Window insulation

Windows have the lowest R-value of any component of a house. R-value is the measurement of how much a material resists the conduction of heat. Windows therefore are the element that have the potential to lose the most heat. This is especially so if you have already insulated the ceiling, underfloor and walls of your home.

There are two main types of window insulation:

Curtains or blinds

Double glazing

Curtains or blinds

Most houses have some form of curtains or blinds on the windows. For these to be effective insulation they need to trap a layer of air behind them and have enough thickness to trap some air inside them, too.

Blinds are usually very thin and often don’t really seal off the air behind them. Generally they will provide very limited insulation unless they are specifically designed thermal blinds.

To provide good insulation curtains should be made from thick, thermally backed material and preferably be double layered. Your curtains should be a tight fit against window frames and either be floor to ceiling or have pelmets on them. This is to prevent a cooling cycle forming. This is where warm air behind your curtains gets cooled by the window, falls out of the bottom of the curtain and sucks more hot air in at the top.

Close-fitting thermal curtains that cover the entire width of the window, fall to the floor, and have pelmets that are installed tight against the wall can reduce the heat loss through single-glazed windows by about 60%. This applies only with the curtains drawn, and curtains are therefore not a substitute for double-glazed windows.[1] 

For double glazing, good curtains can reduce heat loss through windows by 40-50%.

Having your curtains open during the day in winter and closing them just before it gets dark will help keep your rooms warm.

Double glazing

Double glazing is a way of creating an insulating layer of air between two panes of glass.

A well designed double-glazed window with a wooden, PVC or thermally broken aluminium frame:

  • Can halve energy loss through the window
  • Is a cost effective way to save energy, even in the warmer parts of New Zealand
  • Can, depending on the glazing, cut down external noise
  • Reduces condensation.

The R-values required for window in new houses and renovations are most easily met by installing double glazing. But it’s worth considering double glazing any time that you are renovating. Be aware that some structural changes will be required to the wall framing to accommodate the additional weight of double-glazed windows.

Use double glazing where other window coverings are impractical – skylights, for example, or where you want to have a clear view of a feature outside.

If you need to prioritise, install double glazing to the main heated areas of the house and where there are large windows. Use good curtains elsewhere.

Find out more about the benefits of double glazing in the section Designing a new home.

Take action

Hang curtains and blinds

Good, thermal backed and lined curtains can significantly reduce heat loss through windows.

Curtains should be bigger than the window frame and close fitted with no gaps for warm air to escape. They should either be ceiling to floor, or have a pelmet to reduce air circulating behind the curtains. Use thermal or close-woven materials and linings.

Draught proof your windows

Draughts and air leakage can increase your heating costs and make your home uncomfortable.

Double glazing

If you are able to, install double glazing where possible.

More information

Independent and unbiased research, testing, consultancy and information resource for the building and construction industry is available at:

 

Notes

[1] Lloyd, B., Bishop, T., Callau, M.: Retrofit alternatives for State Houses in Cold Regions of New Zealand - Report Nº 2", Physics Department, University of Otago, Dunedin, New Zealand, September 2007.
NZS 4218:2004 "Energy Efficiency - Small Building Envelope", Standards New Zealand, 2004

 

From the Resource Centre

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