Suspended Ceilings

Suspended ceilings are used extensively in many public and commercial buildings to create an attractive ceiling below a structural floor


Suspended ceilings are normally installed to create a decorative ceiling below a soffit or roof void. However, its installation will almost inevitably create a void between the ceiling and the soffit of the floor or roof above. Unrestricted voids need to be avoided as they create a route along which fire and sound may travel.

Due to its inherent product characteristics, stonewool mattress is an ideal product to subdivide these voids since it will restrict the passage of sound and fire.

In certain circumstances, a suspended ceiling system is used in combination with sound absorbing materials such as glasswool or stonewool to control the acoustic characteristics of a room or area. In areas where a high performance is required, such as broadcasting studios or auditoria, a perforated suspended ceiling tile is installed which is then backed with a stonewool wool quilt enclosed in a scrim cloth. Stonewool is an excellent acoustic absorber.

Knauf Insulation Products

  • Rocksilk Smoke and Fire Barrier is made from rock mineral wool, and formed into a flexible mattress faced on one side with galvanised wire mesh stitched in position.
    Rocksilk Smoke and Fire Barrier is also available foil faced on one or both sides.
  • TP116


Knauf Insulation provides cavity barrier systems for subdividing voids above suspended ceilings, providing both fire resistance and acoustic insulation. Knauf Insulation products can also be used to overlay suspended ceiling system to improve the acoustic performance of the ceiling. The table below illustrates cavity barrier systems offering both fire resistance and sound insulation

Detailed Design Considerations

Fire Protection

Concealed spaces or cavities, particularly those above a suspended ceiling, provide a ready made route for smoke and flame spread. Cavity barriers above suspended ceilings are used to prevent smoke and flame from bypassing fire-resisting walls and partitions.



Building Regulations usually require cavity barriers to be provided above suspended ceilings in non-domestic buildings where the fire-resisting construction is not carried full storey height:
  • In a protected escape route
  • Where the corridor should be subdivided to prevent fire or smoke affecting two alternative escape routes simultaneously – see diagram below

Client Requirements

The requirements in the Building Regulations are to protect life, not property. To limit the degree of damage to property, a building owner may wish to install cavity barriers at more frequent intervals.

Resistance to Smoke

Cavity barriers are primarily intended to delay the passage of fire. Indicative tests have shown that stonewool, reinforced with a 25mm wire mesh, acts as a high efficiency filter to prevent the passage of smoke particles.

For applications where a particularly high standard of resistance to smoke or fumes is required, then a foil faced cavity barrier should be used. Similarly, a foil faced cavity barrier may also be considered necessary in air plenums.

Acoustic Insulation

The void above a suspended ceiling is a potential route for flanking sound above partitions. This is especially true where a lightweight, modular ceiling is installed.

The sound absorbent characteristics of glasswool can be employed to minimise the ‘acoustic weakness’ of suspended ceilings. Glasswool can be used to provide acoustic absorbency at ceiling level, and so reduce the reverberation time in the room below.

Sound Absorbent Cavity Barriers

There are many situations, such as hospital consulting rooms, where it is important that the sound performance of a partition is not compromised by the void above the suspended ceiling. In these cases, a stonewool cavity barrier, in combination with the suspended ceiling, can limit flanking sound – see illustrations below.

Perforated Metal Ceilings

High degrees of sound absorption can be achieved using perforated or slotted panels with a glasswool or stonewool backing. The presence of an air space above a suspended ceiling or behind a wall lining greatly increases the absorption value of the insulation backing.

Although insulation installed directly above a perforated metal ceiling will help to reduce flanking sound, this form of construction is mainly used to reduce the reverberant noise within a room or area.

Sound Absorption

Sound absorption refers to the attenuation of reverberant noise within the same room or area as the noise source. This normally involves lining all or part of the room surfaces with a material which absorbs sound.

When a sound wave hits a surface, some of its energy will be reflected and some will be absorbed. A material’s ability to absorb sound efficiently can be gauged from its sound absorption coefficient, which is defined as the ratio of the sound energy absorbed to the sound energy incident. A material which absorbs 65% of the sound energy striking it has a sound absorption coefficient of 0.65.

Glasswool and rock wool have equally good sound absorption characteristics due to their fibrous nature, making mineral wool ideal as a sound absorption layer.

Typical Specification Clauses

Half Hour Cavity Barrier

A single layer of 50mm Rocksilk Smoke and Fire Barrier to be installed in the positions marked on the drawings, and fixed in accordance with the manufacturer’s recommendations. All joints either tightly butted, overlapped, or with edges returned, and stitched in a continuous spiral loop with 1mm diameter galvanised lacing wire (* delete as appropriate).

One Hour Cavity Barrier

Two layers of 50mm Rocksilk Smoke and Fire Barrier (*Foil faced), separated by a 50mm air space to be installed in the positions marked on the drawings, and fixed in accordance with the manufacturer’s recommendations. All joints either tightly butted, overlapped, or with edges returned, and stitched in a continuous spiral loop with 1mm diameter galvanised lacing wire (* delete as appropriate).