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? Air is a pretty poor thermal performer.

Replacing air with ComfortWall provides the following higher performance benefits:

Heat flow – empty cavity

In summer (as shown in the top illustration), the empty cavity allows heat from the external leaf flows inward, heating the cavity, then the internal leaf – and then the inside of the home. With likely high cavity ventilation rates (air flowing through the cavity) during the day, the air in the cavity will tend to be at the same temperature as the outside air – reinforcing the inward heat flow.

In winter, with an empty cavity the process is reversed – with the addition of internal walls acting as a thermal bridge allows heat from the warm internal walls to flow outward. Whilst cavity ventilation rates are likely to be lower, they will tend to increase heat flow because the air in the cavity will be kept near the temperature of the outdoor air.

Heat flow – ComfortWall filled cavity

Building research now recognises externally insulated walls as having the highest “real-world” or operational performance.

Filling the cavity with ComfortWall applies this principle to cavity masonry because there is continuous insulation layer (no joints etc) on the outside of the inner leaf of brickwork, the cavity space is fully filled (no ventilation heat loss/gain) – and the insulation is tight fitting around frames and like (no air leakage).

Hygrothermal modelling shows that with ComfortWall installed, summer and winter heat flow through the insulated cavity masonry reduces by up to 00%.

Walls play a critical role in the overall thermal performance of the home. House Energy Rating Software (HERS) modelling shows that, when the 50mm cavity is fully with ComfortWall, the star rating of the home can increase by up to 00%, reducing air-conditioning and/or heating costs by up to 00%.

Building Code of Australia (BCA) Compliance

The BCA Volume One Part 3.12 prescribes the thermal performance that must be achieved by new homes. BCA 3.12.0 Application of Part 3.12 provides two pathways for compliance: 3.12.0(a)(i) – a minimum 6-Star rating verified using HERS, or BCA 3.12.0(a)(ii) – comply with the Elemental Provisions (Parts 3.12.1 to

There Total R-Value of cavity masonry with ComfortWall installed is:

R0.69 (cavity masonry) + R1.32 (50mm ComfortWall) – R0.17 (cavity air space) = R1.84.

For new homes, the Total R-Value of R1.84 can be used for the HERS option, however, depending on the option used, may not satisfy the Elemental Provisions. For renovations, the Total R-Value of R1.84 can be used to satisfy the WA Alterations & Additions Protocol for Energy Efficiency.

Operational Performance

Compliance versus Operational Performance

Builder know the real test of any construction is how it performs over the life of the building. The technical term for this is operational performance – the difference between design (theoretical) and actual (operating) performance when the real-world effects trade practices, climate and the like are allowed for.

Insulation options for cavity masonry range from bulk insulations such as ComfortWall to reflective foils. Whilst both satisfy BCA Building fabric thermal insulation (design performance), long-term operational performance can vary significantly.

The operational performance of reflective foils is likely to be affected by the combination of changing cavity air temperatures, cavity air flows, tarnishing and/or dust accumulation.

ComfortWall is manufactured by the ROCKWOOL Group, the world’s largest stone wool manufacturer. Their leading edge technology and continuous quality control sampling produce a stone wool of the highest quality.

ROCKWOOL stone wool is dimensionally stable, will not slump or distort over time and will not expand or contract with changes in temperature and/or relative humidity. Meaning that the operational performance of a masonry wall with a ComfortWall filled cavity will not diminish over time.