Window film for cars blocks harmful UV rays, keeps the cabin cooler and adds privacy — but the law sets firm limits on where it can legally go, and the warranty covers less than most owners assume. This guide covers film types, legislation, installation and warranty cover.

What is window film, and how does it work?

Window film — also called tint or solar film — is a thin polyester film applied to the inside of a car's windows to reduce UV radiation, heat build-up and glare while increasing cabin privacy. The film is built from several layers: a protective top coat on the outer surface, a dye or metal layer that controls how much heat and light passes through, and an adhesive layer that bonds the film to the inside of the glass.

What types of window film are available?

Most window film sold today falls into one of four categories, and the difference comes down to heat rejection, durability and whether the film interferes with electronics:

  • Dyed film: the cheapest option. A dye layer absorbs sunlight and cuts glare and visibility into the cabin, but it rejects noticeably less heat than the other types and can fade to a purple or brown tint over time.
  • Metallised film: contains microscopic metal particles that reflect heat and light effectively. The drawback is that the metal layer can interfere with GPS, radio and mobile signals, making it a poor fit for cars with antennas built into the glass.
  • Carbon film: rejects heat without any metal content, so it does not interfere with electronics. It has a matte, non-reflective finish and holds its colour better over time than dyed film.
  • Ceramic (nano-ceramic) film: the most advanced and most expensive type. It rejects the most infrared heat, does not interfere with electronics, and keeps its clarity the longest — most of the longest warranties (5–10 years or lifetime on installation) are issued on ceramic film.

What do VLT, UV rejection and TSER actually mean?

VLT, UV rejection and TSER are the three numbers that actually describe a window film's performance — not how dark it looks:

  • 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 up to 99% of harmful UV radiation. This protects skin, interior and dashboard plastics from fading — and is the property that does most of the actual protecting, regardless of how dark the film looks.
  • Heat rejection (TSER): indicates the film's ability to reject total solar energy. Higher TSER means a cooler cabin on hot days.
  • Glare reduction: reduction of glare from sunlight and oncoming headlights.

UV rejection — not VLT percentage — is what actually protects skin and interior from fading. A lighter film with high UV rejection protects better against sun damage than a darker film with low UV rejection.

Is window film legal? What does the law actually say?

In the UK, aftermarket window film on the front windscreen and front side windows must not reduce light transmission below the legal minimum — commonly cited as 75% VLT for the windscreen and 70% for the front side windows — while the rear windscreen and rear side windows are not regulated and can legally be tinted to any darkness. A professional installer will always advise you correctly and only fit film where it is legal for your specific windows.

These thresholds are UK-specific and vary by country — some set different limits, and some ban aftermarket front film entirely regardless of VLT, similar to Denmark. If you are outside the UK, or moving the car across borders, always check the current local regulations before fitting or keeping film on the front windows, rather than relying on figures written for another market.

How is window film installed?

Installation follows broadly the same steps regardless of film type: the glass is cleaned of dust and grease, the film is cut to match each window's shape — often using a computer-controlled plotter for a precise fit — and applied wet with a soap-and-water solution so it can be positioned before it grips the glass. A squeegee pushes out the water and air bubbles, while a heat gun shrinks the film around the curved edges of the rear windscreen and rear side windows.

The film needs several days to cure fully before the windows are washed or rolled down — the exact time depends on the film and weather, so follow the installer's guidance. Most installers hand the car back the same day it is fitted.

What does the warranty on window film cover — and what does it not cover?

The warranty on window film typically covers faults caused by poor film quality or poor installation — not physical damage from outside. Premium window film usually comes with a product warranty of 5–10 years, sometimes a lifetime warranty on the installation itself. The warranty normally covers bubbling, discolouration (a purple or brown tint is typically a sign of poor quality) and delamination at the edges.

The warranty typically does not cover mechanical damage from windscreen wipers, scratches from jewellery or keys, or damage caused by incorrect cleaning — for example using an abrasive sponge or a razor blade directly on the film's surface.

Why does documentation matter for window film?

Documenting window film means being able to prove exactly which film is fitted, when it was installed, and by whom — which can make a real difference at an MOT test, when selling the car, or in an insurance claim. Without documentation, it is one person's word against another's if the film turns out to be too dark for a particular window, or if a warranty dispute arises.

A digital warranty certificate solves this: film specifications, VLT percentage, installation date and warranty terms live in one place that neither fades nor gets lost in a glovebox. Selling the car does not automatically end the warranty either — read what happens to a warranty when you sell the car — and a documented, active window film warranty is one more concrete point a buyer can see and trust.

Frequently asked questions

How dark can window film legally be on rear windows?

In the UK, rear windows are not subject to a minimum VLT, so owners can in practice choose any darkness there, while the front windscreen and front side windows have a legal minimum light transmission. Rules differ elsewhere and can change, so always check the current regulations in your own country before fitting film, rather than assuming UK figures apply.

Can you be stopped or fined for illegal window tint?

Yes — driving with front window film that falls below the legal VLT minimum can result in a roadside check flagging the vehicle, a fine, and a requirement to remove or replace the film. Responsibility for compliance sits with the car's owner regardless of who installed the film, so choose an installer who knows and follows the current rules for your market.

How long does window film last before it needs replacing?

Quality film typically carries a product warranty of 5–10 years, and with correct care it often lasts longer than the warranty period itself. Fading to a purple or brown tint, bubbling, or delamination at the edges are signs the film is wearing out and should be replaced.

Can window film be removed without damaging the glass?

Yes, correctly removed window film leaves no damage on the glass. A professional installer typically uses heat to soften the adhesive before peeling the film away, then removes any adhesive residue with a suitable cleaner. Attempting removal without heat risks tearing the film and leaving stubborn adhesive residue that is harder to clean off.

Is dyed window film the same protection as ceramic film?

No. Dyed film mainly reduces glare and adds privacy, while ceramic film also rejects significantly more heat and typically more UV radiation, without interfering with GPS or radio signals. If the priority is protecting interior and skin from UV, ceramic film is generally the better — though more expensive — choice.

A digital certificate with the film's specifications makes it easy to document the work for an MOT, resale or an insurance claim — exactly the kind of detail that makes a customer feel properly looked after. See how UV and heat damage car paint and interiors beyond the glass, and see what your customer gets with a system like Sevv.