Can you put window film on double glazing?

What is thermal stress?

When the sun hits glass, the centre of the pane warms up, while the edges stay cooler because they're held in the frame. Glass expands as it warms, so the difference in temperature between the centre and the edges puts the glass under stress. Window film, particularly darker or more absorbent film, can make the pane absorb more heat and increase that difference. If the stress is too great for the glass, it can crack, usually starting from an edge.

Which glass needs the most care?

• Large panes, where the temperature difference across the glass can be bigger
• Glass that's already tinted or heat absorbing
• Units with a low-E or solar control coating
• Glass that's partly shaded, for example by a deep overhang or a tree, so one part is hot and another cool
• Older units, or units with damaged or chipped edges
• Laminated glass, often used in rooflights

None of these rule film out. They mean the film has to be chosen carefully.

How to choose film safely

Film manufacturers publish compatibility guidance showing which of their films can go on which types of glass. A good installer checks the type and specification of your glass, any existing coatings, the pane sizes and the orientation, and recommends a film that's approved for that glass. If a film isn't suitable, they should say so.

Will film affect my double glazing warranty?

It can. Some glazing manufacturers exclude damage caused by applied film, or require a compatible film. If your windows are still under warranty, check the terms, and we'll take them into account when we recommend a film.

Can window film fix misted or blown double glazing?

No. Condensation between the panes means the seal on the unit has failed. Film goes on the surface and can't fix that. The unit needs replacing.