Wood has always been sensitive to moisture. It absorbs and looses moisture until an equilibrium with the surrounding air has been reached. For each wood moisture content there is one specific relative humidity and temperature, when wood does not gain or loose any moisture. This moisture content is called the EMC (equilibrium moisture content).
Once the EMC has been reached and relative humidity and ambient temperature are not changing, wood is dimensionally stable. No more shrinking or warping will occur – ideal conditions have been reached.
When wood absorbs and looses moisture below the Fiber Saturation Point (around 25-30%) wood swells or shrinks. To make matters worse shrinking and swelling is often accompanied by warping. Now we have a moisture problem.
See chapter: Shrinking, Warping and the Perfect Board.
Following are examples showing the difference in the equilibrium moisture content (EMC) between dry winters and moist summers:
At 70°F and 75% relative humidity, wood moisture will reach 14%.
At 70°F and 35% relative humidity, wood moisture will reach 7%.
At 70°F and 20% relative humidity, wood moisture will reach 4.5%.
If wood pieces with different moisture contents (some drier, some with more moisture) were placed in a constant climate of 70°F and 35% relative humidity, all pieces would end up with a moisture content of 7%, independent of wood species or initial moisture content. Since in-house conditions are usually between 30-45% relative humidity, the magic moisture content for stable woodwork, furniture and hardwood floors inside a building is 6-8%.
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Moisture Problems are predictable+Compare wood moisture, relative humidity and ambient temperature with the EMC Chart to find out if wood will absorb moisture, keep its moisture content or loose moisture and shrink:+ + – Use a moisture meter to determine the wood moisture content. – Use a Thermo-Hygrometer to measure relative humidity. – Use the EMC Table to find the corresponding EMC for the relative humidity of the surrounding air.No shrinking or warping will occur when the wood moisture and the EMC of the surrounding air are the same. Wood is stable. |
Prevent Moisture Problems
Compare wood moisture, relative humidity and ambient temperature with the EMC Chart to find out if wood will absorb moisture, keep its moisture content or loose moisture and shrink:
– Use a moisture meter to determine the wood moisture content.
– Use a Thermo-Hygrometer to measure relative humidity.
– Use the EMC Table to find the corresponding EMC for the
relative humidity of the surrounding air.
No shrinking or warping will occur when the wood moisture and the EMC of the surrounding air are the same. Wood is stable.
