Wood comes from trees.  Everyone knows this, but we sometimes overlook this fact when the natural properties of wood behave differently than we might wish.  In a tree, wood serves many functions.  Most importantly, it provides a strong, durable, and resilient structure to support the tree’s towering growth.  It is this extraordinary structure that makes wood desirable for the construction of homes, furniture, and fine cabinetry.  Wood also serves to conduct water throughout the tree, and it is this function that can cause a lot of distress if not fully understood.

By its nature, wood is hygroscopic – it loves water and will absorb as much as it can.  The amount of water wood holds balances with the amount of water available in the surrounding environment.  To keep this balance, wood continuously exchanges moisture with the air around it.  Unfortunately, this exchange doesn’t stop after wood is harvested and shaped into its final form.  Wood continues to balance its water content with its environment, even decades after it was harvested. 

This interaction can create problems, because wood changes in size as its moisture content changes.  Wood acts much like a sponge.  As wood absorbs water, it swells in size.  When water is driven out of it, it shrinks. 

The relative humidity of the surrounding air is the most important factor in determining the moisture content of wood.  Relative humidity measures how much water vapor is in the air.  Wood will either absorb or give off water to balance its moisture content with the air around it.  When humidity is high, there is a lot of water vapor in the air, and wood will naturally absorb this water.  When humidity is low, the dry air will pull water back out of the wood.  Temperature plays a lesser role in the moisture content of wood.  Table 1 at the back of this Product Awareness Declaration shows the equilibrium moisture content of wood at common temperatures and relative humidity levels.  Holiday Kitchens recommends that homeowners maintain a controlled temperature and humidity where their cabinetry is installed.  This range is summarized below and is highlighted on Table 1. 

Another factor to consider is the species of wood.  The species has little effect on the moisture content – that is fairly constant for all woods that Holiday Kitchens offers.  However, the degree of shrinking and swelling is tied closely to the species of wood.  Red Oak, for example, will change in size more than Alder.  Table 2 at the back of this Product Awareness Declaration shows the effect of moisture changes on size for many species of wood.

Unfortunately, it isn’t as simple as wood gets larger when it is wetter and smaller when it is dryer.  Wood does not expand or shrink equally in all directions.  To understand why this is, you have to understand the structure of wood.  Wood is composed of a complex network of fibers, cells, and microscopic tunnels – which we see as grain.  The microscopic tunnels in wood run in generally one direction from the bottom of the tree to its top.  Imagine these tunnels as tight bundles of tiny straws running the length of the tree.  When looking at the end of a piece of wood, it is like looking into those bundles of straws.

The purpose of these tunnels is to transport fluids through the tree.  Of course, these tunnels are not made of plastic, but of absorbent tree cells.  As they swell, they grow wider, but not any longer; and when they shrink they grow narrower, but not shorter.  This means that wood will mostly change in size across the grain while remaining relatively the same in the direction of the grain. 

This difference in how size changes in wood is important to understand because it impacts how changing moisture content affects the components of cabinetry.  Cabinet boxes are built from either plywood or particle board core materials (depending on which Holiday line you order), both of which are specifically designed to resist changes in dimension due to moisture content changes. 

Other components are affected more significantly.  For example, if you have Contemporary (solid wood) doors, the change in humidity will be uniform across the width of the entire face of the door.  The entire width of the door may change, affecting the reveals on each side of your doors, but very little at the top or bottom. 

On the other hand, five piece doors (framed with a panel) are designed to move with humidity changes while maintaining their overall square shape and avoiding warping.  If you look at the cross rails (the top and bottom pieces of the door), you will see that the grain runs across the door, whereas the grain in the panel and in the stiles (the left and right pieces of the frame) run up and down.  Wood, as we discussed, changes dimension across the grain.  This means that the rails will remain about the same size, but the panel could shrink or grow in width.  The frame of the door is constructed with spacers in the groove that holds the panel to allow for reasonable changes in the width of the panel.  Still, if the moisture content gets too low, the panel will shrink and could expose edges of the panel that haven’t been finished.  If the moisture content gets too high, the panel could expand beyond what the spacers in the frame can allow, causing the outer frame to crack.

Some fluctuations in temperature and humidity are bound to happen, and cabinet construction methods have been developed to compensate for this.  In winter, the air is colder and can’t hold as much moisture so all wood products will dry out to some degree.  In summer, the air is warmer and holds much more moisture, causing wood to swell in response.  Wood will always respond to the natural cycle temperature and humidity.  This affects all wood in the home: the doors, the trim, the floors, the furniture, and the cabinetry.  Holiday Kitchens designs its cabinetry to be able to adjust with these constantly cycling conditions.  For example, joints in wood products will move slightly over time, and are designed to do just that so the wood itself is not damaged. 

The effects of this natural cycling can be minimized by good temperature and humidity control in the home.  Holiday Kitchens strongly recommends maintaining an environment with temperatures between 60º-90ºF and a relative humidity of 35-50%.  These ranges recognize that there will be some natural cycling, but should prevent serious damage from occurring. 

Failing to control wood’s moisture content is a leading cause of damage to cabinetry.  Excessive changes in moisture content create tremendous stresses on your cabinetry as the wood grows or shrinks.  The majority of cracks, splits, open joints, and other damage to cabinetry results from failing to maintain a steady range of temperature and humidity.  Outside of Holiday’s recommended ranges, you risk seriously damaging your cabinetry.

Brief fluctuations in temperature or humidity, such as opening the door in winter or mopping your floor, will not adversely affect your wood cabinetry.  It takes many days for wood to reach a balance with its environment, so only prolonged shifts in temperature and humidity will have serious effects.

Wood, like all things, is not perfect.  But by understanding and adjusting for its natural characteristics, it can provide decades of beauty and function.  Simple humidity and temperature control is the best way for you to ensure your Holiday Kitchen cabinetry lasts a lifetime.  

CHART 1:  Equilibrium Moisture Content of Wood