Low-E window film
Low-E film (low-emissivity film) is a thin polyester film with a micro-thin, transparent metal coating. It is stuck directly to your existing glass to reflect radiant heat - either from outside or inside - which reduces the amount of energy you need to heat or cool your home.
How it works
In New Zealand homes, low-E film is usually fitted to the inside of single-glazed windows to reduce heat loss in winter. Normal glass allows most of the heat radiation coming from inside your house to pass through to the outside. By fitting a low-E film this radiant heat is largely reflected back into the room, thereby reducing heat loss through windows.
Low-E films can also reduce the amount of the sun's heat and UV light coming in through the windows. This can reduce overheating and fading of furnishings in summer, but can also reduce the amount of free heat your house can soak up in winter.
Condensation and performance
BRANZ research shows low-E window film can reduce heat loss through single glazing by up to 25%. For comparison, double glazing, secondary glazing and DIY window insulation kits can reduce heat loss through glazing by about 60%.
Low-E window film on single glazed windows works best in houses that rarely experience condensation on windows, according to the BRANZ research. That’s because fitting low-E film to single-glazed windows can cause increased condensation which then largely negates the effectiveness of the low-E film. As the low-E film reflects heat, the glass itself becomes colder and condensation is more likely to form on your windows.
Like condensation on a bathroom mirror, the condensation on the low-E film means the glass surface is no longer reflective. While condensation is present, the low-E surface provides very little benefit over normal glass. Once the condensation has dried up, the performance of low-E window film is restored.
Tackling damp at source, and good ventilation and heating practices may help to reduce the amount of condensation that forms when low-E window film is fitted.
The same condensation and performance issues also apply to single-glazed windows with low-E glass. In double glazed windows, low-E glass can be very effective as the coating is placed on an inner surface of the sealed unit during manufacture, where condensation is not able to reduce its performance.
Alternatives for reducing window heat loss
If you have wooden single-glazed windows you may want to try out DIY window insulation kits which are inexpensive and effective.
If your windows need substantial repair or replacing it's worth installing double glazing.
Thermal curtains and blinds can also significantly reduce heat loss through your windows, provided they are well fitted.