I must admit I’m sceptical of a blanket recommendation to install window films in climates where you have cold winters, though. A window film will reduce the heat gain from the Sun in the wintertime, and this can amount to a substantial saving in heating costs.

If the main purpose of the film is to prevent heat from entering the house, the film should be applied to the outside of your window. If the main purpose is to retain heat inside your house, it should be applied to the inside of your window. There are also films available that stick to the window by static cling, which means they could be removed as seasons change. I’m not sure if they’re as efficient as the professionally installed, permanent ones, though.

I would personally only recommend installing reflective film on your windows if you live in a warm climate, where you need to keep excess heat out of the house for most of the year.

There’s a whole bunch of other stuff you can do to keep your house cool in the summer heat and reduce your airconditioning bill, though – I’m currently writing a post series about this on my blog: http://www.renewablesathome.com/energy-conservation/green-home-cooling-mini-series