Timber expands and contracts as the moisture content in the wood changes. Wood movement along the grain is insignificant, but across the grain expansion can cause large dimensional changes. This movement can cause joint failures in furniture that has been poorly designed. The coffee table below belonging to my parents, is an example of joint failure which was caused by timber expansion. The timber expanded significantly across the grain, but the edging (with its grain running axially) did not expand much at all causing the joint to break and a gap to form.
As humidity levels in a house change throughout the seasons, timber furniture absorbs or releases moisture to the air to stay in equilibrium with the environment. This is termed the Equilibrium Moisture Content (EMC) and can be acurately predicted for different relative humidity levels, as shown in the graph below. There is also a slight variation in moisture content with temperature, but this is minimal and can be ignored.
(Image from http://www.conservationphysics.org/isothrm/isothrm.php)
So what does this all mean? Well, if the relative humidity (RH) in your room changes from 40% in winter to 60% in spring, the EMC in your furniture may change from 6% to 12%. This results in expansion across the grain of timber of up to 20mm for a 1m wide table (the exact amount of expansion varies depending on timber species). If your furniture is not designed to account for this wood movement, joints will eventually separate and fail.
A good quality furniture finish will slow down the rate of moisture transfer with the air and go a long way to protecting it. No finish prevents moisture exchange but it can significantly slow down the rate of transfer (see this blog for more on finishes). Timber takes several weeks to reach an equilibrium with the humidity in the air, so one rainy day will not cause furniture to change size.
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