The void created between
the ceiling and the soffit of the structural floor is often
utilised to carry building services, but it will also provide a
route for smoke and fire transfer in the event of a fire. The void
will also create a route for sound transfer to flank around
soundresisting partitions.
The Building Regulations require cavity barriers to subdivide these
voids if they extend beyond certain dimensions. In addition cavity
barriers are also required to be placed above fire resisting
partitions. However, Building Regulations do not require the
control of sound flanking around partitions via the ceiling void.
The demand for this is expected to come from the occupiers of the
building.
Advantages
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, dense rock mineral
wool mattress is an ideal product to subdivide these voids since it
will restrict the passage of sound and fire.
However, the method of installation used to install the cavity
barrier is of crucial importance. Performance figures obtained in
fire tests are based on both the material and the method of
installation. In general, rock mineral wool fire barrier systems
should utilise commonly available supporting components.
In certain circumstances, a suspended ceiling system is used in
combination with sound absorbing materials such as mineral wool 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 mineral wool quilt enclosed in a scrim
cloth. Mineral wool 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.
Summary
Knauf Insulation provides cavity barrier systems for
subdividing voids above suspended ceilings, providing both fire
resistance, as required by the Building Regulations, and acoustic
insulation. The systems fully comply with the Building Regulations
requirements for fire resistance. 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.
The Building Regulations 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
In addition, voids above suspended ceilings need to be subdivided
if they exceed certain dimensions.
Detailed requirements are complex. For full details about the
position and spacing of cavity fire barriers refer to:
- Section 10 of Approved Document B (England and Wales)
- Part D6 of Technical Standard D (Scotland)
Cavity barriers should achieve at least 30 minutes fire
resistance, providing a minimum of 30 minutes integrity and 15
minutes insulation.
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. There are no specific British
Standard test criteria for resistance to the passage of smoke.
However, indicative tests have shown that mineral wool, 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 mineral wool can be employed
to minimise the ‘acoustic weakness’ of suspended ceilings by
installing mineral wool cavity barriers above partitions.
Mineral wool can also 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 mineral wool cavity barrier,
in combination with the suspended ceiling, can limit flanking sound
– see illustrations below.
To achieve the sound reduction figures in the above illustrations,
the partition must have at least the same level of sound insulation
as the cavity barrier and ceiling combined.
Perforated Metal Ceilings
High degrees of sound absorption can be
achieved using perforated or slotted panels with a mineral wool
backing. The presence of an air space above a suspended ceiling or
behind a wall lining greatly increases the absorption value of the
mineral wool backing.
Although mineral wool 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.
Glass mineral wool and rock mineral wool have equally good sound
absorption characteristics due to their fibrous nature, making
mineral wool ideal as a sound absorption layer.
Fixing Systems
Any item
used for fire protection can only be considered to be offering a
specific period of fire protection when it is installed in the
manner specified in the appropriate fire test certificate. This
includes the specification of all associated fixings and supporting
framing. The specification of these items for use with Rocksilk
Smoke and Fire Barrier are detailed in the table above.
Typical Specification Clauses
Half hour cavity barrier
A single layer of 50mm Rocksilk Smoke and Fire Barrier (*Foil
faced) 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)