Will window film break my glass?

This is the question the industry does not advertise, and the one every property manager should ask. Solar control film works partly by absorbing solar energy, which makes the glass hotter. Glass expands where it is hot and stays cooler at the shaded edges inside the frame, and that differential creates tensile stress at the edge. Past a threshold, the pane cracks.

Tolerance varies enormously by glass type. Fully tempered glass can accept a very large temperature differential. Annealed glass accepts only a fraction of that, and heat-strengthened, laminated, wired, patterned and sealed insulating units all have their own constraints. Sealed units add a second issue, because trapped heat affects the cavity and the edge seal.

The mitigation is not complicated, it is just disciplined: identify the glass type, thickness and construction for each opening, note orientation and shading, then select a film whose manufacturer's compatibility guidance covers that combination. Where the guidance says no, the honest answer is a different film, exterior shading, or glass replacement.

We do this at survey stage. If a film and glass combination is outside manufacturer guidance, we will not install it, because a cracked pane is a far more expensive conversation than a declined quote.

What we record before specifying

• Glass type: annealed, heat-strengthened, tempered, laminated, wired or patterned
• Construction: single glazed, sealed insulating unit, or existing laminate
• Thickness and any existing tint or low-emissivity coating
• Orientation, shading patterns and partial-shade conditions
• Frame material, glazing bead depth and edge condition

If someone quotes solar film for your building without asking what the glass is, that is the warning sign.