WAVE goodbye to leaks

By - , Build 117

BRANZ is expanding on previous weathertightness research with a new multi-disciplinary programme looking at how buildings manage contaminants, including moisture.

BRANZ has had an active research presence in weathertightness since the 1980s, and with the advent of the leaky home crisis, work intensified in 2003 in the shape of a major 6-year research programme funded by the Foundation for Research, Science and Technology (FRST) and the Building Research Levy.

Understanding of cavities and drying

That programme is now complete and has been responsible for some real breakthroughs. It was the first to experimentally verify the ventilation processes that occur in cavity walls and how these can affect drying from different parts of the wall. It also managed to replicate the experimental drying results using a computational model – this means that the results no longer just apply to the test building on the BRANZ site.

The result is that we now know the critical parameters that determine the success of a cavity design – we can, for example, see what happens if we vary cavity depth, use vented battens or move to a less sheltered location. Fortunately, the cavities specified in E2/AS1:2005 (on the back of traditional practices) have been proven as having a huge capacity for managing moisture outside of the framing line.

New programme covers weathertightness and more

These are great results – although one only has to turn on the television or read a newspaper to appreciate that leaky homes are still a huge issue for New Zealand. As such, weathertightness forms a vital thread in a new multi-disciplinary programme at BRANZ, known as the WAVE (Weathertightness, Air Quality and Ventilation Engineering) project. This is a 6-year programme funded by the Building Research Levy and FRST.

In short, this programme recognises that the performance of the building envelope is linked to the environment within the building and aims to form a complete model of how buildings manage contaminants (including moisture). This would allow designers and builders to assess the impact of any features they want to implement.

The ultimate aim is to have homes that do not leak and have safe indoor environments. The programme consists of four complementary streams: weathertightness, ventilation, interstitial moisture and indoor air quality technology. This article focuses on the weathertightness activities within the overall WAVE project.

Many of the activities have been directed by the needs of industry as recognised at forums such as the Claddings Institute of New Zealand breakfast meetings.

Going beyond low-rise buildings

The focus of the previous weathertightness programme was largely residential buildings. The new programme will begin to look at issues that affect taller buildings as well as maintaining momentum in the low-rise area.

Work on air barriers will address confusion that exists in industry relating to building wraps, rigid air barriers and sheathing. Building surveyors have reported a growing caseload of leaking apartment buildings and medium-sized commercial buildings. It appears that design details that work in low-rise buildings are being pushed too far; the air barrier components are not adequately managing the wind pressures across the cladding at the higher altitudes.

The management of this pressure is a key factor in determining the amount of rain that may enter a wall cavity. Although the work will focus largely on the impact of the choice of air barrier on water management, the study will also emphasise the practicalities of the different options.

In the low-rise area, packages of work will establish the relative drying potential of direct-fixed weatherboard and sheet claddings. This work will clarify the theory behind the performance of capillary grooves and compare water leakage rates through various types of weatherboard.

User-friendly design tools

Running alongside the programme is a series of educational activities. The thrust of this is about encapsulating the results from the separate weathertightness projects in computer models – these feed into the overall model that the WAVE project aims to create but will also be distilled into user-friendly design tools.

An example will be the creation of WALL-DRY. This tool encapsulates all of the knowledge from the previous programme, such as how much water walls can cope with, but will also include new knowledge from subsequent work. These ‘bolt-ons’ will include the rain loads for different parts of the country, results from the drainage plane programme (see Build 103 December 2007/January 2008, pages 56–57) and the work packages stated above. This tool will be used in educational forums such as the NZIBS weathertightness course and BRANZ seminars.

The end of guesswork?

Moisture plays a large part in other aspects of the WAVE programme too. The interstitial moisture (condensation within the building structure) stream will look at moisture management in roof spaces, and the ventilation stream will investigate the moisture removal effectiveness of various ventilation strategies.

The previous weathertightness programme may have finished but its legacy will live on; the experimental building techniques that were developed in that period will prove vital to the success of the new programme. If the WAVE project is successful, it will provide a complete performance basis for weathertight design – removing some of the guesswork from current practice and ensuring a dry and safe future for the New Zealand building stock.

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