Know Your Cavity Part 3: The Air Drying Of Cavities

In the last ‘Know Your Cavity’ series article, the ‘4Ds’ design philosophy used in New Zealand was discussed. This latest article deals with the third of the ‘D’s, which requires that walls be able to “dry” water from the back of their cladding or sarking membranes. Typically in Australia, this drying capability is achieved by adding a ventilated cavity behind the cladding. By Jesse Clarke

There are several standard designs for draining and ventilating brick veneer cavity walls and ventilated fibre cement. However, there is no established design process that is used to decide on the ventilation provisions that will work best with other wall claddings or non-standard systems.

Wall cavity systems should always have a water resistant drainage plane. In Australia, this is usually the sarking material placed on the external side of the studwork, most commonly water barrier sarking membranes (AS/NZS4200.1). Ideally these will have low water absorbency and air barrier properties.

The classification of cavity type depends on the quantity of ventilation that can be achieved in the cavity. This is primarily dependent on the battening method, and the top and bottom vent configuration.

There are three general classifications for wall cavity systems. A Drained and Ventilated Cavity separates the cladding from the wall by a cavity that is vented at the top and bottom and detailed to allow water to drain from the base of the wall. Open Rainscreen Walls follow a similar design, but without deliberate openings at the top of the cavity. A Drainage Plane separates the cladding from the inner wall by a narrow cavity of only a few millimetres thickness that is only intended to drain water from the back of the cladding.

 

Brick veneer is the most widely used drained and ventilated wall type in Australia, with a sound history of managing water leakage. Recommendations for cavity depth and ventilation opening sizes vary little between standards and the practical literature on design and construction, with minimal change to the systems underlying principles.

The perpend openings required by AS 3700:2001 amount to 76mm high opening by the width of the mortar joint at maximum centres of 1200mm. This equates to every 5th brick and a total of 633mm² of ventilation opening per metre of wall. Usually, centres are closer to every 3rd brick, typically 900-1000mm² of ventilation opening per metre of wall.

Figure 1 Typical Brick Veneer Drained and Ventilated Cavity

A typical eave detail allows the wall cavity to open into the roof cavity (Figure 1). Considering the purpose of the wall cavity is to remove moisture by using air to transport it away, it stands that air vented into the roof cavity increases the risk of moisture-related issues in roof spaces.

Large amounts of moisture can enter the roof space, particularly when the wall cavity is subject to excessive wetting from poor window flashings or non-sealed façade materials. To counter this, ventilation openings to the external air may be incorporated at the top of the wall or as a continuous 10mm opening, enhancing moisture resilience. Protecting the structure from moisture has always been one of the primary goals of product manufacturers, builders and practitioners. This is the primary reason for the use of sarking membranes to protect many common building materials that cannot be exposed to high levels of moisture for long periods of time. Buildings are not perfect, leaks still occur, and the role of drying is very important to the moisture balance.

The BCA has several deemed-to-satisfy provisions for weatherboard and fibre cement sheeting products that are only separated by incidental gaps between the cladding and sarking membranes and could be considered ‘drainage plane systems. These deemed-to-satisfy systems do not have purpose built cavities and are therefore missing the benefit of accelerated drying potential in the case that moisture ingress should occur. These systems are not recommended if durability is a high priority.

The capacity for ventilation drying is intrinsic 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 such as a fibre cement sheeted wall (drainage plane system).

Cavities that are only vented at the bottom work well. These allow air to circulate between cavities, relying on other air leakage paths in the wall, such as gaps between battens, or purpose-built vented battens. Most weatherboard claddings have natural leakage paths between lap joints and provide for ventilation drying – even where they are directly fixed to the frame.

According to research conducted in Canada, walls with cavities dry three times faster than comparable panels without cavities. Ventilation openings at both top and bottom improve the drying process compared to bottom only vents, and walls with wider cavities dry faster.

Walls with higher vapour-permeable sarking dry water trapped in stud cavities faster than walls with low vapour permeance sarking. Lower vapour permeance generally restricts the drying through the sarking to the exterior.

Solar radiation increases drying in ventilated walls, but has little effect on claddings directly fixed to the studwork. In Australia, south facing walls are the most common place to find moisture related problems, where solar heating is limited and ventilation drying is predominantly driven by the wind.

Wall systems with large open vents provide high ventilation rates at low wind and solar exposure, whereas wall systems with smaller vents require much higher driving pressures for effective ventilation. Plastic bug screens typically installed in the vent openings are restrictive to flow and will significantly reduce this ventilation rate by an order of magnitude.

Effective ventilation is dependent on both the wall system and exterior climate. High winds and high temperatures produce higher flow rates. Fast-drying wall designs can be repeatedly wetted over several years and remain in almost perfect condition without damage.

Ventilation rates in construction cavities are measured in how many times per hour the air within the cavity replaces itself. This is related to the volume of the cavity and the volume of air flow into the cavity.

Figure 2 Drying comparison for a 50mm cavity with different ventilation rates (Derived from Schumacher et al. 2003).

Figure 2 shows the effect of air change rate in a 50mm cavity and the ability of the wall system to dry trapped moisture in a wall. In the case shown over 100 grams (100ml) of moisture was able to be removed per square metre of wall area every day. It can be seen that ventilation drying has an important role in moisture resilience of walls.

If a wall receives little sunlight, ventilation rates will be predominantly driven by wind conditions during daytime hours. Research conducted by BRANZ and Lund University in Sweden shows the relationship between the external wind speed and the airflow rate for five cavity configurations shown in figure 3.

Cladding systems that are fully open along the bottom and top ends of the cavity with vertical battens achieve maximum airflow. Even with very low wind speeds, cavities which have unobstructed airflow and suitable openings at top and bottom will still achieve good air change rates and high drying potential.

Figure 3 Indicative cavity air change rate by wind speed (Derived from BRANZ and Lund University)

Drained and ventilated cavities and open rain screens provide the best protection against moisture-related issues. Top and bottom vent dimensions, batten configuration and cavity width will determine the ability to improve drying potential.


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