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Moisture and Wall Systems

Moisture and Wood

Water is present in large quantities in trees and in freshly cut (green) lumber. In some species of wood use in construction, the moisture content can be as high as 100% - meaning that the weight of the water in the wood is equivalent to the weight of the wood when it is completely dried.

As wood seasons, its moisture content decreases. Dry wood, as required under most building codes is defined as wood having a moisture content of 19% or less. Wood containing more than 19% moisture is called green lumber.

The Equilibrium Moisture Content (EMC) in lumber refers to the point at which the moisture content in the lumber stabilizes. After a period of not more than one year, properly installed framing lumber will reach a moisture content of 7 to 10%. Wall drying test research has demonstrated that the rate of lumber drying is affected by both the temperature of the cavity and by the permeability of the exterior sheathing.

As the framing lumber dries after installation, you must address several concerns , including:

  • dimensional changes of the lumber
  • shrinkage,
  • twisting,
  • bowing
  • cupping
  • decay of the lumber where moisture is trapped in the wall