Ventilation drying in cavities

This Issue This is a part of the Weathertightness feature

By - , Build 110

Recent BRANZ research has shed new light on how evaporative drying in wall cavities depends on cavity dimensions, vent sizes and climate.

Figure 2: Average drying rates at 14 meteorological stations.
Inside the BRANZ weathertightness testing building.
Figure 1: Influence of cavity type on drying performance.

Developing the science behind cavity drying has been difficult. In the early days of leaky buildings, existing brick veneer and cavity stucco designs were simply adapted to a wider range of claddings because there was no obvious way of choosing more appropriate cavity dimensions and vent sizes.

It was particularly hard to work out how to measure ventilation rates in the small spaces inside wall cavities, but BRANZ researchers developed a method using a tracer gas and then successfully applied it to the cavities of many wall types (see Build 100 June/July 2007, pages 66–67).

Small gaps important

The researchers were surprised to find that infiltration paths, such as small gaps between cavity battens and lap joints in weatherboard wall claddings, contribute significantly to cavity ventilation. This means traditional thinking that ventilation in wall cavities depends only on purpose-built vents at the base of walls and at the head of windows is incorrect. This is similar to traditional understanding of home ventilation – this also depends on air infiltration more than expected.

Air infiltration in wall cavities is not a problem, but builders and designers need to be aware that changes which make the cavity more airtight may inadvertently change its drying characteristics.

Investigating drying rates

This new understanding of ventilation and infiltration in wall cavities has been used to calculate drying rates in a wide range of cavity types and geographical locations and with other climatic variables, such as wind exposure and solar radiation.

The cavity types shown in Figure 1 are:

  • drained and ventilated brick veneer wall – 50 mm deep cavities, no battens and vents top and bottom
  • open rainscreen vented battens – cavity depth of 20 mm, vented battens and vents at the bottom only
  • open rainscreen vented battens and solid corners – same as the previous cavity but with solid battens at the corners
  • open rainscreen non-vented battens – same as the previous cavity but with solid battens throughout
  • 6 mm drainage mat – this is a drainage plane cavity with vents top and bottom.
Inside the BRANZ weathertightness testing building.

What was found

The main conclusions from the research are described below and shown in Figures 1 and 2. These figures show the influence of cavity type and location on drying performance.

CAVITY GEOMETRY IMPORTANT

The research shows that the geometry of cavities and vents – that is, their size and location – has more influence on ventilation and drying performance than climate. This is shown in Figure 1 where the cavity types with top and bottom vents achieve higher average drying rates than bottom-only vented cavities.

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DRYING RATES AND CLADDING TYPE

High drying rates are necessary for more porous and absorbent cladding types. Cavity types that successfully drain water out are clearly less reliant on ventilation drying than walls with absorbent claddings.

Brick veneer walls, for example, are typically absorbent and hold water that has to be disposed of by evaporation. Their drying capabilities are consistent with high moisture loads.

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CAVITY DEPTH

In practice, cavity depth has little to do with ventilation drying, and cavity depths have to be quite small before they start to limit ventilation rates.

Figure 1: Influence of cavity type on drying performance.
Figure 2: Average drying rates at 14 meteorological stations.

Cavity depths tend to be chosen to minimise moisture bridging paths to the building wrap, that is, the cavity depth in a brick veneer wall will need to cope with mortar irregularities whereas, in a fibre-cement clad wall, the deciding issue is likely to be insulation bulging the wall wrap towards the cladding.

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CLIMATE EFFECTS

Average drying rates in four cavity types have been plotted as a bar graph in Figure 2 for 14 city locations. Average wind speeds for the 14 meteorological stations are also plotted, and although the ventilation rate is primarily dependent on wind speed, the drying rate is clearly a more complex mixture of humidity, air temperature and solar radiation. Overall, these location-related effects are less significant than the geometry of the cavity and its vents.

More questions to answer

This research only explores the ventilation drying aspect of cavity performance – the data does not complete the argument for any particular cavity design. In particular, it does not answer the question of how cavity drying rate needs should be derived from the weathertightness characteristics of a cladding. That is a difficult question that depends on workmanship as much as on the properties of cladding materials and is still being investigated.

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Download the PDF

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

Figure 2: Average drying rates at 14 meteorological stations.
Inside the BRANZ weathertightness testing building.
Figure 1: Influence of cavity type on drying performance.

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