Wood Flooring & Moisture
Wood simply reacts to changes in moisture, so it shows you what moisture changes are occurring. Wood is hydroscopic, meaning it absorbs and loses moisture in relationship to its environment. Adding moisture raises the moisture content, swelling the wood. The opposite is also true. One can think of wood being like a person’s skin, swelling with excessive moisture like after a long bath, and shrinking and cracking if too dry like during cold winters. This continues forever unless the wood fibers get crushed and lose their elasticity. The expansion and contraction of solid hardwood is determined by 4 factors: moisture content, grain orientation, fastening, and coefficient of expansion.
Humidity
A wood floor’s environment determines its moisture content. The most important factors in controlling the moisture content of the wood flooring is the moisture content of the sub floor and the humidity of the air below the subfloor as well as the humidity of the air above the wood flooring. It is critical to monitor the humidity of the air in a basement along with the humidity of the air on the main level. The humidity above and below a wood floor must be similar or the wood will react. Usually problems occur from the subfloor taking on moisture and slowly expanding the wood floor above. This causes cupping until the subfloor and wood floor are dried down causing the flooring to have gaps. These gaps result from the boards expanding and forcing the outer boards towards the walls. When the boards dry, they are narrower in width, but there is no force to push the boards back into their original position. The shrinkage shows up as gaps. This is also the reason it is critical to control the humidity during humid weather.
The moisture content of a substrate such as cement, affects the wood flooring the same as does a wood subfloor. Problems arise with the cement slabs because cement is very porous and also hydroscopic. The cement must be cured and dry, and it must have a sufficient vapor barrier to prevent moisture saturation in the future.
The humidity in a home is the most important factor in minimizing expansion and contraction. The humidity in a home must be controlled at normal levels at least 48 hours before the installation of a floor, during installation, and for the life of the floor. If the humidity were to stay constant, then the wood flooring wouldn’t expand or contract unless it was in contact with a direct moisture source such as water. Unfortunately, our extreme climate, with the hot humid summers and cold winters, doesn’t allow us to keep the humidity in our homes constant. Humidity in a home should be controlled year round within a 20% range, between 35% and 55%. The RH in the winter should be 35% to 45% and the RH in the summer should be 45% to 55%. This results in an average RH of 45%. The wood’s moisture content is 7% during the winter with 35% RH. It’s Moisture content is 10% during the summer with a humidity of 55%. See Table 2. This 20% change in humidity only allows the wood moisture content to change 3%. Referring to Figure 1, with a 3% change in moisture content the wood flooring changes width 1/64” on a 2 ¼ wide board. Due to these normal humidity and moisture content changes, all wood flooring will have gaps during low humidity and cupping during high humidity.
Controlling the humidity throughout the year in our extreme climate can be a challenge. The humidity can be raised a number of ways during the winter. The most common is with a humidifier, either directly on the furnace or a stand alone unit. Some low tech ways include boiling water on the stove, leaving the bathroom fan off during bathing, cracking open the dishwasher during the dry cycle, and even venting the air from the clothes dryer inside. The humidity may have to be lowered temporarily in the winter during extremely cold weather. Otherwise, too much condensation may build up on a home’s windows. Lowering the humidity in the summer can be done by using an air conditioner and/or using a separate dehumidifier if the AC can’t keep up or it’s not hot enough for it to run. The humidity in a home must be kept as constant as possible without overcooling during warm weather and over humidifying during cold weather. Either of these will cause condensation inside the home.
If the humidity does fluctuate outside this range temporarily, it won’t greatly affect the wood. Conversely, if the wood is exposed to either higher or lower humidity than normal, then it will react after approximately a week, depending upon how severe the moisture fluctuation. Controlling the humidity is the key to avoiding cupping resulting from too much moisture, and gaps from too little moisture.
Grain Orientation
Wood expands and contracts more in certain directions than in others. Wood expands and contracts the most, tangential to the trunk of the tree, or with the grain. A wood board expands and contracts mostly in width. The amount a board’s dimensions change is affected by the direction the board is cut from the tree. There are three saw cuts used to make logs into boards. Flat sawn or plain sawn boards are cut with the grain almost entirely parallel to the surface of the board. This cut provides a wide, defined grain. Quarter sawn boards are cut from a log with the grain nearly vertical. This produces a very narrow grain. Rift sawn boards are between these two. Rift sawn boards have grain that is angled approximately 45 degrees to the surface. Rift sawing is used to give wood a narrow grain like quarter sawing, but it is a more efficient way of using the entire log. Quarter sawn flooring expands and contracts the least.
Fastening
Installing and fastening solid wood flooring is the next most important factor in controlling movement. The more fasteners the better. This is where the width of the board becomes important. The narrower the board, the more fasteners a floor will have to hold it together. As an example a floor with 2 ¼” wide boards will have over twice as many fasteners as a floor with 5” wide boards. More fasteners hold the wood in place better, but just as importantly, they actually force the boards together during installation. This sideways preload is crucial to limiting expansion and contraction sideways and also vertical movement due to tongue and groove separation. The tongue and groove board edges are tapered, so as they separate the boards can actually move up and down vertically. Narrower boards get more fasteners, so they move less. The wider boards have a beautiful, unique appeal, but they can move more because they have fewer fasteners holding them in place.
Coefficient of Expansion
A particular wood’s coefficient of expansion is the last and least important factor influencing expansion and contraction. A coefficient of expansion is a mathematical way of showing that certain species of trees have wood fibers that expand and contract more than others when they are exposed to the same change in moisture content. The higher the coefficient of expansion, the more that wood can move with moisture content fluctuations. In real life the differences between wood species is seldom visible. The slight differences are negated by the side preloads during installation and the boards’ close proximity to each other. For example a wood species with a coefficient of expansion that’s half that of another wood will not have half as many gaps during the dryer winter months (See Figure 3).
Expansion and contraction of solid wood flooring is minimized mostly by controlling humidity, using narrower boards, and to a lesser extent, using rift or quarter sawn material and the wood species chosen. Solid wood flooring will experience minimal movement and remain beautiful if these guidelines are followed.
Water Damage
Water damage is common with new and existing appliances. Refrigerator connectors are typically a compression type fitting and they will usually leak it loosened and retightened. Dishwashers love to leak at the door seal and at the hose connections on the bottom of the machine. These leaks often require replacing the floor in the entire kitchen. A door seal leak is usually apparent and it can often be caught quickly. Leaking hoses typically leak under the cabinets for some time before any surface floor damage is visible. This is costly and time-consuming to fix so avoiding the problem is important. Often times the factory clamps are weak spring clamps that don't provide a long-term positive seal. Sometimes the clamps aren't even installed correctly and the unit leaks as soon as it's hooked up. Use caution not to avoid warranties, but the surest fix is to remove the factory clamps on the hoses and use permatex sealant on the exterior of the pipe before slipping on the hose and properly tightening the clamp. Do this before initial installation and you or your plumber should be able to catch most problems before they arise. If moisture damage occurs we can almost always repair and refinish the floor. We remove the wet hardwood to dry the subfloor, install new hardwood, and refinish the hardwood floor.
No Guarantee or Warranty
The information contained in this publication represents general accepted descriptions of the relationship between water and wood. However, wood is a natural material subject to sometimes unpredictable reactions as it gains and loses moisture. As such, it is impossible for any description to encompass all possible reactions. Kensok’s Hardwood Floors, Incorporated accepts no risk or liability for application of the information contained in this publication.
For more information, contact NWFA (National Wood Flooring Association) or NOFMA (National Oak Flooring Manufacturers Association)