Wood is a natural, hygroscopic material that readily exchanges moisture with the surrounding air. Wood movement is governed by two environmental factors: temperature and humidity. While temperature alone causes a specific, predictable change in wood’s dimensions, moisture in the air often overrides this effect. The dimensional changes observed in wood during cold seasons are primarily related to changes in its internal water content rather than the drop in temperature itself.
How Wood Reacts to Temperature Alone
The purely physical response of wood to a drop in temperature is contraction, or shrinkage. Wood exhibits thermal contraction when it is cooled and thermal expansion when it is heated. This effect is a minor part of wood movement, governed by its coefficient of thermal expansion.
For wood that is completely dry, a decrease in temperature will cause a slight reduction in size. However, wood has a very low thermal expansion coefficient compared to other building materials like steel or concrete. From a practical standpoint, the dimensional change caused by temperature alone is usually negligible. This thermal effect is consistently overshadowed by the influence of moisture content fluctuations.
The Controlling Factor of Moisture Content
In cold weather, the air often holds less moisture, resulting in lower relative humidity (RH), especially indoors after heating. Wood constantly seeks equilibrium moisture content (EMC) with its environment. When the air becomes drier in winter, wood releases moisture and dramatically shrinks. This moisture-driven shrinkage is the overwhelming reason wood floors gap and furniture joints loosen in the cold season.
The water within wood exists in two forms: free water, which sits in the cell cavities or lumens, and bound water, which is chemically held within the cell walls. Wood only begins to shrink once it loses its bound water, a point known as the fiber saturation point (FSP), typically around 30% moisture content. The loss of bound water causes the microscopic cellulose fibers in the cell walls to pull closer together. Free water can leave the wood without causing any dimensional change, which is why wood does not shrink until it dries below the FSP.
In a typical heated home during winter, the RH can drop low enough that the wood’s EMC falls to 6%. This significant drop from a summer EMC of around 9% to 12% causes the noticeable seasonal shrinkage. Wood shrinks most significantly across its width (tangentially and radially) and very little along its length (longitudinally). This unequal movement creates internal stress, which often results in the cracking or splitting that homeowners observe and attribute to the cold.
Strategies for Preventing Cold Weather Damage
Since moisture loss is the primary cause of wood damage in cold weather, mitigation strategies focus on controlling the wood’s moisture content and slowing its exchange with the air.
Indoor Humidity Control
A primary defense for indoor woodwork is maintaining consistent relative humidity, ideally between 35% and 55%. Using a humidifier in heated spaces during the winter helps prevent the air from becoming excessively dry, thereby keeping the wood’s EMC more stable. Wood furnishings should be kept away from direct heat sources, such as radiators or heating vents, which accelerate localized drying and cause uneven shrinkage.
Acclimatization and Installation
For new wood installations, proper acclimatization is important. Lumber should be brought into the installation environment several days or weeks before use to allow it to reach its EMC with that space. This practice minimizes the shock of a sudden change in moisture level after installation, reducing the risk of warping or cracking.
Protecting Outdoor Wood
Outdoor wood, such as decks and fences, requires protective finishes to reduce the rate of moisture exchange. Applying a sealant, oil, or protective coating acts as a barrier against excessive moisture loss or gain. These coatings also help prevent water from penetrating the wood’s surface and freezing, which can lead to damage. Regular maintenance checks allow for early identification and repair of any existing cracks or damage.