As it is not possible to provide a detailed scheme
of insulation for any specific case without inspecting the premises
and performing certain measurements, the observations given here
must necessarily be general and may not produce a significant
improvement in all instances.
Before embarking on a programme of supplementing
the inherent sound insulation of the separating floor, it is worth inspecting
the floor construction to determine whether there are any specific
weaknesses from a sound insulation viewpoint. Such weaknesses
include gaps between boards in timber floors, inadequately isolated
flues or ventilation ducts, pipework etc. Sound will also be
transmitted via the walls which flank the separating floor, particularly
if they are lightweight, this is likely to limit the
maximum sound insulation attainable.
Sound transmitted through separating floors generally
falls into two categories; airborne sound (voices, music etc)
and impact sound (footsteps). Impact sound can be greatly reduced
by a soft floor covering such as carpet on thick underlay but
airborne sound would not be affected significantly. An effective
way to improve the insulation of a floor against both airborne
and impact sound is to construct a new ceiling supported on independent
joists below the existing ceiling. The new ceiling should be
of a minimum 25mm plasterboard (in two layers of at least 12.5mm)
with a cavity of 150mm or greater between the old and new ceilings
and a layer of mineral wool or glass fibre insulation at least
50mm thick and of about 50kg/m3 density in the cavity.
Such treatment is clearly only possible where
the existing ceiling is high enough to permit a new ceiling at
least 150mm below it. Where this treatment is not possible due
to restricted head height then the sound insulation may be improved
to some extent by filling the space between the floor joists with
a heavy, loose fill material ('pugging'). This can consist of
dry sand or high density (>120kg/m3) mineral wool in pelleted
form but great care must be taken to ensure that the floor and
ceiling structure can support the considerable extra weight.
Isolated 'floating' floor structures can also
be constructed on the existing floor but these require careful
design for the particular installation and only general guidance
can be given in this brief document. In many cases, the resultant
raising of floor height can cause considerable problems with doors,
skirting boards, power sockets etc. Typically such constructions
comprise a floating layer of 18mm thick timber or wood based
board with tongue and grooved edges spot bonded to a substrate
of 19mm plasterboard. The loadbearing resilient layer should
be 25mm mineral fibre having a density of between 60-100kg/m3 and
should extend up around the floating layer to provide isolation
from the walls, the gap between the floating layer and the walls
being around 10mm. The lower the density of the mineral fibre the
better the sound insulation though this does result in a 'softer'
feel to the floor. A 3-5mm gap should be left beneath the skirting
boards to prevent bridging of the floating layer, filled with a soft
setting silicone sealant.
The improvement in sound insulation obtained depends
greatly on the existing sound insulation and in some circumstances
may be negligible. In many cases, however, the use of a new independent
ceiling will give about 10dB improvement in insulation which roughly
corresponds to a halving in the loudness of the transmitted noise.
Other Sources of Information:
'The Building Regulations 2000 - Approved Document
E'.
BR 238 - CIRIA report 127 'Sound Control for Homes',
Section 4: Dwelling-houses and flats formed by material
change of use, pp 53-59.
Office of the Deputy Prime Minister, TSO.
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published by The Building Research Establishment and the
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