How to Tell What Type of Glass You Have

Guidance is compiled from safety-glazing standards and glass-manufacturer sources, cited in Sources.

Why it matters before you film

Your glass type decides what film is safe: whether it can crack from thermal stress, whether a coating is already there, and whether the warranty applies. A few minutes of identifying beats a cracked pane. Here's how to tell — and where each test is definitive versus just a clue.

Is it tempered (safety) glass?

The one definitive test: look for a permanent etched or sandblasted mark ("bug") in a corner. Safety-glazing rules require tempered glass to carry a mark that "cannot be removed without destroying it," showing the maker, the standard (ANSI Z97.1 and/or CPSC 16 CFR 1201) and the type.[1][3] If you find that mark, it's tempered — done. It's most likely in doors, glass next to doors, shower/tub enclosures, and large low panes, where code requires safety glazing.[4]

Clues only (not proof): through polarized sunglasses in daylight, tempered glass often shows faint dark spots or strain bands from the tempering process; reflections may show a rippled roller-wave distortion near the edges.[8][5] These indicate heat treatment but can be faint or absent — and the mark can be hidden by the frame, so "no mark found" means "unknown," not "annealed."[1]

Single or double pane?

Look at the edge of the glass where it meets the frame. A visible spacer bar (a metal or foam strip, often with a desiccant inside) between two panes means it's a sealed double-pane insulated unit; no spacer means single pane.[8] A quick confirmation: hold a small flame or flashlight near the glass and count the reflections — a single pane shows about 2, a double-pane shows about 4.[6]

Is it Low-E / coated glass?

Use the flame test (best after dark): hold a lit match or lighter a few inches from the glass and look at the row of reflected flames. Uncoated glass reflects them all the same orange. If one reflection is a noticeably different color — often pink, violet or blue — that surface carries a Low-E (or metallic) coating.[6] Counting which reflection is off-color tells you roughly which pane surface it's on.[7] Caveat: the flame test confirms that a coating exists and roughly where, but it can't tell a durable hard-coat from a film-sensitive soft-coat Low-E, or confirm the gas fill — so treat it as a strong clue, not the last word.

Is it laminated?

Laminated glass is two glass layers bonded to a plastic (PVB) interlayer. The definitive tell is a layered "sandwich" edge — but once the glass is framed, that edge is usually hidden, so this is often hard to confirm on installed windows; a duller "thud" when tapped is only a weak clue.[8] Laminated glass shows up in some entry doors, skylights and security/hurricane glazing.

No mark, not laminated? Treat it as unknown

Ordinary annealed (float) glass is the default when there's no tempering mark and no lamination — but remember the mark can simply be hidden. Never test by trying to break, drill or score the glass. If you can't confirm the type by inspection, treat it as unknown and check the window or film manufacturer's compatibility guidance before applying film — especially for a dark/solar film, where the annealed-vs-tempered question is exactly what decides the thermal-stress risk.

Which tests to trust

Affiliate disclosure: HowToApplyNow may earn a commission from qualifying purchases made through links on this page, at no additional cost to you. Products are added only after we vet the program and the terms — and only when a specific product's own instructions support their use for this task.

FAQ