Good Repair Guide: Insulating timber windows

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. 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.

The thermal performance requirements of building elements given in this book are from the 4th edition Building Code acceptable solution H1/AS1 and verification method H1/VM1. Note that 5th editions of H1/AS1 and H1/VM1 replaced the 4th edition on 3 November 2022.