Plasterboard in Areas Subject to Intermittent Heat

We are often asked “Which product should be used in an area subject to intermittent heat such as from gas hobs and around fireplaces?”

Much confusion surrounds the specification of plasterboard in areas subject to heat generated by devices such as gas or solid fuel heaters, gas or electric cooking hobs, radiant heating systems, halogen lighting or flues.

The protection of combustible surfaces surrounding gas cooking appliances is covered by NZS 5261.  Consult the current version of this standard to ensure compliance.

However, as a guide the following options are acceptable for wall surfaces within 200mm of the periphery of a gas element to a height of 150mm above the element for the full dimensions (width and depth) of the cooktop surface area: 

• 5mm ceramic tiles on GIB® plasterboard
• 5mm toughened glass on GIB® plasterboard
• or any system that can be demonstrated to meet the requirements of Clause 2.6.2.6 of NZS5261.

Except as outlined above it is commonly assumed that because fire resistant plasterboard is the product generally specified for fire rated construction, it is equally appropriate to require it in areas exposed to heat sources as described above.

This is not the case. Gypsum plasterboard used in a fire rated system degrades but retains its integrity long enough to achieve the stated fire resistance rating (e.g. 60/60/60).

The effectiveness of a fire rated system is a combination of the framing design, correct plasterboard specification and strict adherence to plasterboard installation and finishing.

AS/NZS 2589; 2007 which is the joint Australian and New Zealand standard that covers the installation and finishing of Gypsum plasterboard linings states

Gypsum linings shall not be exposed to temperatures exceeding 52°C for prolonged periods.  Note: Heat generating devices may include halogen lighting, cooking elements, radiant heating and solid fuel exhausts.

All plasterboard contains water which is chemically bound into the gypsum crystal structure but which is released as moisture vapour or steam when the gypsum is exposed to elevated temperatures. Regular exposure to temperatures of 52°C or greater will evaporate the water and attack the crystal structure causing calcination. Calcination results in the gradual deterioration of the integrity of the gypsum core. The degree to which this deterioration occurs is proportionate to the temperature and time of exposure.

Gypsum plasterboard also has a paper face, commonly finished with wallpaper or paint. This surface finish is often not appropriate for use in close proximity to a heat source.

Gypsum plasterboard (of any type) must be protected to prevent it being subjected to temperatures exceeding 52°C for prolonged periods. This protection can take the form of a stainless steel sheet, ceramic tiles or heat resistant glass. The requirements for protection around gas hobs are covered by NZS 5261; 2003

In summary: It is not appropriate to specify or insist on the use of fire resistant plasterboard in heat exposed situations. All types of gypsum plasterboard must be protected from prolonged exposure to temperatures exceeding 52°C.