Could windows be installed better?

By - , Build 163

BRANZ Is investigating different installation techniques for thermally efficient and weathertight aluminium windows and is keen to hear about any new industry solutions.

Figure 1 Aluminium window head detail – cavity construction (adapted from E2/AS1 Figure 85).
Figure 2 Aluminium window sill detail – direct-fixed (adapted from E2/AS1 Figure 83).

ALUMINIUM WINDOWS are the most common window frame type in New Zealand housing, currently making up over 90% of the domestic market.

While market demand for thermally broken (thermally improved) aluminium window frames has increased recently, these windows are typically installed in the same way as standard aluminium windows (see Figure 1).

Window installation different here

Not many other countries install aluminium windows as we do in New Zealand. People who have followed European practices often struggle to understand why we have such different installation practices. This is particularly an issue when thermally broken aluminium or uPVC window frames are installed, since our installation practices reduce the thermal effectiveness of these frames.

New Zealand window frames often provide an easy path for heat to bypass the insulation provided from insulating glass units (IGUs), reducing the thermal effectiveness of the wall element. This is not ideal, but it is a compromise between several competing issues.

Figure 1 Aluminium window head detail – cavity construction (adapted from E2/AS1 Figure 85).

Why we install windows as we do

There are several factors behind our window installation methods:

  • In New Zealand, narrow timber-framed external walls are typically used. These are much less tolerant of moisture than European external walls, which often include masonry that can absorb some water leakage.
  • New Zealand windows typically open outwards, which has benefits in keeping water out.
  • New Zealand windows are designed to allow greater window opening areas and greater solar gain. They typically use an external frame profile with a flange that covers the ends of thin claddings at the window opening or butt into a brick veneer.
  • The New Zealand climate is less extreme than parts of Europe and North America, so thermal issues have been comparatively less important.
  • The leaky building crisis brought about changes in window installation methods, such as adding air seals and sill support bars.
Figure 2 Aluminium window sill detail – direct-fixed (adapted from E2/AS1 Figure 83).

Avoiding leaky windows

Deterioration of materials below window sills was often the first indicator of weathertightness failure in building claddings during the leaky building crisis. This led to the introduction of a drainage cavity behind the cladding in the Acceptable Solution to New Zealand Building Code clause E2 External moisture.

When following the Acceptable Solution for buildings with a higher weathertightness risk, the drainage cavity connects with the area around an aluminium window – the trim cavity. This ensures that the window trim cavity is drained and ventilated to allow any penetrating moisture to dry out and minimises the risk of moisture damage to structural framing.

Where claddings are installed on buildings at lower risk of weathertightness damage, claddings can be direct-fixed to the structural frame, and window trim cavities are drained and ventilated directly to the outside (see Figure 2).

Considerable research findings were built into this practical weathertightness solution for window installation, incorporating the lessons learned from the Canadian weathertightness problems of the 1980s and 1990s.

Our current E2/AS1 window installation methods minimise weathertightness risk and maximise glazing areas but do not consider the maximising of the thermal performance of window frames. Window installation practices in the northern hemisphere have arisen from different drivers, with thermal performance being more important than weathertightness performance.

Identifying new installation techniques

It is challenging to marry these different approaches, which is one reason why BRANZ is planning a research project to identify new installation techniques for New Zealand residential windows.

The task is to improve the thermal performance of our window installation practices while providing a robust weathertight solution. There are potentially many different types of window installation solutions that achieve this and could be useful to the building industry.

Need to be tested for weathertightness

Currently, some windows used here do not have the typical external flange around their outer frame and use alternative tested methods to ensure weathertightness, such as a fixing flange and tapes.

All proposed new approaches should be tested for weathertightness before they can be readily accepted into New Zealand building practice.

Get in touch

We are interested in hearing of existing industry solutions that work for New Zealand’s residential construction.

Ideally, BRANZ would like to test and publicise the best of these online for all of industry to use and benefit from.


To contribute to this research, please email

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Articles are correct at the time of publication but may have since become outdated.

Figure 1 Aluminium window head detail – cavity construction (adapted from E2/AS1 Figure 85).
Figure 2 Aluminium window sill detail – direct-fixed (adapted from E2/AS1 Figure 83).