An estimated 1 million houses in New Zealand have timber-framed windows, and nearly all of these are single glazed. Although timber-framed windows have a thermal performance approximately 25% better than single-glazed aluminium, all single-glazed windows are the major source of heat loss.
They contribute as much as 50% of the heat loss for a house built to the New Zealand Building Code (NZBC) between 1979 and 2008. This figure is worse for pre-1979 houses.
Based on this, the installation of new, higher-performance windows would seem to be the best option to reduce the heat loss. However, it is also the most expensive, costing at least double that of any retrofit options. A number of the retrofit options for improving the thermal performance of timber windows achieve results that are comparable to full window replacement. Replacement will be necessary where existing timber windows are in poor condition.
As well as energy savings, other benefits include extending the life of existing windows, avoiding production of new materials, reducing waste, lowering the carbon footprint and preserving the character of a home.
This Good Repair Guide looks at the various options, and the advantages and disadvantages of each, when considering whether to thermally upgrade existing timber windows or replace them.