Empty or filled cavity[0] – which has the better performance?

Traditional local practice has been to build with an empty masonry cavity – but this is not common practice elsewhere in the world. The reason – air has pretty average acoustic, thermal and moisture performance.

But cavity masonry has excellent fire performance. Masonry is non-combustible and cavity masonry is very good at resisting (stopping) fire from getting to the other side of the wall (inside the home).

This excellent performance is compromised when combustible insulation is placed in the cavity.

Non-combustible or combustible

These are BCA defined terms – but also two of the most misunderstood and incorrectly used material descriptions.

The BCA states that materials are either combustible or non-combustible.

As suggested by the description, combustible materials burn.

Terms used to imply that an insulation is not combustible are: non-flammable, flame resisting, fire retarding, have a fire retardant additive – and even flame repellent. When these descriptors are used, the material typically will not spontaneously combust when exposed to a flame – but will ignite and then burn when exposed to the elevated temperatures and flames of a fire.

Definition: a material that has been tested to AS 1530.1 and certified to be non-combustible and construction (a wall) that is made wholly of materials that are non-combustible.

AS 1530.1 is a very severe test – one that few materials can pass. It was originally developed to test materials used for ship building to protect the lives of people at sea (where is the exit door on a ship?).

The Certificate opposite verifies that ComfortWall has been tested to AS 1530.1 by the CSIRO and is non-combustible.

Reaction to fire

Reaction to fire

Resistance to fire (ROCKWOOL stone wool)

Reaction and resistance to fire

Building fire performance has two parts:

  1. Reaction (combustibility) – as shown in the upper image, will material used to build the house burn and, if so, how does it “react” when exposed to a flame (smoulder, ignite, melt and/or create toxic smoke)?

When ComfortWall is added to the cavity, being non-combustible, it will not burn. Therefore there is not risk that it will ignite when exposed to the concentrated flame and heat of a plumber’s oxy-welding equipment.

  1. Resistance (Fire Resistance Level, FRL) – as shown in the lower image, will the wall “resist” or stop a fire from getting inside the house so that the occupants have time to escape?

When ComfortWall is added to the cavity, fire engineering analysis shows that if the outer leaf was exposed to fire conditions with temperatures up to 1,0000C for 90 minutes, there would be less “bowing” or deflection because the insulation kept the inner leaf cooler and resulted in a more even temperature gradient across in the outer leaf of masonry.

Building Code of Australia (BCA) Compliance

The BCA Volume One Part 3.7.1 Fire Safety prescribes “resistance” performance for external and separating walls, but does not prescribe any “reaction” performance for materials that are located in the cavity of a masonry wall.

Empty cavity masonry is non-combustible and provides a high level of fire resistance. If insulation is added to the cavity space, this high standard of performance is retained when the insulation is non-combustible.

For bushfire prone areas, a cavity filled with ComfortWall provides the added safety of not being at risk of ember attack because the material is non-combustible and because it acts to block embers from entering the cavity through any gaps in the external leaf that can form over time around windows, doors and service penetrations.