Water & Wood

WATER AND WOOD

1996 NATIONAL WOOD FLOORING ASSON AT I O N A L W O D F L O R I N G A S O C I A T I O N T E C H N I C A L P U B L I C A T 0
Water and Wood Basics
The easy explanation that students learn in school — trees grow with roots in the ground and leaves in the air — still serves as the basis for understanding the never-ending relationship between water and wood. The roots collect moisture and nutrients from the soil and ship them through vessels or fibres up the trunk and branches to the leaves. These vessels are similar to the “strings” in a stalk of celery. They are similar, too, to a group of soda straws gathered together, running up and down the tree. That’s the simple version of how a still-standing tree is made up of vertically aligned fibres. Cut the tree down, and the fibres are horizontal. Saw it and manufacture strip flooring, nail the floor down and most of the fibres are still horizontal, running the length of the boards. In the live tree, the fibres are loaded with moisture, as sap. After being cut, the tree begins to dry out, just like a rose will wilt after it’s picked. As the tree’s fibres dry, they shrink in thickness or diameter, but almost none lengthways. This shrinkage, characteristic of all woods, is critical in understanding the effect of moisture on wood flooring.
Moisture content in solid wood is defined as the weight of water in wood expressed as a percentage of the weight of oven-dry wood. Weight, shrinkage, strength and other properties depend on the moisture content of wood. In trees, moisture content may be as much as 200 percent of the weight of wood substance.
After harvesting and milling, the wood will be dried to the proper moisture content for its end use.
Wood fibres are dimensionally stable when the moisture content is above the fibre saturation point
(usually about 30 percent moisture content). Below that, wood changes dimension when it gains or loses moisture. Here are some quick points about shrinking and swelling:
• Shrinkage usually begins at 25 to 30 percent moisture content, the fibre saturation point. Shrinkage continues to zero percent moisture content, an oven-dry state.
• Swelling occurs as wood gains moisture, when it moves from zero to 25 to 30 percent moisture content, the fibre saturation point. Different woods exhibit different moisture stability factors, but they always shrink and swell the most in the direction of the annual growth rings (tangentially), about half as much across the rings (radially) and only in minuscule amounts along the grain (longitudinally).
This means that plainsawn flooring will tend to shrink and swell more in width than quartersawn flooring, and that most flooring will not shrink or swell measurably in length.
• Generally, flooring is expected to shrink in dry environments and expand in wetter environments.
• Between the fibre saturation point and the ovendry state, wood will only change by about .1 percent of its dimension along the grain (lengthways in a flatsawn board). It will change by 2 to 8 percent across the grain and across the annular rings (top to bottom), if quartersawn; and 5 to 15 percent across the grain and parallel to the annular rings (side to side), if plainsawn.
• Wider boards tend to move more than narrower boards. Movement in a 5-inch-wide plank is more dramatic than in a 21/4-inch strip. The ideal moisture content for flooring installation can vary from an extreme of 4 to 18 percent, depending on the wood species, the geographic location of the end product and time of year. Most oak flooring, for example, is milled at 6 to 9 percent.

A wood’s weight and moisture content
Wood is hygroscopic — meaning, when exposed to air, wood will lose or gain moisture until it is in equilibrium with the humidity and temperature of the air.
Moisture content (MC) from 5 to 25 percent may be determined using various moisture meters developed for this purpose. The most accurate method in all cases, and for any moisture content, is to follow the laboratory procedure of weighing the piece with moisture, removing the moisture by fully drying it in an oven (105 degrees C) and re-weighing. The equation for determining moisture content is weight of wood with water - oven-dry weight MC% = x 100 oven-dry weight

AT I O N A L W O D F L O R I N G A S O C I A T I O N T E C H N I C A L P U B L I C A T I O
Wood Flooring Over Radiant Heating

Radiant heating is a growing source of heating in North America, both in residential and commercial installations. Consequently, it’s important for installers to understand how radiant heating works with hardwood flooring installations. Radiant heating does not heat air directly, as do more conventional forms of heating, such as baseboard convectors or forced air circulation. Radiant heat is “omni-directional.” Unlike warm air, which tends to rise, radiant energy tends to travel in all directions. A large area of mild surface temperatures, such as a warm floor, is capable of transferring as much heat as a small surface area, such as a steam radiator, at high surface temperatures.
Radiant heat beneath wood flooring involves tubing in concrete, or tubing under plywood subfloors. The most important factor in a successful wood flooring installation over radiant heat is a dry slab and a dry subfloor. The only sure way to dry a slab and subfloor system is to turn on the radiant heating system before installing the wood flooring.
If this isn’t done, moisture left in the slab will enter the wood flooring as soon as the heat is turned on. The result is floors that will expand, contract, shrink, crack, cup and bow excessively. If the heat can’t be turned on, then everyone involved — down to the homeowner — should understand and accept the compromises that will appear down the road. Opinions on the amount of time required vary widely. Some say the heating system should be turned on at least 72 hours before installation, with a preferred time of five to six days. That assumes that the slab has been in place for at least 60 days. (See “Testing concrete” on page 16.) If the slab is relatively new, the recommendation is to have the heating system turned on for 30 to 60 days before installing wood floors. As always, follow the recommendations of your wood flooring manufacturer. Wood dries rapidly when the heat is first turned on. It dries to a lower moisture content toward the end of the heating season. When the radiant heat is turned off, moisture once again starts to seep into the wood subfloor and radiant slab. Abruptly turning on the radiant heat in the fall will subject wood flooring to rapid and easily noticed movement: Evidence of this movement will be cupping or crowning of the boards. Finally, shrinkage cracks will appear between individual floor boards. Alternatively, gradually turning the heat on before the first really cool day will begin the seasonal movement more gradually.
Thus, the movement of the floor will be much less noticeable. As always, humidity controls can help offset flooring expansion and contraction.
Not all species of wood are good candidates for an installation over radiant heating. It’s best to follow the manufacturer’s recommendation for a species’ suitability over radiant heat. When possible, choose a species that is known for its stability. Quartersawn or rift-sawn flooring is preferable to plainsawn in the search for stability. Strip flooring is also a better choice than plank flooring, because narrow boards expand and contract less than wide boards do. Using narrow boards also means there are more seams in a floor to take up movement. Because of its dimensional stability, laminated flooring is another good choice.
Radiant heating systems are currently designed to run cooler than they did years ago, although water supplied to the systems generally range from 90 degrees to 140 degrees. In years past, when water temperatures exceeded 140 degrees, wood fibres were repeatedly traumatised, causing stress fractures, gaps and twisting. Repeated heating and cooling also broke down the adhesive that bonded the hardwood to the slab.
But today, a set of thermostat controls can help avoid those problems. It is recommended to have three thermostats — one to control the tubing water supply temperature; one to control the room temperature with different zone controls; and one for outside the house. This three-thermostat system is kindest to wood flooring, because it moderates the floor temperature.
People tend to crank up the heat when they’re cold, but with three thermostats, the system adapts itself to conditions both inside and out. The outside thermostat gears up the system for the arrival of colder weather, and a thermostat adjusting the control water temperature on the tubing will keep the temperature at the homeowner’s comfort level.