Blinds or window film: which keeps the heat out better?

How blinds work

An internal blind stops sunlight reaching the room behind it, but the sun has already passed through the glass and warmed the blind and the air between the blind and the window. Pale, reflective blinds do better than dark ones. External blinds and shutters are far more effective for heat, because they stop the sun outside.

How window film works

Solar film rejects part of the sun's energy at the glass, so less heat enters the room in the first place. It's working whenever the sun is on the glass, whether you're home or not, and you keep the view and the daylight.

Side by side

• Heat: film usually does better than internal blinds; external blinds do best.
• Glare: both help; blinds can block it completely.
• View and daylight: film keeps them; blinds lose them when closed.
• Privacy at night: blinds; most films don't give night-time privacy unless they're frosted.
• Rooflights and conservatory roofs: film avoids the dust and awkward access of roof blinds.
• Upkeep: film is cleaned with the glass; blinds need dusting and can fail mechanically.

When to use both

Film for everyday heat and glare, and blinds for privacy in the evening or full darkness in a bedroom. The combination often means blinds stay open far more of the time.