Bamboo and Wood Floors – The Effect of Water

The largest percentage of complaints when it comes to both wood and bamboo are moisture related.  The moisture is usually due to a site related condition.  Frequently a moisture problem is caused by conditions such as water under the structure, leaking pipe, excessively wet concrete or too much moisture in a buildings structural components at the time the floor is installed.  Improper maintenance also results in the introduction of excess moisture.

The major component in many cleaning chemicals is water.  Water can have a detrimental effect on both wood and bamboo floors which are hygroscopic materials.  When bamboo and wood are exposed to air their tendency is to either dry or gain in moisture.  The loss or gain in moisture will continue until the material is in equilibrium with the humidity and air temperature.  If too much moisture is gained these materials will swell and if too much moisture is lost they will shrink.  While bamboo is somewhat more stable then wood in this respect, both are affected.

Wood is dimensional stable at its saturation point of 25-30% moisture content.  Below this point it begins to shrink until it reaches 0%.  As it starts to gain in moisture it expands until it reaches the saturation point again.  Once back at the saturation point of 25-30% it again stabilizes.

When wood floors are manufactured it is kiln dried to an average of 6-9% Bamboo floors are usually kiln dried to 8-10%.  Both wood and bamboo when made into a floor are dried significantly below the saturation point.  Once at this point the flooring is expected to stabilize with the environment in which it is installed.  Since the floor still has a natural tendency to gain or loose moisture, it is imperative that the environment be maintained at a suitable level.

The expansion and contraction of floor material is not equal in all directions.  As an example, a solid piece of wood that was over dried to 0% and allowed to increase to its saturation point of 28%, will increase by about 0.1% longitudinally or along the length/grain.  The same piece of wood will increase 2 -8% perpendicular to the annular growth rings (racially or in strip width for flooring that is quarter sawn.  If the floor is plain sawn flooring the increase will be parallel to the trees annular growth rings (tangentially or in strip width.)  With engineered flooring the expansion will be significantly less.  The reason for this is that alternating layers of the laminate are prepared to run in opposite directions.  Since they are not all running in the same direction they have a natural tendency to pull against one another, which helps to maintain stability. While the engineered floor is more stable in this respect, equilibrium moisture content still remains critical.  The moisture content of wood and bamboo that is in storage or installed at a point below its fiber saturation point has a natural tendency to fluctuate.  This fluctuation is a result of changes in the relative humidity and air temperature within the environment in which the material is stored or installed.

An understanding of equilibrium moisture content (EMC) is important.  EMC is the point at which moisture is not being gained or lost.  Table 1 demonstrates the relationship between temperature and humidity and its resultant EMC.  Wood and bamboo will provide excellent performance for many years if properly maintained.  EMC is a major factor in its maintenance. 

Table 1: Moisture content of wood in equilibrium with stated dry-bulb temperature in Fahrenheit and relative humidity. *

*From the Wood Handbook: Wood as an Engineering Material, 1999, the Forest Products laboratory, U.S. Department of Agriculture.

The equilibrium moisture content (EMC) is not the same for all locals.  Weather conditions, heat and humidity are not the same in all parts of the country.  Take a look at the Moisture Content Map (table 1).  Take a look at Oregon, the authors state and you will see 8-13 along the coast and 5-7 in the rest of the state.  What this map is telling you is that the normal point of EMC along the coast is 8% in January and 13% in July and inland it is 5% in January and 7% in July.  Now take a look at your own state and you may see and entirely different set of variables.

Moisture and humidity are changing throughout the day - throughout the year.  Additionally there are long-term seasonal changes in relative humidity and temperature.  Since wood and bamboo are hygroscopic material they are also experiencing continuing change throughout the day - throughout the year. 

In a dry climate and during the heating season wood and bamboo will shrink and this is normal.  Expansion is also normal in wetter climates and when exposed to wet conditions.  To maintain the floor at the proper EMC as exhibited in the tables; the environment needs to be carefully monitored. Wood and bamboo perform best in a controlled environment that has a relative humidity between 30-50% and room temperature of 60-80%.  These are the same levels that humans are most comfortable at.  In addition to temperature controls on a furnace, the use of a dehumidifier to remove moisture from the air or humidifier to add moisture to the air will be important in many parts of the country.

As stated earlier, engineered flooring (laminated flooring), due to its multiple layers that are glued together, will shrink and expand less than that of a solid floor. 

With an engineered wood floor the grain of each layer runs at a 90-degree angle to that of the adjacent layer, working against each other to maintain stability. 

Bamboo wants to bend and curl just like planks from a tree.  With an engineered bamboo floor the strips need to be directionally balanced.  On a 3-layer laminate, this is accomplished by arranging the strips so that on top and bottom layers the hard side is facing outward. Some manufacturers will randomly orient strips within the middle layer.  With bamboo, some manufacturers will have all of the layers running in the same direction and this is unacceptable.

While engineered wood products have a built in resistance to shrinkage and expansion and can be installed in areas with a wide ranges in humidity the EMC must still be controlled.