The polystyrene in the Thermacell blocks contains a natural fire retardant. When exposed to a source of heat, our blocks only shrink - they do not ignite. According to the head of the fire testing section of the CSIRO, in the event of fire Thermacell blocks will not lose their integrity, because they are steel reinforced. Walls constructed solely of bricks and concrete, by contrast, do not stand up in a fire test – we have all seen the tragic results of conventional brick homes caught in a bushfire. The bricks absorb the heat and expand, the mortar between the bricks breaks down quickly, and the walls collapse, exposing the timber frame. This in turn is quickly destroyed causing the roof to collapse.

Thermacell walls are generally rendered with fibreglass-reinforced cement render. This combination will generally survive for about 1 hour in a fire. However, the inside of the wall is covered with plaster sheeting, and this too will last about an hour in a fire. Therefore, it would take an extremely hot fire over a fairly long period to get through the render on the outside and the plaster sheet on the inside of the walls. Thermacell walls can also be clad with concrete tiles such as Hooblerstone or veneered limestone, or by products such as Magboard, which is a tested fireproof material.

Thermacell has been shown time and time again to be a suitable building material in fire-prone areas, and as such it has been approved by councils in declared fire areas such as the Murrindindi Shire Council (Kinglake area) of Victoria.