Relative Humidity, Wood Moisture and EMC Chart
Recommended ambient conditions in buildings are 30-50% relative humidity at 60-80°F.
If ambient conditions stay within this range, the amount of expansion and contraction of
wood fl oors and furnishings at 6-9% moisture content is limited. (Table from US Dept of
Agriculture “Wood Handbook, Wood as an Engineering Material”)
Example: For a relative humidity of 35% and a temperature of 70°F, the EMC is 6.9%. A wood fl oor at 6.9%
moisture content is stable, if the surrounding air has a relative humidity of 35% and a temperature of 70°F.
In short a fl oor is stable, when MC and EMC are the same.
Dimensional Stability of Wood: Wood moisture and relative humidity are crucial
factors for dimensional stability of wood. Use moisture meter and thermo-hygrometer
to measure both. If wood shrinks, warps, cups, checks or delaminates, it always has to
do with changes in wood moisture and/or relative humidity. No shrinking and warping
occurs, when an equilibrium (see EMC chart) with the surrounding air has already been
reached, which means wood is not losing or absorbing any more moisture.
Predictions and Explanations: Measure relative humidity, temperature and moisture
content. Compare the measured values with the EMC chart above to predict, if wood is
stable or will loose or gain moisture.
-- A hardwood fl oor shows gaps from shrinking. The moisture content at the present time
measures 6.2% and the rel. humidity is at 30%. The only explanation for the gapping
is, that the fl oor was installed at a higher moisture content. Once exposed to the dry
air, the fl oor dried out and the gapping occurred.
-- If a dry fl oor with a perfect moisture content of 6.2% is exposed for a long time to air
with a relative humidity of 60% at 70°F, it can be predicted that the fl oor will absorb
moisture until 11% has been reached. Depending on the wood species, a 5% increase
in moisture content can be accompanied by a substantial amount of expansion.
No more surprises! Use the EMC chart and your Ligno-Scanner to predict problems.
The chart is also helpful to explain to your customers, why relative humidity in their home
matters.
EMC CHART
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