How Moisture Affects Hardwood and Strand
Woven Bamboo Flooring
Wood and Bamboo flooring is a natural product that responds to humidity variations by
expanding and contracting even though the wood has been dried, cut and processed into
flooring. Sometimes these changes may be noticeable. During warm and humid summers wood
expands. During dry winter weather, hardwood contracts. This seasonal movement is normal for
hardwood flooring. The best way to reduce such movement is by installing humidity controls and
making sure that they function before flooring is installed.
Wood and bamboo is a hygroscopic material. This means that when wood or bamboo is exposed
to air, it will dry or pick up moisture until it is in equilibrium with the humidity and temperature of
the air. Moisture absorption causes wood or bamboo to swell. Moisture loss causes the floors to
shrink. Shrinkage of the floors begins at 25-30% moisture content. Swelling occurs as the floors
increases in moisture content from 0% to 25-30% moisture content. Above 25%-30% the floors
are dimensionally stable.
Hardwood or Strand Woven bamboo does not shrink or swell equally in all directions. A change
in moisture content of a flooring piece from 0% to 28% will increase the size of the piece
approximately 0.1% along the
length
of the board, and 5% to 15% change in size in
width
of the
strip with plain sawn flooring.
Winter Time (Not Applicable to Hawaii)
In the winter time when homes are heated and the air is dry, the flooring gives up some of its
moisture and contracts as a result. When this happens, thin gaps can appear between planks.
This is normal, and a homeowner should be prepared for it to occur. Once indoor heating is
turned off in the spring and humidity levels rise again, most of the gaps will close up.
To avoid these separations, try to control and monitor air humidity levels during the dry season
by installing a humidifier in the furnace or bringing a movable humidifier into the room that has
good air circulation. Optimal humidity level falls in 40-65% range. As long as humidity does not
fall lower than 40%, no gaps at all will appear between the planks. Installing a simple humidity
meter will allow you to monitor and control humidity levels in your home all year round.
