As we've already mentioned, but is so critical that it deserves repeating, most offices and homes in North America, have an average interior EMC of 6% to 9%, a relative humidity of 30% RH to 50% RH. Sometimes, the home or office may be drier, such as during in the middle of winter in the Northern U.S. or during the summer in the dry South Western U.S. On the other hand, sometimes, the home or office along the Southern or Western coastal areas maybe wetter.
The correct moisture content for lumber coming in to a manufacturing facility, for wood products during manufacturing, and for the product during storage and installation, depends on the ultimate in use EMC for the product. Because having lumber even a little too wet cause shrinkage, and shrinkage problem are usually more serious than swelling problems, and because the typical home or office is seldom much drier than 6% to 7% EMC, 28% to 38% relative humidity.
The typical desired average moisture content for lumber, parts and components intended for interior products, is 6% to 7% moisture content. Electronic relative humidity indicators are available for under $30 at electronic supply stores. It is therefore easy and relatively inexpensive to determine the precise humidity and associated EMC in any location where wood products are manufactured, stored, installed or being used.
Although the average piece of wood is 6% to 7% moisture content in most interior locations, wood is a variable material with no two pieces being identical, therefore, there will always be some small variation in moisture content under 1% moisture content in individual pieces exposed to the same EMC.
As a rule of thumb, the ideal difference between the moisture content of wood and the EMC of air is 2% moisture content. However, appreciate that the amount of variability that can be accepted, depends on the product being manufactured and its sensitivity to moisture content changes.
Achieving better moisture content uniformity requires more effort during kiln-drying and better dry lumber storage in handling, which both add to the price of the lumber and components. However, reducing the risk of rejects will be well worth the extra cost and care.