Window changes affect weathertightness

By - , Build 98

Changes in window installation have contributed to some of the weathertightness problems of the last decade. BRANZ is researching the issues.

A long window with instrumentation so tracer gases can be used and temperature and humidity measured.
A cavity drain-closure installed above a window at BRANZ.
A completed window for measuring the performance of the head flashing and sill performance.
Finished window installed on a flat sill according to the WANZ WIS methodology.

Changing construction practices and materials have led to slight changes in how windows are installed, sometimes resulting in uncertain weathertightness performance. The requirement for a drained and ventilated cavity has also had implications for the installation of window systems.

Sloping sills in E2/AS1

The publication in 2004 of the Acceptable Solutions for the New Zealand Building Code Clause E2 External moisture, showed some window installation details that required sloping sills in direct-fixed claddings, and also required ventilation into batten cavities above the heads of windows. While the industry recognised that this was difficult to achieve without extending the window facings, the requirement for sloping sills was made on the best available information about current practice, risk of failure and practicability. In particular, it followed Canada recognising that flat surfaces should be avoided in places where water can pool.

Following the publication of these details, the window industry suggested that flat sills may actually manage water leaks well enough in direct-fixed cladding without the added difficulty of a sloping sill. They also raised issues regarding the structural support of heavy window/door systems on a sloping sill trimmer within a cavity-based system, and the amount of ventilation that was actually needed at the heads of window systems.

A cavity drain-closure installed above a window at BRANZ.
A completed window for measuring the performance of the head flashing and sill performance.

The details for cavity installation were published by the industry as the ‘WANZ WIS’ following substantial testing and work performed at BRANZ. However, little testing could be done for windows within direct-fixed cladding systems. Since the Department for Building and Housing (DBH) requires sound scientific basis on which to develop compliance information, no quick answer was available.

A long window with instrumentation so tracer gases can be used and temperature and humidity measured.
Finished window installed on a flat sill according to the WANZ WIS methodology.

Research assesses flat sill systems

To find the answers Building Research decided in January 2006 to fund a BRANZ project to assess the claims for windows in direct-fixed cladding with flat sills. The project will look at both the weathertightness and durability concerns surrounding flat sill systems and the ventilation of the heads of cavity-fix windows. It will look to scientifically support the development of practical window installation alternatives.

Six windows have now been installed at the BRANZ research facilities in Wellington (see photos). Both the Window Association of New Zealand and the DBH support this work, and will receive the results.

Four of the windows have been given sensors to determine how long water is retained around the flat sill tray, and two have sensors to determine how much ventilation is needed in the batten cavity spaces above the head.

Test results are being gathered for the normal operation of the window systems in natural weather conditions. When the typical response of all the sensors is known, water will be injected into the ‘dangerous’ places, and the measurements repeated. This will help to establish how quickly water is able to be removed from the flat sills, and if the surrounding timbers ‘wet-up’.

This research will determine whether flat sill systems can appropriately manage water in direct-fixed cladding systems, and recommendations will be able to be made on whether flat sill systems can be included within the approved documents.

The work on the airflows at the head of windows will help determine what sort of head flashings and ventilation requirements are necessary for cavity-fixed windows.

The approved documents of the Building Code are under continuous improvement. This work will support this improvement process, so that the New Zealand building industry is always working from the best information available.

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

A long window with instrumentation so tracer gases can be used and temperature and humidity measured.
A cavity drain-closure installed above a window at BRANZ.
A completed window for measuring the performance of the head flashing and sill performance.
Finished window installed on a flat sill according to the WANZ WIS methodology.

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