Wood Moisture Content & Product Dependability
23 MARCH, 2013 BY JIM HILL
New home Cabinet Doors and remodeled homes with Replacement Cabinet Doors can face a difficult environment if relative humidity is left uncontrolled. Here's why.
Ideally the wood moisture content in Kitchen Cabinet Doors will be matched to the average relative humidity of the region where the wood product will be used. This will allow the wood's moisture content to be stable. When the wood moisture content and the local climate is closely matched, the finish on the cabinet door will keep the moisture content in the door from reacting too rapidly to relative humidity changes and, therefore prevent the damage those humidity swings could cause to an unfinished door.
Wood with moisture content of 7% is said to be at equilibrium (that is it won't take-on or give-off moisture) when relative humidity is at 30%. So wood with a moisture content of 7% will be stable when the humidity is 30%. As the relative humidity increases above 30% wood at 7% moisture content will absorb moisture, increasing the wood's moisture content. When relative humidity decreases below 30% wood at 7% moisture will give off moisture. It's not the gaining or losing of moisture that is potentially damaging to wood products, it's the speed of the change in moisture content. Unfinished wood will see the end-grain change moisture levels at a much faster rate than the center of the wood piece, and wood with large differences in moisture content across the length will develop significant internal stress. This internal stress can result in catastrophic damages, like cupping, warping, and even serious splitting.
Humidity is seldom constant and changes in relative humidity are certain. That's where the cabinet doors finish offers protection. The finish is not intended to completely protect the door from the effects of humidity changes. But it is designed to slow the changes to the wood's moisture content with the humidity fluctuations. When a rain storm approaches the relative humidity will spike but the finish on the cabinet doors will slow that high humidity from being absorbed into your doors so quickly as to cause damage. Moisture will still enter the doors, but before the wood moisture content is significantly increased, the storm will have passed and relative humidity will have returned to a point closer to the regions average level.
A more serious condition exists when an unfinished wood product has acclimated to a humidity level above 70%. If wood which has stabilized at this relative humidity is subjected to a very dry climate, with relative humidity levels around 10-15%, the high moisture content in the wood will boil-off very quickly. This condition where moisture leaves the end-grain faster than the moisture leaves the center (to replace it) is typically the major cause on end-grain splits. While end-grain splits are not even abnormal in hardwood lumber, that same end-grain split in the panel-cut of your Raised Panel Cabinet Door would be a serious defect.
The door styles most likely to show splits are Raised Panel Cabinet Doors. The area most susceptible to damage from rapid moisture loss is the end-grain on the raised panels. These panel cuts are where the panels are machined down from the A three-quarter inch thickness in the center to a one-quarter inch thickness where the panel tongue fits into the groove machined into the Rails. Splits caused by rapid moisture loss are common in these panel end-grains. Splits in the three-quarter inch thick panel center are much less common.
All traditional cope & stick cabinet doors have exposed end-grain on the stile ends which can show splitting with rapid moisture loss, although not as likely as the raised panel end-grain.
Mitered doors have the stile end-grain slightly protected because of their design so stile end-grain splits are somewhat less likely than in traditional doors.
It's important to remember that we are talking about the worst-case of an Unfinished cabinet door being exposed to an extreme climate change. While this perfect-storm of events is likely to damage unfinished cabinet doors, there is a preventive solution; Finish your cabinet doors as soon as they are delivered!
As a rule of thumb, wood products manufactured in a damp climate and shipped into a dry climate, unless finished very soon after delivery, have a high degree of potential danger, while wood products made in a dry climate can usually be shipped into a wet climate (or any other climate) with minimal likelihood of damage. This is because most climate-caused damage to a cabinet door comes from rapid moisture loss, and damage from rapid moisture gain is far less likely. That is one of the mail reasons we built our factory in Arizona. Arizona's dry climate allows our products to be shipped anywhere in the country with very little risk of climate related damage to the product.
The sealer and lacquer will slow the moisture migration, even in extreme climate conditions, to a point where your new doors will be a dependable, reliable, and beautiful addition to your home for generations.
When considering a location to place your unfinished cabinet doors prior to finishing, ask yourself this question; would this be a place I would store an expensive piano or other piece of fine wood furniture?