Living trees
are full of water and the amount within a freshly felled tree usually outweighs
the wood itself.
The moisture content
of oak, or any other timber, is defined by dividing the weight of the water it
holds by the dried weight of the timber, and it's then shown as a percentage.
There are two
ways that wood holds water;
- Within the wood cells (free water)
- Within the wood cell walls (bound water)
Wood being dried does not shrink until all the free water has disappeared, meaning that the bound water then starts to ‘come out’ also.
When this
starts to happen, this point of moisture loss is called the Fibre Saturation Point
(FSP), and when the FSP of any piece of timber is reached, only then will it
start to change dimension.
This is
especially important information for any oak flooring owner to know because it’s
important for them to know how their oak flooring is going to respond to either
absorbing moisture from the air humidity being high or being forced to lose moisture from the air
becoming too dry.
The FSP
varies from specie to specie but when wood is used and installed as flooring it
will eventually dry to below its FSP, because of the indoor conditions like
temperature and humidity within a controlled domestic environment.
It will
continue to shrink until it reaches equilibrium with those surrounding
environment conditions.
The whole
point of acclimatisation is to make sure that the oak flooring reaches this state
of equilibrium before installation and not afterwards, ensuring that further
dimensional changes are minimised.
Oak flooring
will shrink or expand mostly along its growth rings (tangential) and least
radially to them. Longitudinal movement is also much less and usually not to be worried about with oak flooring…
Even though this dimensional movement is both normal and expected, it can be controlled by monitoring and maintaining the humidity that surrounds the oak flooring.
A moisture
meter should always be used to confirm the suitability of the new oak flooring
and its surroundings before any installation begins, and keeping the Relative
Humidity within the area to between 35-60% is as vital for good floor performance.
Relative
Humidity is calculated as being the amount of water vapour in the air at any
given temperature, and is expressed as a percentage.
The amount of
moisture within oak flooring and its surrounding air creates a vapour pressure,
the higher the level the higher the pressure.
When the
vapour pressure within the air is lower than the vapour pressure within the oak
flooring, the oak flooring will be forced to lose moisture and vice versa when
it’s higher.
Therefore, by
creating and then maintaining an environment for the oak flooring where the air
vapour pressure is the same as the oak’s vapour pressure, will prevent moisture
absorption or desorption and thus prevent dimensional movement.
It’s
obviously vitally important to install new oak flooring that has a moisture
content that suits the surrounding conditions of where it’s to be installed,
because it will always want to meet and stabilise at the moisture content
dictated by its environment.
If installed
with higher moisture content than the environment it's being installed within, it will lose moisture and
shrink afterwards, causing unsightly gaps between the boards, and if installed
with lower moisture content it will absorb moisture from the surrounding
environment and expand.
The surface
finish of an oak floor will help to prevent gain or loss of moisture for a
while during short periods of humidity fluctuation but this element of
protection will reduce over days and weeks if the Relative Humidity continues
to be either too high or too low.
Even the most
reputable oak flooring supplier can only control moisture content of the flooring
they supply when it’s in their possession, and there’s no way of them being
able to control it during transit or after it’s delivered to site.
That’s why
the installer and end user must make sure that all new oak flooring is
acclimatised on the site where it’s to be installed, after the site conditions
have been confirmed as being suitable to accept a natural product like oak
flooring.
How long this
acclimatisation process takes is directly relative to the site conditions and
the moisture content of the oak flooring on delivery.
When the
possibilities of floor failure can depend on moisture so much, it’s difficult
to understand why more people don’t buy a Thermo-Hygrometer to continuously
monitor humidity and temperature surrounding their oak flooring because these
small inexpensive and unobtrusive units can be the difference between floor success and failure.
If acclimatising
your new oak flooring or monitoring its surrounding conditions seems too much
trouble, consider the fact that we receive calls from end users and home owners
every week during cold winter months claiming that our flooring is failing but
in fact it’s actually the individual that’s failing…failing to understand how a
natural material like oak flooring responds to the conditions that it’s forced
to endure within their home.
The simple
answer to avoiding costly problems is to ensure that all oak flooring is
installed at the right moisture content for the environment it’s to ‘live’ in,
and then those conditions are maintained as consistently as possible
thereafter.
