Knock on Wood
Get the nitty-gritty on wood species, their characteristics and how trees react to becoming part of your log home.
More than two dozen species of wood are commonly used to build log homes in North America. Almost all are softwoods: evergreens such as pine, cedar, fir, cypress and spruce, though some hardwoods, like oak, are also used. Though each log home producer favors certain species, the successful use of so many varieties is a clear indication that there’s not one type of tree that makes a better log home than another. Instead, the choice rests in the type of wood your log home company and you prefer. Despite their differences, these various woods share certain characteristics.
Technically, wood is the hard, fibrous substance beneath the bark of a tree. It owes its character to the hollow, spindle-shaped cells that constitute it. These cells are arranged parallel to each other along the tree trunk, and this arrangement affects some properties of the wood, notably strength and shrinkage. The wood’s fibrous nature influences how it is used.
Trees grow by adding new wood. Wood that has already been formed does not continue to grow, but each year, a new layer of wood, called an annual growth ring, is added. The portion of the ring formed in the spring is light in color and is called earlywood. The portion formed later in the growing season is darker and is called latewood. Latewood is generally denser and stronger than earlywood.
The wood formed just inside the bark is known as sapwood. Depending on the size and species of the tree, sapwood can measure one to three inches beneath the bark. As a rule, the more vigorously growing tree species have wider sapwood layers. Second-growth trees of marketable size consist mainly of sapwood.
Sapwood contains mostly living cells that carry sap, the tree’s food, from the roots to the leaves. It’s
not durable, and if exposed to moisture and other factors, it can decay. But in terms of log home construction, sapwood usually absorbs preservatives readily, so when wood is impregnated with a good wood preservative, the presence of sapwood can be an advantage. If thoroughly treated, sapwood will usually be at least as decay resistant as the treated heartwood, maybe even more so.
Inside the sapwood is the heartwood. Heartwood consists of inactive wood cells that have been changed slightly, both chemically and physically, so that they no longer conduct sap. Heartwood is usually more decay resistant than sapwood.
Wood has several distinguishing qualities that affect its use for log home construction.
Grain usually refers to the log’s annual growth rings or to the arrangement of the wood fibers. Annual rings are said to have either a fine or a coarse grain. Close-grained wood, such as ponderosa pine, has narrow, inconspicuous annual growth rings and closely spaced pores. In contrast, coarsegrained wood, such as southern yellow pine, has wide, conspicuous annual growth rings.
Texture, used synonymously with grain, usually refers to the size, appearance and quality of fibers in the wood. The earlywood (cells grown in spring and summer) in coarse-grained wood is light in color and soft in texture. Latewood (cells grown in fall and winter) is much darker and harder.
The arrangement of a log’s fibers, referred to as either straight or spiral grain, can affect the log’s structural properties. Some arrangements are considered more desirable for building than others. A straight grain, where the fibers run parallel to the length of the wood, can be highly desirable. This wood tends to remain straight while it is drying. A spiral grain, where fibers swirl around the tree trunk, is less desirable because cut timber tends to twist as it dries. A tree may form a spiral grain as it grows and seeks sunlight.
Wood’s strength derives from its cellular makeup. Its cells ( hollow, cellulose blocks) are bonded by a substance called lignin. The resulting material is stronger, pound for pound, than steel. Lignin also tends to be an elastic material that adds to wood’s resiliency.
For some wood used in log homes — such as rafters, beams and posts — strength, resiliency, hardness and shock resistance are important considerations. But for logs stacked in the walls, strength and resiliency aren’t as critical, as one single log won’t carry the load alone. Log home producers refer to engineering calculations to determine the proper size or species of wood for each job.
Wood’s weight varies greatly from the time it’s first cut and saturated with water until it is dry. Wood’s weight is an important consideration in determining the mass of individual logs as they’re lifted into position, and, more importantly, the load of the entire house as it rests on its foundation.
THE BENEFITS OF THERMAL MASS
Wood has several thermal properties that affect its energy efficiency.
In the simplest, most easy-to-understand terms, wood reacts to heat retention and release in much the same way as stone. Take a good sized rock and set it outside in the summer sun all day. Then as dusk falls and temperatures cool, bring it inside. Depending on its size, that stone will continue to emit the heat it absorbed for hours. Logs work the same way. This is thermal mass.
Conductivity is an inverse measure of the insulating value or resistance to heat flow of the material. The lower the conductivity, the higher its insulating value.
Wood conducts heat more slowly than other building materials. Structural lumber, such as southern yellow pine, has a conductivity of only 0.8 Btu per inch, per hour, per square foot, per degree of Fahrenheit. By way of comparison, the rate for steel (a poor insulator with high conductivity) is 320 Btu.
Thermal resistance is the insulating value of a material. This resistance to heat flow, usually expressed as R-value, varies among wood species and depends on the wood’s density and other qualities. While R-values alone aren’t a proper measure of the energy efficiency of a log wall, they are widely used. This is why looking at the full picture of a log’s thermal properties — not simply R-value — is vital to determining a log home’s energy efficiency.
The moisture content of timbers used in log home construction is crucial to predicting the amount of settling that will occur in your wall system. Moisture content is the amount of water contained in wood, expressed as a percentage of the weight of water relative to the dried weight of the wood. Wood may contain “free water,” between wood cells,
and “bound water,” found within the cell walls. Just as human bodies are an average of 60 percent liquid, living trees typically contain more moisture than wood.
The moment a tree is cut, water begins to evaporate from the wood as it seeks equilibrium with the relative humidity of its surroundings. Wood doesn’t begin to shrink until all the free water has evaporated. The point at which no free water remains and shrinkage begins is known as the fiber saturation point. Determining moisture content lets producers predict how much log shrinkage they must accommodate. The lower the moisture content, the more stable the wood.
Green wood has 30 percent or greater moisture content. This is the condition of newly harvested, healthy trees. The wood fibers are totally saturated with water.
Surface-dry wood has 25 percent or less moisture content. This is the condition of wood two to four weeks after cutting and debarking. The outer 1/8-inch of the surface
feels dry to the touch. Many wall logs are sold in this condition.
Air-dried wood has 19 percent or less moisture content. It normally takes at least a year to air-dry wood to this percentage.
Kiln-dried wood has 15 percent or less moisture content. Because of their thickness, the logs used in homes require two to three weeks in a kiln to reach this benchmark. If the center of a log is reduced to this level, very little additional shrinkage will occur.
As wood approaches its moisture equilibrium, it changes dimensionally. Shrinkage is a byproduct that can affect log home construction. Why? Wood shrinks unevenly. Most shrinkage occurs tangentially, (i.e., in the direction of the growth rings). Tangential shrinkage causes logs to check, or crack, and the combination of tangential and radial shrinkage can cause an 8-foot-high log wall to shrink as much as one to two inches when green (moist) wood is used.
Most shrinkage will occur between 25 percent and 15 percent moisture content levels. Even kiln- dried logs, which have had most of their moisture removed, and thus already shrunk, still will shrink slightly.
Log home producers, designers and builders take wood shrinkage into account. (It’s particularly vital to handcrafters who often work with large-diameter logs that take a considerable amount of time to air dry.) They can calculate the amount of shrinkage logs will undergo and use construction techniques that account for it. If your home is properly built according to professionally designed plans, shrinkage shouldn’t pose a problem.
Most log settlement and shrinkage happens within five years of construction. After that, a log home finds its equilibrium within its environment.
Log home producers can calculate shrinkage and use construction techniques that account for it.