Vysal Underfloor Heating : Floor Finishes : Hardwood Flooring : Humidity And Dimensional Changes

Floor Finishes > Hardwood Flooring > Humidity and dimensional changes

Humidity and dimensional changes

A well established, if perhaps not so well-known, phenomenon is that "A piece of wood is stable when its moisture content becomes equal to that of the surrounding environment when it neither gains nor loses humidity. Consequently, the balance of equilibrium varies on the basis of the temperature and level of ambient relative humidity." As a result, the dimensions of wooden flooring boards are affected directly by changes in ambient humidity. The moisture content of wood will rise if the relative humidity (RH) is higher than usual, and conversely, the wood will lose some of its moisture content (MC) if the ambient air is too dry. These variations in the moisture content of the wood will cause the wooden flooring boards to expand or contract. Wood is dried to a moisture content (MC) of 6% to 9% before the flooring boards are shaped. Relative humidity (RH) during the pre-finishing phase averages 40%. The manufacturer's warehouse is normally heated and ventilated so that finished products are stored in ideal conditions while awaiting shipment. Even when dried correctly, wood continues to react to changes in relative humidity.

The table below shows how moisture content (MC) of wood varies with relative humidity (RH%) and temperature.
Temperature	RH 25%	RH 30%	RH 35%	RH 40%	RH 45%	RH 50%	RH 55%	RH 60%	RH 65%	RH 70%	RH 75%
15˚C	5.40%	6.20%	7.00%	7.80%	8.60%	9.40%	10.20%	11.00%	11.80%	12.60%	13.40%
21˚C	5.40%	6.20%	6.90%	7.70%	8.50%	9.20%	10.10%	10.90%	11.70%	12.50%	13.30%
26˚C	5.30%	6.10%	6.80%	7.60%	8.30%	9.10%	9.90%	10.70%	11.50%	12.30%	13.10%

Definition of normal moisture content (MC).

An ideal MC must be fixed to enable calculation of variations in dimension. However, a normal MC value is not an absolute value, because there are differences in the MC of boards that have been dried at the same time. Stable, good quality boards are characterized by a MC between 6% and 9%. There are different causes for variations, for example, in the size of the raw lumber, differing humidity rates in trees before they are cut and dried, etc.

For practical purposes, it is possible to use an average figure based on the data used to prepare the table above which shows how moisture content (MC) of wood varies with relative humidity (RH%) and temperature. Thus we find that typically wood is stable in a relative humidity environment varying between 35% and 40% and the MC ranges from 6.9% to 7.7%.