Interior cladding

Fibre Reinforced Facade Encyclopedia: Design and detailing

Interior cladding is any panel, board or lining applied to an internal wall, ceiling, bulkhead or column as a finished surface rather than as structure. The governing fire measure for these surfaces is not non-combustibility. Under the National Construction Code (check the edition adopted in your jurisdiction) an internal lining is assessed by its material group number, determined under AS 5637.1, and by its smoke growth rate index.

At a glance

Fire inside a building is a group number question

For internal wall and ceiling linings the National Construction Code asks how quickly a surface will spread flame and generate smoke in a room fire, not whether the material burns at all. That is expressed as a material group number: Group 1 is the best performing, Group 4 the worst, and the permitted group depends on the building class, the location within the building and whether the building is sprinkler protected. Group 4 materials are not permitted as linings in most regulated locations.

The number is determined under AS 5637.1, either directly from a full AS ISO 9705 room corner test or by prediction from an AS/NZS 3837 cone calorimeter test on the specific product. For Group 1 and Group 2 linings a smoke growth rate index limit also applies. See group number for how the classification is derived and reaction-to-fire for how it sits against the combustibility tests used externally.

Materials used internally

• Plasterboard and fibrous plaster: the default, cheap, easily made flat, limited in three-dimensional form.
• Glass fibre reinforced gypsum: cast to a mould, so it holds complex geometry, curves and relief while staying light.
• Fibre cement sheet: robust, tolerant of humidity, limited in form.
• Timber and timber veneer: warm and common in fitouts, and the material most often caught by group number limits without treatment.
• Metal panel and perforated metal: durable and acoustically useful, flat or singly curved.
• Fibre reinforced polymer: used internally for sculptural elements and heavy-traffic surfaces where a moulded polymer composite is appropriate.

The choice usually comes down to geometry. If the surface is flat, plasterboard wins on cost. Once the design calls for curvature, relief, coffering or a seamless sculptural form, a cast material becomes the practical option.

Where glass fibre reinforced gypsum sits

Glass fibre reinforced gypsum, GRG or GFRG, is alpha gypsum reinforced with glass fibre and cast as a thin shell against a mould. It is an interior material and only an interior material; gypsum takes up moisture, so it has no place in an external wall, a wet area or an unconditioned space. ShapeShell-RG is cast as a 3 to 6 mm shell at about 23 kg per square metre, which is light enough to be carried on a standard suspended ceiling grid or a light stud frame.

The advantage over plasterboard is that the mould face becomes the finished face. Curves, coffers, relief patterns, integrated lighting reveals and complex ceiling geometry come out of the mould rather than being built up on site from framing and skim. Joints are stopped flush so that a run of panels reads as a single continuous surface, or expressed as a deliberate shadow gap where movement or access argues for it.

Fixing, movement and access

Interior panels move far less than external ones, because the temperature range is small, but they still move. Gypsum composites respond to humidity, so long continuous runs need a movement strategy: an expressed joint at intervals, a slip connection at the head, and a deliberate break where the lining crosses a structural movement joint. A seamless 20 metre run stopped solid at both ends will crack at the weakest joint rather than at the joint the designer chose.

Fixing is usually concealed. Cast-in inserts, secret fix clips or a proprietary rail let a panel be removed without damaging its neighbours, which matters because internal linings conceal services. Plan demountable panels at valves, dampers, lighting drivers and access hatches at design stage. A panel cut open on site to reach a service is a panel that will never look right again.

Acoustics, services and durability

A thin cast shell reflects sound. Where reverberation control is required, the lining is perforated and backed with an acoustic absorber, or the cavity behind is filled and the panel micro-perforated. Perforation changes the fire test position as well as the acoustic one, so test the perforated build-up rather than the solid sample.

Durability internally is about impact and cleaning rather than weather. In circulation spaces, lift lobbies and public areas, a thin gypsum shell is vulnerable at corners and at skirting height, and is usually protected with a hard nosing, a solid surface skirting or a change of material in the lower 1,200 mm. Where heavy traffic or wash-down is expected, a polymer composite is the more sensible lining.

Standards and references

• AS 5637.1 Determination of fire hazard properties: wall and ceiling linings
• AS ISO 9705 Fire tests: full-scale room test for surface products
• AS/NZS 3837 Method of test for heat and smoke release rates using an oxygen consumption calorimeter
• AS 1530.3 Simultaneous determination of ignitability, flame propagation, heat release and smoke release
• NCC 2022 Volume One Specification 7 and the fire hazard properties provisions (check the edition adopted in your jurisdiction)
• AS/NZS 2785 Suspended ceilings: design and installation
• AS/NZS 2589 Gypsum linings: application and finishing
• EN 13501-1 Euroclass and ASTM E84 Class A, where an international equivalence is requested

Related terms

• Glass fibre reinforced gypsum (GRG / GFRG)
• AS 5637.1 group number
• GFRG panels
• AS 1530.3 early fire hazard test
• Wall lining
• Alpha gypsum

Questions specifiers ask

Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.