Understanding the Film-to-Glass Thermal Stress Score

Every film and glass combination creates a thermal stress load on the glass. Too much stress and the glass can crack — a phenomenon called thermal breakage. Flexfilm's Film to Glass Compatibility Calculator uses an industry-standard film-to-glass stress chart to estimate that load for a specific installation, combining a primary stress score with additional site-condition points. Use the calculator directly for a real score on your specific glass and site conditions — the breakdown below explains how it works.

Primary Stress

A base score assigned to each film-and-glass combination, reflecting how much extra heat the film traps against the glass. Darker, more reflective films on multi-pane glass carry higher scores.

Additional Stress Factors

• Summer temperature: peak heat raises glass surface temperature; above 109°F adds points, and over 115°F adds more.
• Glass condition: scratches and chips are stress points that film's added thermal load can turn into failures; chipped edges are an automatic Not Recommended.
• Pane size: larger panes absorb more heat overall — panes between 50 and 100 sq ft add a point, and panes over 100 sq ft are automatically Not Recommended.
• Gasket condition: a deteriorated rubber gasket restricts the glass's ability to expand and contract safely and adds a point; no gasket at all adds two.
• Frame material: steel or concrete frames conduct and retain more heat than wood, vinyl, or aluminum. Wood frames should be inspected for exposed fasteners, paint buildup, warping, or deteriorated glazing compound before filming.
• Shadow type: shadows heat glass unevenly. Straight shadows add the least stress; "L"-shaped shadows add more; "T" or "V"-shaped shadows add the most, since they create the sharpest temperature differential.
• Altitude: higher elevations mean more intense solar radiation due to a thinner atmosphere, adding stress at higher bands (2,000–7,000 ft, 7,000–10,000 ft, over 10,000 ft).
• Window treatment: blinds, curtains, or shades within 4 inches of the glass trap heat and add a point; treatments farther than 4 inches don't.

What the Total Score Means

• 10 or under: any glass — standard annealed is fine.
• 11–14: requires heat strengthened glass, roughly twice as strong as standard.
• 15–18: tempered glass only.
• 19 or higher (or an automatic NR): installation is not recommended regardless of glass type. Certain conditions — chipped glass, very large panes, complex shadow patterns — trigger an automatic Not Recommended rating on their own.

This scoring approach is a planning guide, not a guarantee — results are informational, and Flexfilm isn't liable for glass breakage or seal failure resulting from an installation. When in doubt, have your glass verified by a licensed glazier before installing film.

Helpful Links

• Film to Glass Compatibility Calculator