Yes, on most windows, as long as you choose a film that reflects more of the sun than it absorbs. The risk with the wrong film is thermal stress: the glass gets hotter than it was built for and can crack, or the seal of a double-pane unit can fail and fog. Here's how to tell what glass you have, which films are low risk on each type, and what never to film.
Why a film can crack glass
Window film works by reflecting and absorbing sunlight before it reaches the room. The absorbed part heats the glass. The middle of a pane sits in the sun and warms up; the edges sit in the frame and stay cool. That difference pulls on the glass, and ordinary annealed glass can crack from the edge if the difference gets large enough. Tempered and heat-strengthened glass handle it: Solar Gard's film-to-glass guide says windows made of tempered or heat-strengthened glass are not at risk for breakage.
Double-pane (insulated) units add a second problem. The sealed air space keeps the heat in the inner pane, and the hotter unit stresses the edge seal; a failed seal shows as fog or moisture between the panes. Low-E coatings change the picture again, because they reflect heat back toward one pane or the other depending on which surface they're on.
The rule that follows: the more sun a film absorbs, the more care the glass needs. Dark absorbing films (black opaque, dark neutral tints) add the most stress. Reflective silver films add the least. Exterior films shed the heat outside the glass, which is why they're rated for nearly everything.
How to tell what glass you have
- Double pane. Look at the edge of the glass inside the frame: a metal or plastic spacer bar between two panes means a double-pane unit. At night, hold a flashlight or lighter at an angle to the glass. Two panes give four reflections, one from each glass surface; a single pane gives two.
- Low-E. In that flashlight test, a low-E coating shows as one reflection with a different colour, usually a faint purple, blue or green. Count the reflections from outside: surface #1 is the outside face, #2 the inside face of the outer pane, #3 the outside face of the inner pane, #4 the room side. Most low-E coatings sit on #2 (common in cold climates) or #3 (common in hot climates). Most windows sold in the last twenty years have one; the label on the glass or the spacer bar often says so.
- Tempered. A small etched mark in one corner of the pane. Patio doors, windows near tubs and showers, and large panes near the floor are tempered by code.
- Laminated. Two panes with a plastic layer between them, visible at the edge as a thin line; common in hurricane windows. Treat it like double pane and ask us.
Never film these
The film makers agree on this list, and so do we.
- Cracked, chipped or badly scratched glass, or a double-pane unit that is already fogged.
- Wired glass, textured or patterned glass, etched glass, and glass block. Film can't bond evenly to them.
- Plastic windows: acrylic (Plexiglas) and polycarbonate. Adhesive film is for glass. Static cling is the only film for plastic.
- Glass that already has a film on it. Remove the old film first.
- Triple-pane units, except with the films marked for them below. Most films are rated tempered-only on triple pane.
- Glass thicker than half an inch, or with one pane over 8 mm, without the maker's assessment.
- Glass with paint, lettering or graphics, spandrel glass, and skylights, unless you ask us first.
Things that add risk
SunTek's installation guidelines score each window; these conditions push a borderline film into "ask first" territory, and two or three together can rule it out.
- A pane over 50 square feet (over 100 is out for most films).
- A window that sits partly in shadow for hours, from a wall, an overhang or a tree: half the glass hot, half cool.
- Heavy curtains or blinds closer than 4 inches to the glass, which trap heat behind them.
- Steel or concrete frames, worn gaskets, or no gaskets at all.
- Altitude above 2,000 feet, and summer highs above 109°F.
- A building with a history of cracked glass or failed seals.
Which of our films are low risk
For Solar Gard and SunTek films, this follows the makers' own film-to-glass charts. For SnapTint-branded films we use the same logic, based on how much solar energy each film absorbs on its spec sheet. "Low risk" is the makers' term; "ask first" means send us the window details and we'll check.
