Window Film and Thermal Fracture

If window film is going to have any bearing on glass breakage, it will occur within the first 2 seasonal cycles (or 2 years from installation date). If the glass has come through the 2 year 'cycle' without cracking then another reason must be considered (e.g. structural settling, impact, tension stress, existing but out of sight cracks or edge weak points).

Check the crack formation...

You can determine whether the glass edge was damaged prior to breakage to aid in discussing the problem.

First, thermal stress fractures can be identified by the fact that, at their origin, they propagate at 90° angles to the glass edge and to the two glass surfaces.
In addition, the fracture line always runs straight from the edge for 1-2 inches before beginning to propagate across the window or dividing into two or more fracture lines. Unfortunately, unless the broken pane of glass is removed from the window, you will not be able to see the edge of the glass. However, the other details given above can help you determine if the breakage is possibly thermal stress related.
In addition to the above characteristics, thermal stress fractures always originate along the middle ½ to ¾ of a side, away from the corners. Thermal stress fractures always originate from the glass edge, never in a radial pattern as with breakage caused by impact from an object. Therefore, if you find broken panes as shown here in Figures 2 and 3, the fractures are not caused by thermal stress.
Only fractures that have all three previously discussed characteristics of thermal breaks should be investigated further. Again, these characteristics are:

Fracture begins on the middle ½ to ¾ of pane edge, not near a corner.
Fracture is at a 90° angle to edge of glass, straight out from frame (and to both surfaces of glass, but this can only be identified by removing glass from window).
Fracture runs at a 90° angle from the glass edge for 1-2 inches before propagating further or branching.
If a fracture has the above three fingerprints and is probably caused by thermal stress, the fracture involved should be looked at further to determine if the glass had been weakened by edge defects prior to film installation.

If the glass pane is substantially weakened by edge defects, a true thermal stress fracture will only meander partially across the window. If the edge defect only partially weakens the glass then a single fracture will completely cross the window. However, if the glass is of good quality with no edge defects, and is broken by thermal stress, see the attached image. In each of these circumstances, the fracture origin can start on any edge of the pane, but away from corners.



In the first two diagrams in the following posts glass breakage occurred, but only because the glass was damaged. Depending on the degree of edge damage, the glass could have fractured from any window retrofit causing an increase in glass temperature (adding close fitting blinds or curtains for instance). Only in the third diagram do you see where a pane of glass has broken from application of a solar control film that absorbs more heat than good quality glass can withstand.


More times than not homeowners are not informed of close fitting window dressings impact in aiding thermal stress... when film is applied, drapes and blinds should remain open during sunshine hours to reduce a radiator effect from being produced. This happens because there is reduced air circulation immediately adjacent to the film/glass surface, which would keep the glass surface cooler.

Thermal stress on annealed glass can occur with temperature differentials of 40 degrees F. between pane center and pane edge.

Thermal stress break pattern "less than normal strength" annealed glass

...Thermal stress break pattern "much less than normal strength" annealed glass

...Thermal stress break pattern "normal strength" annealed glass

found this EDGE DEFECT on the last FLAT job i did. found it as i was cleaning the glass, i stopped and took pictures for evidence. notifed the owner of the defect adn he said TINT IT ANYWAYS.

this is the kind of hidden stuff we deal with as tinters. glass shops want to blame us for their crappy installs if the glass breaks. bad thing is sometimes these defects are behind the gasket where you cant see them. cheep glass and faulty glass install are behind the majority of this stuff.

this was a brand new building.

Wow, thanks for the info. My cracks actually happened during the first summer after installation. On one window, there is a clear two inch 90 degree line before it curves. The other two windows have multiple cracks all from the same place (they have been getting worse as this dispute has been going on for a year). The windows are a quality brand in a very expensive house (over $2mil).

The windows do not open and have been in place for over nine years, they withstood a vicious hail storm that almost tore off our roof and siding, so, I think they are good windows (Marvin). We never got a warranty card, only a bill. We were told they were guaranteed. We were never told the guarantee would not be sufficient to cover the cost of the windows due to their size. Is there a professional organization I could contact for a list of standards? Thanks.

The other two windows have multiple cracks all from the same place (they have been getting worse as this dispute has been going on for a year).

These smack of poor glass edge quality since thermal stress cracks will display a single 1-2 inch long crack, exactly perpendicular to the edge, before meandering off in other directions.

The film adds thermal stress, yes, but if the glass edge is poor quality cut it will crack from the weak point, installed film or not... film adding heat absorption only brings out the edge flaw issue more quickly than if the glass were left untinted.

If it's been 2 years since the install, chances of additional pane breakage decreases with time. You may have been much better off with the next lighter in the film in the series.

Yes indeed... and many who are now aware of this incident are placing their bets...

Clients been on a merry-go-round for a year... chances are pretty good it won't stop until he ponies up from his own resources. Sad, but I have seen this before and not with just one film name...

Many film makers cover glass breakage on a limited basis (usually $500 per window)... only a minority of film manu's are involved in dealer interactions concerning specific and careful use of film that is to be installed on different glass size shapes and densities...

One that I know of actually has film to glass recommendation charts available to every purchaser of their product, as well as an extensive technical service department that can augment this chart with verbal telecom confirmation for any project greater than 2500 sq ft (and certainly for those who are unsure of any given situation).

Maybe the other bidder on your project had this available to him or her... I do not know.

Good installing contractors do inspect the glass, however, cannot see the mistakes made by the glazer when the glass or window was installed. Those that do would certainly point out any defects that present a challenge to them installing film and some go to the extent of reducing risk through encouraging the client to compromise on performance for a safer install... or ask that the window in question be repaired before going forward with the install.

Is this dual pane or single pane, does the glass have any shadowing on its surface anytime during the day, close fitting window dressing suffocating the glass surface from cooling through air movement, compass direction (East being the most vulnerable), altitude above sea level... all are considerations important to film selection.

One thing I have learned about window film is that one cannot always deliver what the consumer expects in performance without risk or compromise of sort.

I am unaware of any other manu's with cases in litigation over glass breakage issues.

I do hope the best for all involved in your scenario... and would most certainly have the window glass edge inspected through frame removal. It is essential to know the film is not always the culprit, though glass and window makers will speak to the contrary.