Know Your Cavity Part 2: Air flow in Ventilated Cavities

This is second of four major articles in our ‘Know Your Cavity’ series which analyses key hygrothermal issues for the performance of wall construction systems. The articles outline the material properties and design parameters which can be used to create effective drying to complement the thermal control properties of a construction system. This second article answers questions surrounding cavity type, drainage and airflow functions suitable for specific regions.

The modern house should be designed to be durable and capable of being maintained – and the single most important factor affecting durability is deterioration of materials by moisture. Houses should be protected from wetting during construction and operation and also be designed to dry should they get wet.

The Building Research Association of New Zealand (BRANZ) promotes a design and construction methodology which follows the four ‘Ds’ – Deflection, Drainage, Drying and Durability.

Deflection and Drainage are all about using correct flashings and drainage planes to protect the construction from wetting and excess accumulation of moisture in the construction system.

Drying is all about improving the resilience of the structure due to botches in the design or construction process which allow for water ingress (as much as we would like to think this never happens).

Durability is about selecting the right materials which can handle slightly moist conditions for increased duration.

 

The US follows a similar approach, but the science is communicated in a slightly different manner: drain screens or rain screens, and water sensitive materials are used to achieve constructions suitable to the climate. Drain screens provide as little as 1mm drainage cavity to drain off any rain which should penetrate through the cavity.

The drain screens allow water to escape from an opening at the bottom of the façade under the influence of gravity but are closed along the top edge of the cavity. Rain screens are different in their approach as they incorporate air cavities behind the cladding usually greater than 20mm, and will be open along the bottom and top edges for the cladding, allowing for drainage and improved ventilation (increasing drying potential) behind the cladding. The ventilation rate – and therefore drying potential – is governed by the cavity thickness and somewhat by the climate and sunshine on the cladding.

In the US they often refer to moisture sensitive materials which should be protected from wetting. The more susceptible or vulnerable to moisture damage the materials comprising an assembly are, then the more ‘robust’ the rain control strategy required. Moisture sensitive materials may include plasterboard, untreated wood based bracing boards, timber weatherboards and cellulose insulation. Conversely moisture insensitive (durable) materials may include concrete, brickwork, Hebel, synthetic membranes, Cemintel and some insulation types such as PIR boards and Glasswool.

Additionally, assemblies have different drying characteristics based on material properties, insulation levels, interior and exterior climatic conditions and cavity air flows. In general, vulnerable assemblies with poor drying characteristics require greater rain control than moisture insensitive materials with high drying potentials.

In Australia, ventilated cavities have been fundamental in allowing airflow within construction systems to encourage drying for many decades. This practice is still an extremely valid method to create moisture resilient construction systems. Cavities built in behind cladding systems in Australia are generally intended to provide drainage of liquid water and drying of excess water.

The widespread deployment of ventilated cavity systems driven by historical practice and the theory backed by the building codes have allowed us some ‘forgiveness’ when using the ‘she’ll be right’ technique of design and construction which often overlooks the detail of deflection and drainage theory including overhangs, eaves, flashings with drip edges and extended window sills.

Drainage will remove much of the bulk moisture by gravity, when a drainage path is provided. However moisture can still remain adhered to or absorbed in materials within the wall assembly. The amount of moisture that can be safely absorbed or stored depends on the material properties. Drying can occur by vapour diffusion, evaporation, desorption, or by air convection (i.e. ventilation).

Vapour diffusion is shown to be a relatively slow process particularly when low permeance materials are used within the wall assembly. Evaporation or desorption can only occur when moisture is able to get to the surface of the material (often only at the cladding or interior surface), and be removed by the flow of air.

Allowing evaporation or desorption to occur at layers within the wall assembly, particularly at the sarking membrane and removing the excess moisture by ventilation to the exterior provides an effective means to remove additional moisture directly from sensitive materials and improve the drying potential of some wall assemblies. This effect is promoted through the use of vapour permeable membranes as discussed in Know Your Cavity – Part 1 in the cooler and temperate parts of Australia.

Figure1  Functions of a wall cavity on thermal and moisture performance of a wall system, Adapted from Mikael Salonvarra et al. (2007)

As a rule of thumb, systems not incorporating a drainage cavity should only be used in locations where the rainfall is below 500mm per annum, which limits the use of these systems to the driest inland areas of Australia. Systems which are only drained should only be used if rainfall is below 1000mm per annum.

