Double glazing alternatives: film, secondary glazing and curtains

Yes, there are cheaper alternatives to double glazing, and some work surprisingly well. EECA says a DIY shrink-film kit on timber windows can reduce heat loss by almost as much as double glazing in some cases, but it lasts about one winter and won't stick to aluminium frames. Secondary glazing and good thermal curtains are the other two options worth your money.

Does double glazing window film actually work?

The shrink-wrap kind does. You stick double-sided tape around the inside face of the frame, stretch a clear insulating film over it, and shrink it tight with a hairdryer. That traps a still layer of air in front of the glass, which is the same principle as a sealed double-glazed unit, just with plastic instead of a second pane.

• It needs a timber frame. EECA notes the film won't stick to aluminium windows.
• The gap between the inside face of the frame and the glass should be at least 5mm, so the film doesn't touch the glass.
• It is designed to last one winter season, though it can last several years if left alone.
• Pierce it and it stops working. Windows that you open every day are a poor fit.
• It is ideal in a rental, where you can't change the windows and want something you can peel off at the end of the tenancy.

Be careful with the name. 'Window film' also covers the tinted and solar-control films that go directly on the glass. Those reduce glare and summer heat but add almost nothing to insulation in winter, because there's no air gap.

What is secondary glazing?

Secondary glazing adds a second pane, glass or clear acrylic, on the inside of your existing window, held in its own slim frame or with magnetic strips. The original window stays exactly as it is. EECA describes it as a more cost-effective option than full double glazing that depends on your existing windows being in good condition.

Acrylic (Perspex-style) panels on magnetic strips are the version people fit themselves. They are light, can come down in summer, and on a timber window they can make a real difference in a cold bedroom. The trade-offs are cleaning two surfaces, the look, and that an opening window now has a panel in front of it.

Do thermal curtains help as much as double glazing?

On a single-glazed window, good curtains are the cheapest heat you will get back. EECA's advice is double-layered curtains with thermal linings that fit tightly against the wall or frame, run full length to the floor and the full width of the window, with pelmets above. It says closing curtains before it gets dark can save $80–90 a year in energy costs, and that honeycomb blinds are the warmest blind option.

Curtains only work while they're closed, though. In the day, and in rooms where you want the light, a single pane is still a cold surface, and that is what causes condensation on bedroom windows.

How do the alternatives compare?

When is real double glazing worth it instead?

When you own the house, plan to stay, and the windows are a big part of the cold. Retrofit double glazing puts a sealed unit into your existing frames, keeps the look of the joinery, and doesn't need replacing each winter. EECA says up to 40% of a home's heating energy can be lost through windows, and that double glazing brings that to 25% or less.

Start with the cost guide to see what a fair per-window rate looks like, read how retrofit double glazing works, then put your own window sizes into the calculator.