The science behind dry walls

This Issue This is a part of the Weathertightness feature

By - , Build 138

A BRANZ program has the answers for those keen to understand how vents in cavity walls help keep walls dry. By using this knowledge, keeping it simple and following the 4Ds – drying, deflection, drainage and durability – walls will be weathertight.

Figure 1: The ventilation drying display in WALLDRY.
Figure 2: Comparing the capacity for ventilation drying in some wall types.
Figure 3: Leakage rates in red compared with the capacity for ventilation drying.
Figure 4: Average wind and rain on the building façade taken from WALLDRY.

ONE OF THE WAYS that cavity walls manage water is by allowing air to circulate behind the cladding. An interactive computer application called WALLDRY-NZ, developed by BRANZ scientists, now provides a better appreciation of the capacity for ventilation drying and of the effectiveness of vents.

Interactive comparison

Drying is one of the 4Ds of water management, forming the cornerstone of weathertight buildings and of the cavity wall solutions offered in E2/AS1. The remaining Ds are:

  • deflection – a cladding that deflects water
  • drainage – an engineered drainage path on the back of the cladding
  • durability – construction materials that are sufficiently durable for their role.

WALLDRY looks at deflection and drying and compares them in a useful and interactive way.

In Figure 1, WALLDRY shows an outline of a house with outward-facing bar graphs representing the capacity for ventilation drying along with the building location and wall design details in drop-down menus.

Figure 1: The ventilation drying display in WALLDRY.

The house is located in Wellington on an exposed site and has top and bottom vented brick veneer walls. On the north side, ventilation has the capacity to evaporate water from the back of a saturated brick veneer at a rate of 200–300 grams/day.m2 of wall. That is a lot of water and confirms that ventilation drying deserves its place as one of the 4Ds.

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Best results with top and bottom vents

One of the strengths of WALLDRY is that it allows an easy comparison of the capacity for ventilation drying in different wall types (see Figure 2 for the Wellington building and site).

It is clear that the capacity for ventilation drying is sensitive to wall design. The best cavity walls, with purpose-made vents at the top and bottom of the wall, cope with up to 100 times the ventilation drying of a direct-fixed, airtight cladding.

Cavities that are only vented at the bottom also work well. These rely on other air leakage paths in the wall, such as gaps between battens, allowing air to circulate between cavities.

Figure 2: Comparing the capacity for ventilation drying in some wall types.
Figure 3: Leakage rates in red compared with the capacity for ventilation drying.

Vented battens facilitate this process, contributing to ventilation drying in bottom-only vented cavity walls. Most weatherboard claddings have natural leakage paths between lap joints and provide for ventilation drying – even where they are direct-fixed to the frame.

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Predicting leakage rates

The logical question is, ‘How do these ventilation drying rates compare with likely water leakage rates?’ This is difficult to answer because the amount of water that enters a wall depends on how well the wall has been assembled and on the build quality of flashings.

Setting aside the issue of flashing quality, it is possible to measure the water leakage characteristics of claddings in the laboratory, particularly weatherboard and brick veneer claddings, and then use this data to predict water leakage rates for a range of claddings and climate zones.

WALLDRY has assembled a range of leakage rates and displays them for comparison with the capacity for ventilation drying. Figure 3 makes this comparison for the brick veneer building on the exposed site in Wellington. Here, the red portion of the bar graph represents wetting and the blue portion drying.

In this case, there is more blue than red, so ventilation drying should carry away most of the water that leaks through the veneer onto the back face, especially in this case where the walls are protected by 600 mm eaves. In practice, another of the 4Ds – drainage – would also help manage water leakage.

In this building, the leakage rates are higher from the south because of Wellington’s prevailing rain (see Figure 4). While the prevailing wind is from north-north-west, the average rain loads are clearly higher on the south orientation.

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WALLDRY and compliance

Does WALLDRY have a role in design and Code compliance tasks? For a variety of reasons, the answer is currently no.

First, WALLDRY cannot yet account for all of the factors in weathertight design – particularly build quality and the competence of flashings – although it will identify head room in a wall design to cope with some quality issues.

Second, WALLDRY was designed to aid in teaching the science of water management and as a vehicle for presenting findings from research at BRANZ. Once you have the intuition about what’s important in terms of water management, there is no need to spend more time using WALLDRY.

Designing a wall to manage rainwater is a bit like buying a belt to hold your trousers up. The most basic one that follows the principles will do the job just fine.

Figure 4: Average wind and rain on the building façade taken from WALLDRY.

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Download the PDF

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

Figure 1: The ventilation drying display in WALLDRY.
Figure 2: Comparing the capacity for ventilation drying in some wall types.
Figure 3: Leakage rates in red compared with the capacity for ventilation drying.
Figure 4: Average wind and rain on the building façade taken from WALLDRY.

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