Will Window Film Crack Your Glass?

Will window film crack my glass?

Very rarely, when the film has been matched to the glass. Window film changes how much of the sun’s energy is absorbed at the glass, and on some types of glass that extra heat can contribute to a thermal stress crack. The risk sits almost entirely with annealed glass, it depends on the specific combination of film and glazing, and window film manufacturers publish compatibility guides so that the combination is checked before anything is installed.

The European Window Film Association states the position fairly: once a film installation has been professionally assessed for compatibility with the glass, the risk of thermal stress breakage stays low. It also says the risk can never be zero, because glass can break whether film is installed or not.

The rest of this article explains where that risk comes from, how a contractor should check for it, and how to tell afterward whether film had anything to do with a broken pane.

How does window film cause thermal stress?

Thermal stress is caused by one part of a pane being hotter than another. The International Window Film Association gives the typical example: a cold, east facing window is suddenly heated by sun on a winter morning. The center of the glass warms faster than the edges, which are shaded under the frame. That difference puts stress on the edge of the glass, and if the stress is high enough, the glass cracks. Uneven shading from overhangs or other exterior shading can create the same kind of temperature difference.

Film enters the picture because it changes how much solar energy the glass absorbs. IWFA identifies thermal stress from absorbed solar radiation as the one type of glass stress that film may affect. The European Window Film Association names solar energy absorption as the most important film property for compatibility, and makes a point that matters when comparing products: the same film behaves differently on different glass, so performance figures for one glazing system can be misleading when applied to another.

The film is only one factor. The association also lists glass type, glass color and coatings, thickness, the type of glazing, pane size, exterior shading, blinds and curtains close to the glass, frame type and solar intensity.

Clear safety and security film sits at the low end of this risk. IWFA’s safety and security guide states that clear safety film should have little, if any, effect on thermal stress fractures. In Solar Gard’s current film to glass guide, every clear safety and security film from 4 mil to 14 mil is approved on every glass construction the guide covers, while several of its solar safety films carry cautions or are restricted to tempered or heat strengthened glass. If your project is security film on existing glass, our article on what security window film can and cannot do covers the rest of the decision.

Which types of glass are at risk?

The type of glass matters more than anything else, which is why a contractor should never talk about what “glass” does in general.

Solar Gard’s guide reaches the same conclusion from the manufacturer’s side: any of its films may be applied where every pane in the construction is tempered or heat strengthened. Note the word every. An insulated unit with one tempered pane and one annealed pane still needs to be checked, because the annealed pane is the one at risk.

How do you know what glass is in the window?

You measure it and you look for the evidence. You do not assume.

Solar Gard’s film to glass application asks for the strength of every pane (annealed, heat strengthened or tempered), the thickness of every pane, and the width of any air or gas space between them. The form notes that strength can be identified from the bug, the small permanent mark tempered glass carries in a corner, or with a glass strength meter, and that each pane has to be checked separately.

Coatings are the other half of the question. Low-E coatings are invisible, and whether one is present, and which surface it sits on, changes which films are appropriate. The surfaces of a double pane unit are numbered from the outside in: surface 1 faces the weather, surfaces 2 and 3 face each other across the air space, and surface 4 faces the room.

AAA Glass Tint carries an EDTM Glass-Chek ELITE on every survey. EDTM’s operating manual states that the meter measures glass and air space thickness on single, double, triple and quad pane windows from one side, determines the presence, location and type of Low-E coatings, and identifies and measures laminated glass. The same manual is candid about its limits: in some readings the meter can tell that a coating is on the second pane but not whether it faces surface 3 or surface 4, and on triple pane units it only checks the first two panes for coatings. In both cases EDTM directs the user to test again from the other side of the window.

That is why the meter answers most of the questions but not all of them. A visual inspection of the glass and the glazing system, and a second reading from outside where the glass calls for it, answers the rest.

What is a film to glass guide, and what happens when a combination is not pre-approved?

A film to glass guide is the manufacturer’s table of which films are approved on which common glass constructions. Solar Gard’s guide rates each film on single pane clear or tinted glass, double pane clear or tinted glass, double pane units with a Low-E coating on surface 2 or surface 3, and triple pane clear glass. Each combination is marked approved, caution (a written application and approval required), or restricted to tempered or heat strengthened glass only.

The guide states that it covers the most common glass types and standard rectangular and square windows, plus odd shapes such as arches and triangles up to 20 square feet per window. It does not cover every application. When a combination is not pre-approved, or the glass does not fit the table, the installer submits a film to glass application to Solar Gard Technical Services to secure glass breakage and insulated unit seal failure warranty coverage. Two situations always require an application and written approval: any installation over 2,500 square feet, and round, bent or laminated glass.

