Moisture Content

The moisture content of wood is one of the most important properties of wood along with density.  To determine moisture content, we take the ((original weight – oven dry weight) / oven dry weight) x 100.  Moisture content affects many wood properties such as strength, dimensional stability, acoustical properties, conductivity, decay resistance, and weight.  A couple of topics to emphasize here are (1) dimensional stability and (2) decay resistance.  Wood will shrink and swell as it gains and loses moisture from 0-30%.  The length of the board will remain virtually unchanged but the other dimensions can change considerably depending on the species – more on that in another blog.   Wood that is going to be used for interior uses needs to be dried to the equilibrium moisture content (EMC) of the area that it will be used.  The EMC is a function of temperature and relative humidity.  In most houses, it is 6-8%.  So, if a piece of wood is placed in the house at 1% and another at 99% MC, they both will eventually reach 6-8% MC.  The piece at 99% MC will not experience any shrinkage until the MC drops below 30% (fiber saturation point for most species).  The moisture content can be determined directly, and more accurately, by using an oven and determining initial and ovendry weights or indirectly by using a moisture meter.  Most moisture meters are much more accurate below 30% than above.  Moisture content is also important for wood durability.  Most, but not all, insects and fungi that attack wood prefer to attack wood that is above 30%.  Wood that is kiln dried to 6-8% for interior use or 19% for studs has two advantages:  (1) the kiln drying temperature has killed any insects or fungi in or on the wood and (2) the moisture content of the wood is now sufficiently low to discourage most insects and fungi.