Design Solutions - Winter 2018 - 49

In reality, the woodwork
in an interior space does
not know if the space is
occupied or not and requires
a continuously stable
environment every day and
night of the year.

Why is this happening?
To know how to prepare
for dimensionally stable
woodwork, let's explore
causes of wood movement
and the methods and ways
to plan to manage moisture
in buildings that contain fine
woodwork.

WOOD MOVEMENT
CONTRIBUTORS

RELATIVE HUMIDITY
AND COMFORT

Generally, most
people and things have
a comfort level in an
interior environment where
the relative humidity is
maintained between
30-50%. Not too high,
leading to sweating, not
too low, leading to dry eyes
and static electricity. Wood
fibers and cells take on and
give off moisture to achieve
equilibrium moisture content
in harmony with the ambient
relative humidity.
When wood absorbs
moisture to add water to its
cells, the wood fibers swell
measurably. When wood
expels moisture, the fibers
shrink resulting in joints
opening or cracks developing
and flat members suddenly
begin to cup and bow. Wood
fibers move relatively little
when the ambient relative
humidity is between 25-55%.
The damage done by
very high humidity; 80% or
higher or very low humidity;
20% or lower is often
permanent. The damaged
wood members will not
regain their original shape
or dimensions after the
buildings' relative humidity is
returned to the middle range
again.

HOW DOES THIS WORK?

Aren't we living in an
amazing time? Just 100
years ago, everyone heated
with coal and later oil
and natural gas burning
furnaces. Dry heat was the
norm. These days, one can
include dehumidification and
humidification options to the
HVAC systems often even
within existing buildings.
What is it worth to add
these valuable assets? Just
consider the cost of replacing
or attempting to repair the
woodwork of the building.
Relative humidity is the
expression of the amount of
moisture the air contains at
a given absolute humidity at
a given temperature. Warmer
air can contain more moisture
than cooler air. When the air
is warmed without adding
any absolute humidity, the
relative humidity goes down.
In an attempt to come to
equilibrium with the lower
relative humidity the wood
will give up the moisture
present in the wood cells and
dry out.

Conversely, more moisture
will be picked up by the wood
cells during high humidity
occasions. These can be as
brief as a week of hurricane
weather or as long as a whole
summer. If fine woodwork is
installed in a space that is
unoccupied for more than a
year with the environmental
controls turned off, the
natural forces of unchecked
relative humidity swings from
season to season are going
to play a noticeably large role
in woodwork's dimensional
stability.

While uncontrolled
relative humidity plays a
role in the vast majority of
wood dimensional stability
issues, there are other
considerations at play.
Heating and cooling are
just a part of the picture.
On occasion, it happens when
just one item in a building has
a wood movement problem
while other woodwork is
unaffected while the relative
humidity of the building may
be within range.
There could be other
factors at work. Is the wood
near a water feature? The
relative humidity of that
localized space may be much
higher than the rest of the
building. Is the south or
eastern sun shining through a
wall of glass directly onto the
wood surface? One side of that
wood may be hot to the touch
and the other side might feel
cool as the back side of a
pillow while it enjoys the air
conditioning of the building.

winter 2018

I

designsolutions

49



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