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Moisture control
Timber is a
natural product and ‘hygroscopic’ This means that its moisture
content will vary to equalise with changes in humidity of its
surroundings. As it varies to remain in equilibrium with its
surrounding atmosphere, it will either expand as it absorbs
moisture, or shrink as it loses moisture. Generally, the wider the
individual boards, the greater is this movement. Gaps of 1mm between
boards are not uncommon, particularly during drier months, and often
closing during wetter months. Large windows, heaters and other heat
sources will influence the moisture content in a floor. The heat
source may lead to localised movement such as in front of a heater.
In elevated floors, the underside of the boards may need to be
sealed or protected.
An alternative method of protection in
these elevated floors, is to fix the flooring over a sheet flooring
substrate. Neither of these precautions will provide a suitable
solution or protection where the sub-floor conditions are constantly
damp. In these situations the cause of the dampness must be resolved
via good drainage and ventilation.
It is recommended that
timber flooring be installed at the average equilibrium moisture
content of that environment. Typically timber floors are supplied
with a moisture content suitable for normal conditions in most
locations. Particular environmental conditions such as air
conditioning, heated concrete slabs etc., may require boards
specified to a lower moisture content than those in a non air
conditioned environment.
It may be necessary to acclimatise
the timber to the service conditions. This may be achieved by
exposing the timber to the room’s normal operating environment for a
period of two weeks or more. For further information refer to the
Harwood and Cypress Technical Guide.
Where timber flooring is
installed on joists in a ground floor situation, the sub-floor
environment may impact on the timber flooring. An assessment of the
ventilation, sub-soil dampness and site drainage should be made.
Where the sub-floor conditions are moist, the boards, being exposed
to these conditions, will absorb the moisture. Large percentage
changes in moisture content will impact upon floor
performance.
For timber floors over a concrete slab, moisture
commonly comes from the slab. Recently constructed slabs or slabs on
the ground are likely to have high moisture content. For these
conditions it is imperative to ensure the slab is suitably dry
(typically 5.5% moisture content) before the floor is installed. New
concrete slabs may take many months to dry, typically 1 month per
25mm thickness of slab, plus 1 month is a rough rule of thumb.
Therefore, deep beams, edge thickenings, etc., will consequently
take longer to dry. The actual moisture content should be confirmed
via appropriate moisture measuring equipment. The testing regime
should take into account such variations in slab
thickness.
Many older slabs in ground contact, although dry,
may be porous.
It is often best to assume the slab on ground
is not dry or that it may be porous, and to install a vapour barrier
between the slab and the timber.
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