| Film | Single pane | Double pane, clear | Double pane with low-E |
|---|---|---|---|
| Solar Gard Sentinel Plus Silver 35, 20 (exterior) | Low risk | Low risk | Low risk, either coating. Also rated for triple pane. |
| Solar Gard Sentinel Plus Stainless Steel 15 (exterior) | Low risk | Low risk | Low risk. Also triple pane. |
| Sentinel Plus Stainless Steel 25 (exterior) | Low risk | Low risk | Low risk on #3; ask first on #2. |
| Sentinel Plus Stainless Steel 45 (exterior) | Low risk | Low risk | Low risk on #3; tempered glass only on #2. |
| Solar Gard TrueVue 40 | Low risk | Low risk | Low risk on #2; ask first on #3. |
| Solar Gard TrueVue 30, 15, 5 | Low risk | Low risk | Low risk on #2; ask first on #3. |
| Solar Gard Silver 50 | Low risk | Low risk | Low risk. Also triple pane. |
| Solar Gard Silver 35 | Low risk | Low risk | Low risk on #3; ask first on #2. |
| Solar Gard Silver 20 | Low risk | Low risk | Low risk on #2; ask first on #3. |
| SunTek Symphony 50, 35, 25, 15 | Low risk | Low risk | Low risk. SunTek rates it for any glass on single, double and low-E double pane. |
| SunTek Silver 35, 20 | Low risk | Low risk | Low risk. |
| SunTek Solar Bronze 35, 20 | Low risk | Ask first | Ask first. |
| SnapTint Clear UV Block | Low risk | Low risk | Low risk. It absorbs only 13% of the sun's energy. |
| SnapTint New Silver Reflection 50, 35, 20 | Low risk | Low risk | Ask first. |
| SnapTint Nano Ultra Vision 70, 50 | Low risk | Low risk | Ask first. |
| SnapTint Nano Ultra Vision 40 | Low risk | Ask first | Ask first. |
| SnapTint Blackout Opaque | Ask first | Tempered glass only | Tempered glass only. |
| Whiteout Opaque | Low risk | Ask first | Ask first. |
| Frosted, patterned and rice-paper films | Low risk | Low risk | Low risk. They absorb little sun. |
Two footnotes from Solar Gard's chart explain the "ask first" cells: an interior film on a double-pane unit with low-E on surface #3 is low risk when the unit's solar heat gain coefficient (SHGC, on the window label) is 0.61 or less, and an exterior film with low-E on #2 is low risk when the SHGC is 0.58 or more. If your label shows the SHGC, send it with your question.
Interior or exterior film?
Interior film lasts longest and gives the most choice, and on single-pane or tempered glass it's the normal pick. On a double-pane or low-E window where the film you want shows "ask first", you have three good options: choose a lighter or more reflective film from the low-risk rows, choose the Sentinel Plus Silver or Stainless Steel exterior film, which stops the heat before the glass, or ask us. Exterior film faces the weather, so expect a shorter life than the same film inside.
Your window warranty, and ours
Many window makers exclude glass breakage and seal failure from their warranty once a film is applied, whoever applies it. If your windows are new, read the warranty or ask the installer before you film them. The film makers' own glass-breakage warranties cover dealer installations, not film you apply yourself. We replace film that fails; we can't warrant your glass. So on a double-pane low-E window, choose a low-risk row or an exterior film, and when in doubt, ask.
Not sure? Ask us
Send a photo of the window and of its edge or label, the glass size, which way it faces, whether it's single or double pane, and anything the label says about low-E or SHGC. Contact us or call (800) 880-1384 and we'll tell you which films are safe on it before you order.
Common questions
Will film fog my double-pane windows?
Not by itself. Fogging means the edge seal has failed, which heat stress from a dark absorbing film can hasten on an annealed unit. The low-risk films above don't add that stress.
Can I put any film on tempered glass?
Yes. Tempered and heat-strengthened glass aren't at risk of thermal breakage, so the only limits are the seal of a double-pane unit, covered by the table, and the "never film" list.
My windows are already low-E. Do I need film?
Low-E glass cuts heat loss and some heat gain, but a west window still gets hot and bright in the afternoon, and low-E does nothing for privacy. Film adds glare control, daytime privacy and more heat rejection; pick it from the low-risk rows.
Can I film the outside of the glass?
Only with a film made for it: our two Sentinel Plus exterior films. Interior films fail quickly outdoors.
What about skylights and sunrooms?
Skylights are usually tempered or laminated and run hot; sunroom glass is often large and partly shaded. Both are "ask first", with photos.