Triple pane glass is where the list gets short. Solar Gard publishes a separate triple pane guide, and on triple pane units with a Low-E coating on surface 2, nearly every solar control film in it requires an application or is limited to tempered or heat strengthened glass. Every clear safety and security film is approved on every triple pane construction it lists.

Low-E glass gets its own caution category. Solar Gard’s guide explains that a Low-E caution marking indicates the film may not be eligible for glass breakage and seal failure coverage, because the risk is higher when that film is installed on a double pane unit with a Low-E coating.

AAA Glass Tint’s standard is to recommend films that are already approved for the glass in question. If a customer wants a film that needs approval for their glazing, we complete the application and submit it to Technical Services. If it is approved, the work goes ahead. If it is not, we recommend a different film.

What conditions rule out the glass breakage warranty?

Solar Gard’s guide lists 20 conditions under which its glass breakage and insulated unit seal failure coverage is not available. They include:

• Textured or wired glass, glass that already has a window film on it, and glass with paint, lettering or vinyl graphics
• Glass that is already damaged, chipped, cracked or scratched, and glass that is broken or has a failed seal before the film goes on
• Any building with a history of glass breakage or seal failure problems
• Glass in concrete framing, metal framing with no gaskets or seals, or framing whose sealant or gaskets are no longer resilient
• Any window where light can be seen between the glass and the frame
• Glass 3/8 inch thick or thicker, counting both panes for laminated glass
• Any glass with a single edge longer than 10 feet, any rectangular single pane over 100 square feet, and any rectangular insulated unit over 40 square feet for most films
• Quadruple pane units, spandrel glass, interior film on skylights, and film that does not cover the full visible opening

These conditions do not always mean film cannot be installed. They mean the glass will not carry the manufacturer’s breakage and seal coverage, and a building owner should know that before the work is approved rather than after something breaks. Most of them can only be found by inspecting the windows, which is one more reason a survey should come before a recommendation.

Will window film void my window warranty?

We cannot answer that for every window manufacturer, and nobody else can either. The only reliable answer is in the terms of your own window warranty, and it is worth reading them before film is installed.

What we can explain is how the film manufacturer’s coverage works. Solar Gard’s Armorcoat PremierPlus warranty includes seven years of glass breakage coverage and five years of insulated glass unit seal failure coverage, where the breakage or failure results from an approved film to glass application. Those two coverages share a combined limit of $1,000 per window, with no deductible. It is worth comparing that limit with what a replacement unit in your building would actually cost.

One condition in Solar Gard’s guide ties the two warranties together. Seal failure coverage is not available if the original window manufacturer’s seal warranty ends for reasons unrelated to the film, or if the seal fails after that warranty’s term has run out. In practice, the age and warranty status of existing insulated units is part of the decision, not an afterthought.

If glass breaks after film is installed, did the film cause it?

Not necessarily, and often not. Glass breaks for many reasons, with or without film on it.

A thermal stress crack has a recognizable pattern. According to the European Window Film Association, it starts at the edge of the glass, away from the corners, and leaves the edge at a right angle to both the edge and the face of the glass. IWFA describes the same break as a short crack running straight in from the edge before it turns into a jagged diagonal line across the pane. The European association is direct about the reverse: if a crack does not show those characteristics, it was not caused by thermal stress. It also notes that low thermal stress breaks are often a result of existing damage to the glass.

Other causes are more common than most people expect. Vitro Architectural Glass lists edge or surface damage from handling and glazing, and movement in the building or framing system, among the usual explanations for glass that breaks with no obvious cause.

Tempered glass has its own failure mode. The National Glass Association notes that fully tempered glass in architectural use can break spontaneously because of tiny imperfections in the glass, most notably nickel sulfide inclusions. Pilkington’s technical bulletin states that this breakage can happen with no load on the glass, even five or ten years after installation. Vitro describes nickel sulfide breaks as extremely rare, and it happens in tempered glass because of the way tempering traps the inclusion.

Put those facts together and a tempered pane that shatters after film is installed was almost certainly not cracked by thermal stress from the film, because tempered glass is not subject to that kind of stress in normal building use. It may have been damaged at the edge, stressed by the frame, or failed from an inclusion that was in the glass when it was made.

What should you ask before film goes on your glass?

Four questions settle most of this before any money is spent.

First, has the installer identified every pane: its strength, its thickness, whether it has a Low-E coating and which surface the coating is on? Second, is the proposed film approved for that construction in the manufacturer’s current guide, or has a film to glass application been approved in writing? Third, does any of the glass fall into a condition that rules out breakage and seal coverage, and if so, has that been explained to you? Fourth, who registers the warranty and keeps the paperwork, and will that paperwork still be findable if you need it in six years?

If a contractor recommends a film without having measured the glass, the answer to the first question is no, and the rest of the recommendation is a guess. The right starting point is a survey of the actual windows, with the film chosen to fit what is found.