What is window film?
Window film — or solar film — is a thin polyester film applied to the inside of a car's windows. The film's primary purpose is to reduce UV radiation, heat build-up and glare — as well as offering increased privacy.
The most important properties
- VLT (Visible Light Transmission): Indicates how much visible light passes through the film. 70% VLT is almost invisible, 5% VLT is very dark. The lower the percentage, the darker the film.
- UV rejection: The best window films reject 99% of harmful UV radiation. This protects skin, interior and dashboard plastics from fading.
- Heat rejection: Indicates the film's ability to reject total solar energy (TSER). High TSER = cooler cabin.
- Glare reduction: Reduction of glare from sunlight and oncoming headlights.
Legislation in the UK
It is important to know the actual rules — they surprise many people. In the UK, the front windscreen must let through at least 75% of light, and the front side windows at least 70%. Rear windows and the rear windscreen are not regulated — window film can be applied freely here regardless of VLT percentage.
A professional installer will always advise you correctly and only fit film where it is legal.
Warranty on window film
Premium window film comes with a product warranty of typically 5–10 years, sometimes a lifetime warranty on glass installations. The warranty normally covers bubbling, discolouration (purple or brown tint is a sign of poor quality), and delamination.
The warranty typically does not cover mechanical damage from windscreen wipers, scratches from jewellery or other physical impact.
Documentation matters
If your car has window film, it is important to have documentation of the film's VLT percentage and manufacturer. At MOT, when selling the car, or in insurance claims, this can make a difference. A digital certificate with film specifications is the safest solution.