Double Glazing, does it pay?



Most houses in New Zealand have single glazed windows meaning that each window has a single sheet of glass. Double glazing means each window has 2 sheets of glass with a sealed gap between the 2 sheets or panels. The gap can be either an air gap or a heavy inert gas such as Argon.

The gas or air acts as a thermal barrier reducing the transmission of heat. Argon is more effective than air by 3-9% but it costs more to install. The type of glass used can also improve the thermal heat loss through a window. 

Low E glass (low emissivity) can reduce heat loss by 20-30% compared to the standard window glass. Since 2008 all new houses built must be built using double glazing. When major renovations to an existing house is undertaken where a building permit is required any replacement or new windows would need to be double glazed. 

Existing houses can however have their windows retrofitted to change them from single glazed to double glazed without removing the existing windows. But does the cost of replacing or retrofitting existing windows pay in terms of saving on heating bills?

Research done by Energywise and by Victoria University generally agree that the payback time to recoup the cost of installation can vary depending upon what part of the country (how cold it gets) and whether it is retrofit or replaced. Typical figures for Auckland are 28-30 years, typical figures for Dunedin are 13-15 years.

The other benefits of double glazing apart from the power bill savings are obviously a more comfortable home, less condensation on the windows and noise reduction. Finally, the re-sale value of a house that has double glazing is likely to be higher than if it had single glazing.

Does it pay? Yes, but it is usually a long term investment.