The Dimensional Effects of Wood Moisture

Understanding how moisture interacts with wood can help you avoid problems from moisture-related wood movement. September 13, 2012

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By Gregory J. Robb

Meta Description: Moisture content (MC) is indigenous to wood, but excess water vapor can significantly change its dimensions. A wood moisture meter helps.

Moisture content (MC) takes the form of water vapor in the air, and in every wood cell. In wood, it generally takes two forms. Bound water is held in wood cell walls, while free water is held within the cell cavities. The MC at which the cell walls are saturated but no free water remains in the cell cavities is called the fiber saturation point (FSP). This matters greatly to anyone interested in the long-term health and performance of the end wood product.

The FSP is important in the kiln drying of wood for the following reasons: more energy is required to evaporate water from a cell wall than from the cell cavity, a wood cell will not shrink until it reaches the FSP, and large changes in many physical and mechanical properties of wood begin to take place at the FSP.

Wood loses or gains moisture until the amount it contains is in balance with the surrounding environment. The amount of moisture at this balance is called the equilibrium moisture content (EMC). The EMC depends upon the relative humidity (RH) and temperature of the surrounding air. When the RH drops, wood releases moisture and this can possibly cause shrinkage in the end wood product and when the RH increases wood absorbs moisture and thereby an end wood product could swell in certain circumstances.

This MC process is entirely natural, so wood lovers must continually monitor the MC of wood. Put simply, poorly managing and monitoring the wood productís MC can possibly cause product failures or future product service problems.

Dimensional Effects in Wood
Some wood species have stable dimensional properties while others exhibit extreme wood shrinkage and swelling during MC fluctuation. Shrinkage of wood is a basic cause of many problems that occur both during the kiln drying process and in service. When water leaves the cell walls at 25-30% MC, the cell walls begin to shrink. Drying stresses develop because wood shrinks by different amounts in the radial, tangential and longitudinal directions.

Even after drying, wood will shrink and swell in service as the RH changes. MC change equals wood dimension change. Why does this matter? Imagine your cabinet drawers changing dimension severely during the higher humidity of the winter months, or your wine cask wood shrinking during summer heat. Any time wood pieces are fitted together, wood will continue to lose or gain MC until it reaches its EMC.

There are two solutions here: First, learn the dimensional characteristics of the wood species, and second, manage the wood productís MC to minimize the effects of dimensional change. The alternative is a continuous battle with moisture-related issues.

Choose Wood Species Well
Teak, Tectona grandis, is a deciduous tree which grows in mixed hardwood forests native to south and southeast Asia, mainly India, Indonesia, Malaysia and Burma. Teak is a yellowish brown timber and it has good grains and texture. It is used in the manufacture of products where weather resistance in important. Teak is used as a material in the construction of both indoor and outdoor furniture. Teakís high oil content, strong tensile strength and tight grain makes its suitable for outdoor furniture. Teakís natural oils make the timber termite and pest resistant. It is also used in boat decks as it is extremely durable and requires very little maintenance.

Honduran Mahogany, Swietenia macrophylla, is an important timber in Latin America. The heartwood color can vary a fair amount from a light orangey brown when freshly cut to a deep mahogany color as it fully ages. Honduran Mahogany has excellent weathering properties, which makes it a popular wood for boat building and window fabrication. It is resistant to brown rot and white rot fungi. The color is relatively consistent, making it an excellent wood for staining. Its easy workability combined with its beauty and stability have made this lumber a favorite in the furniture, cabinetry, musical instrument and boat-building industries.

Monitor Well
Whatever wood species you choose for your wood product application, meet the wood moisture meter, a tool for the assessment and management of the woods MC. Lumber mills, wood installers and consumers utilize some form of a wood moisture meter to ensure that the MC is stable. Wood manufacturers kiln-dry "green" lumber in order to reach its EMC. Savvy contractors allow newly-delivered wood bundles to equalize to its environment so that any new changes in MC do not jeopardize subsequent wood installations. Building inspectors harness wood moisture meter technology to assess the MC in wood and other buildings materials. Wood flooring owners now utilize wood moisture meter products to maintain a healthy MC in their flooring as the seasons change.

By understanding the effects of MC on wood dimensions, combined with the ability of continually measuring the wood with a quality moisture meter, professionals and avid hobbyists can ensure that their wood projects stay on track.

The era of widespread wood moisture meter application has begun: air, water and wood - in harmony.

* - (pg. 150).
** - (pg. 81).

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