the glass, furniture pushed against it, signs and decals stuck to it, or an air-conditioning vent blowing on it.
• Damaged edges. Chips and small cracks at the edge are where a stress crack starts. Glass that’s already cracked gets repaired or replaced before any film goes on.

Tempered and heat-strengthened glass handle heat stress far better than annealed glass. Glass doors and sliders are usually tempered, which takes most of the heat-stress worry off those panes, though a dual-pane slider still needs a film that suits insulated glass.

What the film maker’s guidance does

Makers of architectural film publish film-to-glass guidance for their products: which glass types, thicknesses and pane sizes each film suits, and where it shouldn’t go. It’s the reference for matching a film to a window. Sometimes the answer for a given window is a lighter film than the one you had in mind, and now and then it’s no film on that pane.

Why the installer looks at the glass first

Choosing the film before knowing the glass is how problems start. A proper check covers:

• Single or dual pane. Hold a small light or a pen tip to the glass: each surface shows one reflection, so a dual-pane unit shows four.
• Tempered or not. Tempered glass usually carries a small permanent stamp in one corner.
• Coatings. Low-E and reflective coatings change which films are suitable.
• Size and edges. Big panes and chipped edges raise the stakes.
• Shade. Where shadows fall across the glass through the day.
• Old film. Existing film comes off before new film goes on.

At Offshore the glass is checked before a film is recommended, whether it’s a home, a condo or a business.

What you can do

• Tell the installer what you know about the windows: their age, the maker, any labels on the glass and any warranty paperwork.
• Read the window warranty’s terms on aftermarket film if the windows are fairly new.
• Keep heavy drapes and furniture off filmed glass, and don’t stick signs or decals onto it.
• Have cracked or chipped panes fixed before film goes on.
• If a crack ever appears, note where it started, at the edge or in the middle, and photograph it.

Next step

Home and commercial film is quoted after a measure, with no online price or checkout. Send the address, photos of the windows and any labels on the glass, and the rooms or openings you want filmed through the quote form, or call (808) 707-7064. The home window tint and commercial window film pages cover the film types, and the guide to window film and fading covers what film does for floors and furniture.