There is a persistent misconception in the industry regarding acceptable moisture content of wood and wood products. These assumptions can lead to flawed decisions regarding construction drying times, mold risks, and failure criteria.
Dimensional lumber and sheet materials are subject to varied moisture conditions before arriving at the jobsite. Whether air-dried or kiln-dried, these materials are usually targeted for drying to an average moisture content of 15%, but not to exceed 19%. The Forest Products Laboratory has used the term ‘shipping dry’ as the recommended moisture content “to prevent decay in transit”. The Timber Harvesting and Forest Engineering Glossary defines ‘shipping dry’ as:
“Having a moisture content (oven dry basis) of 14 to 20 percent. Results in reduced shipping weight and less susceptibility to decay. Used in the international lumber trade.”
Moisture content of in-service wood depends on relative humidity at boundary layers adjacent to wood surfaces. When exposed to a constant relative humidity, the moisture content of wood will come to equilibrium with its environment, resulting in a ‘Equilibrium Moisture Content’ for that species wood or wood-based composite. For most areas of the U.S., moisture content in the range of 9 to 14% is considered dry for exterior wood or building envelope components within constructed assemblies. Equilibrium moisture contents for major U.S. cities are listed here: U.S. Equilibrium Moisture Content.
The margins between safe moisture levels and deleterious effects are slim. For example, wood moisture content resulting in corrosion of metal fasteners can be as low as 15%. A study by Imamura and Kiguchi (1999) showed that wood moisture content in excess of 20% is associated with 5% loss of nail shank diameter in four years and a projected 25% loss in three decades. The same study showed 40% loss in joint strength and concluded that 20% moisture content may significantly compromise shear resistance of exterior walls. Adjustment factors for load-carrying capacities of wood are recommended when wood exceeds 19% moisture content or when there is a change from wet to dry conditions or dry to wet conditions. It is important to note that reduced wood strength can occur without visible deterioration, delamination, or dimensional change.
With the advent of ASHRAE 160, strategies for moisture control in buildings now focus on moisture thresholds that prevent fungal growth. For wood, the minimum water activity for most fungi is 0.80, which corresponds to a minimum moisture content of 16%. This is not to say that fungi cannot grow at moisture levels lower than 16%. Some species can maintain growth at water activities as low as 0.70, which equates to a wood moisture content closer to 15%. Wood moisture contents in the range of 28-30% are required for notable wood decay; however, once colonized at these higher levels, some Brown-Rot can sustain decay at moisture content as low as 20%.
A summary of major thresholds for wood moisture is illustrated below (click for an enlarged view).
ASHRAE, 2009. ASHRAE Standard 160-2009, Criteria for Moisture-Control Design Analysis in Buildings. American Society of Heating, Refrigerating and air-Conditioning Engineers.
Carll, C. G. and Highley, T.L. 1999. Decay of wood and wood‐based products above ground in buildings. J. Test. Evaluation. 27:150‐158.
Carll, C. and A.C. Wiedenhoeft. 2009. Moisture–related properties of wood and the effects of moisture on wood and wood products. In: Moisture Control in Buildings 2nd edition.
Cole, I., G. Trinidad, and W. Chan. 1999. Prediction of the impact of the environment on timber components: a GIS-based approach. Proceedings, Durability of Building Materials and Components -8, M. Lacasse and D. Vanier, Eds., National Research Council Canada.
CWC, 2010. Moisture and Wood-Frame Buildings. Canadian Wood Council. http://www.cwc.ca/documents/IBS/IBS1_Moisture_SMC_v2.pdf
IEA, 1991. Annex 14, Condensation and Energy, Vol. 2: Guidelines and Practice. International Energy Agency, Leuven, Belgium.
Imamura, H. and M. Kiguchi. 1999. Prediction of wood decay in the exterior wall of wooden houses by the deterioration of nails. Proceedings, Durability of Building Materials and Components-8, M. Lacasse and D. Vanier, Eds., National Research Council Canada .
Lstiburek, J. 2002. Moisture control for buildings. ASHRAE Journal. February 2002.
Simpson, W. T. 1999. Drying and control of moisture content and dimensional changes. Wood handbook : wood as an engineering material. Madison, WI : USDA Forest Service, Forest Products Laboratory, 1999. General technical report FPL ; GTR-113: Pages 12.1-12.20. http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/fplgtr113.htm
Viitanen, H. and S. Salonvaara. 2001. Failure criteria. In: Moisture Analysis and Condensation Control in Building Envelopes. ASTM MNL‐40.
Winandy, J.E. and R.M. Rowell. 2005. Chemistry of Wood Strength. In: Handbook of Wood Chemistry and Wood Composites. CRC Press.
Timber Harvesting and Forest Engineering Glossary: http://www.srs.fs.usda.gov/pubs/gtr/gtr_so073.pdf