Does heat reflective window film work?

How it works

Sunlight carries energy as ultraviolet, visible light and infrared. Solar films use very thin metallic or ceramic layers to reflect and absorb part of that energy before it reaches the room. Traditional reflective films do it with a visible mirror-like finish. Modern spectrally selective films target the infrared and UV more than visible light, so they can reject a useful amount of heat while staying close to clear.

Where it makes the biggest difference

• Conservatory roofs and glass roofs
• Glass extensions, bi-folds and sliding doors facing south or west
• Rooflights and lanterns
• Rooms with screens or furniture near large windows

On a north facing window, or glass that's shaded most of the day, the difference is small.

The trade-offs

• Clearer film, less heat rejected. The clearest films reject less heat than darker or more reflective ones. It's a balance between looks and performance, which is why seeing samples on your own glass matters.
• Less winter sun. A solar film also reduces welcome solar warmth in winter. In most conservatories and glass extensions that's a fair trade, but it's worth knowing.
• Inside or outside. Exterior films stop heat before it enters the glass, so they can perform better, but they're more exposed to weather. Most residential films are fitted inside.
• The glass matters. Some films aren't suitable for some double glazing. See our guide on window film and double glazing.

How to judge a film

Ask for the manufacturer's data sheet. The figures that matter are the total solar energy rejected, the visible light transmitted (how much daylight still comes through) and the UV rejected. Compare films on those numbers rather than on a single headline claim.