Coupling ventilated air spaces behind claddings with drainage planes in an assembly constructed with moisture insensitive materials with a high drying potential represents the most hardy construction systems and addresses all the driving forces for rain penetration into and through building surfaces and can be used in the wettest regions of Australia including: Darwin, Cape York, Cairns, Mackay, Gold Coast, Tweed Heads, Coffs Harbour, the snowfields and the West of Tasmania.

Figure 2 The amount of rain deposited on a surface determines the type of approach necessary to control rain.

Drying cavities is best achieved by having as much airflow as possible through the cavity. This allows for moisture to be harmlessly removed to the exterior in a more efficient manner. However problems may arise if there is no clear distinction between the ventilated section of the construction system and the non-ventilated cavities within the stud bays.

Generally speaking insulated cavities (stud bays) should be maintained with still air for energy efficiency gains as outlined in the last instalment of know your cavity. The only way to achieve this is to use air barrier sarking or rigid sheathing which are tapped or sealed at joints.

Wind pressure across the wall can easily force air through wall cavities, stud bays and plasterboard penetrations to the inside or vice versa. This has implications for the energy use in the dwelling as well as climate specific risks for interstitial condensation in both hot-humid and cold climates where vapour can be transferred with air and condense if it comes into contact with cool surfaces.

Getting the design right relies on designing an escape route for moisture. Good design incorporates enough drying capacity to deal with excess liquid water ingress, humid air ingress, and water vapour diffusion from humid indoor or outdoor conditions.


Un-Insulated Drying Paths



Historically Australian houses relied upon airflows behind a veneer or cladding to keep the structure dry. The main benefit that the pre energy efficiency buildings had was that the lack of insulation meant that all cavities were larger, leakier and generally warmer due to interior heating or external hot climate. The high heat transfer through the cavity allowed for extremely large drying potential; more heat and more airflow allowed for more drying but resulted in extremely poor energy efficiency.

The cavity and stud bays were amalgamated as one, so the majority of moisture was removed through air transportation of the water vapour to outside.

It is important to remember that this design theory was developed in a time long before air conditioners and insulation even existed.

Drying Paths with Insulation



With increasing energy efficiency regulations and addition of insulation and reflective membranes completely changed the heat, air and vapour transfer characteristics of walls. Cavities became segmented and smaller; air leakage through cavities decreased and vapour permeability of the construction decreased with the addition of metallic foils. Colder cavities, less airflow and incorrectly located vapour barriers have seen emerging moisture related issues in Australia.

Ventilation cavities and insulation cavities were divided by the use of sarking. This means the sarking properties are important if trying to allow drying through air transportation of water vapour through cracks or vapour diffusion through building products.


Hot and Humid Climates



Cavity The drained cavity removes excess liquid water which may penetrate during heavy rain periods. A well-sealed, air, water and vapour barrier divides the stud cavity from the ventilation cavity to provide maximum energy efficiency and weather tightness. In hot humid climates the buildings tend to dry by vapour permeating to the interior through the plasterboard. The well-sealed vapour barrier on the external side of the studs prevents humid air forcing vapour into the construction. The cavity primarily acts as a capillary break preventing liquid water migrating across the cavity.

  • Air Barrier Yes
  • Water Barrier Yes
  • Vapour Control BARRIER (>7MN.s/g)

Mixed Climates



Cavity The drained cavity removes excess liquid water which may penetrate during heavy rain periods. A well-sealed, air and water barrier divides the stud cavity from the ventilated cavity to provide maximum energy efficiency and weather tightness. Higher vapour permeability of the membrane increases the ability for the system to utilise the cavity as a drying path and safely remove moisture to outside during favourable conditions. Increasing cavity size and ventilation rates increases the drying potential.

  • Air Barrier Yes
  • Water Barrier Yes
  • Vapour Control Location Specific

Cold Climates



Cavity The drained cavity removes excess liquid water which may penetrate during heavy rain periods. A well-sealed, air and water barrier divides the stud cavity from the ventilated cavity to provide maximum energy efficiency and weather tightness. Higher vapour permeability of the membrane is required to allow any leaked rain to permeate to the cavity and utilise the cavity as a drying path safely removing the moisture to outside. Increasing cavity size and ventilation rates increases the drying potential.

  • Air Barrier Yes
  • Water Barrier Yes
  • Vapour Control NON BARRIER <0.5MN.s/g Recommended

In all cases the drained cavity removes excess liquid water which may penetrate during heavy rain periods. A well-sealed, air and water barrier divides the stud cavity from the ventilated drying cavity to provide maximum energy efficiency and weather tightness. The cavity drying theory as outlined above hold true for all wall system types the main difference is the selection of the vapour Permeance of the membrane to achieve maximum resilience and durability.