Empty or filled cavity – which has the better performance?

Australian double brick external walls typically have two major components, the masonry and the empty (air) cavity, but this is not common practice elsewhere in the world. The reason? Cold air is a pretty poor moisture performer.

Replacing air with ComfortWall greatly reduces the conditions that encourage mould inside the cavity, but how? What’s really going on inside a masonry wall?


Everyone is familiar with the photograph below – mould growing on bathroom tiles.

When the bathroom is used, air humidity increases (steam = water vapour = humidity) and the moist air coats the surface of the walls. In winter, when the wall is cold, this coating is seen as droplets of condensation. This liquid moisture enables mould spores in the atmosphere to germinate and growth.

A similar thing happens with the external walls of a home.

Just like the bathroom, when we use or occupy a home the air we exhale increases the amount of moisture in the air – and this raises the indoor humidity.

When the doors and windows are kept closed the humidity built up inside the home becomes greater than outside and begins to ‘migrate’ outward through the absorbent bricks. If the air in the cavity is warm enough and dry enough, the humidity diffuses easily and the wall remains dry from moisture. However if that flow encounters any resistance the walls become damp (but not visibly wet).

What causes resistance in the outward flow? Like all technical issues, it is usually a combination of factors such as:

  • residual construction moisture because the house was built during the wet winter
  • a leaking flashing or water pipe
  • rain absorbed from the outside lifting the cavity’s relative humidity above 60% and causing moisture to form
  • cold damp air in the empty cavity that, just like the bathroom, won’t absorb the humidity and condenses moisture onto the cavity brick face.

During winter, all of these factors are at play, so it is easy for dampness to accumulate in the external and internal leaf of brickwork. We can’t see this dampness. But we can see the result of persistent dampness – mould growth, and eventually decay and corrosion (this why old brick wire ties rust).

Designing the wall to managing dampness

Until recently the measure of a good building – or a good wall – was one that kept the rain out – that did not leak.

However, as described above, building research has now proven that leaks are just one part of the story. The key to designing a wall with the capacity to manage dampness is to enhance the way water vapour flows through the wall. With advances in computer modelling software, the hygrothermal (energy and moisture) performance of a wall can be modelled. For local weather conditions and cavity masonry construction, modelling shows that wall dampness is minimised when the empty cavity is fully insulated.

Why? Just like the combination of factors that can cause dampness, there are several critical factors that drive dryness.

In winter, an insulated cavity efficiently retains the home’s heat and keeps the internal leaf of masonry much warmer. This allows water vapour to flow outward into the warmer cavity without beginning to condense on the cavity side of the inside brick. Filling the cavity also stops cold moist air from entering the cavity and this stops the formation of condensation in the cavity. And, with the remainder of the brick work being dryer, rain absorption tends to remain at the outer side and hence the brick dries faster when the sun heats the wall between showers.

This holds true only if the insulation is both vapour permeable and water repellent because the moisture management is dynamic. The role of the insulation, in addition to thermal performance, is to act as a barrier to liquid water – but to be as permeable as air so that water vapour can flow freely.

ComfortWall has been specifically developed to use in a masonry cavity. The engineered stone wool is as permeable as air whilst also being highly water repellent, and being made of rock it does not rot or decay.

Pumped into the cavity at a controlled density, the stone wool fills the cavity, fitting tight to the masonry and around wire ties and pipe. Resulting in a water repellent stone wool insulation layer that is too fine for water droplets to enter but open enough for air to carry vapour through.

Building Code of Australia (BCA) Compliance

BCA Volume Part 3.3.4 Weatherproofing of masonry requires a wall to prevent the penetration of water that could affect the amenity of the occupants and/or cause deterioration of building elements.

Cavity masonry with ComfortWall installed satisfies this performance. In addition, because it is not a foil barrier, ComfortWall is highly permeability, creating a wall configuration that promotes drying to reduce dampness from water vapour.