Log Walls and Thermal Mass
by Steve LykinsWe often get the question, “What is the R-value of a log wall?” While the answer to this question is simple, it does not tell the whole story. Most folks actually want to know how will a log home performs when it comes to heating and cooling. This is where things become a bit more complex. In addition to Rvalue, we must also consider the thermal mass of the log wall.What is thermal mass? Thermal Mass is defined as the ability of a body to store thermal energy (also known as heat capacity or thermal energy storage). By contrast, R-value refers to a body’s ability to insulate against conduction of thermal energy (the ability to insulate). A significant thermal mass within a building, like a log wall, can help to “flatten out” daily temperature fluctuations by either radiating or absorbing additional thermal energy, whichever is supported by temperature differences. All materials and objects have a certain thermal mass, or specific heat capacity, which is a unique property of such material or object. Even air has thermal mass, but not much.

A thermal mass performs best when located on the interior of the home’s thermal envelope and is situated to absorb low-angled winter sunlight, but to avoid high-angle summer sunlight. A thermal mass works best in conjunction with passive solar design. Passive solar design and thermal mass design are most effective in hot, arid desert-like climates, but the concept can also work in Midwest climates like our own, only with less efficiency.

Many readily available materials have good thermal mass properties, but almost all of them have little to no insulting properties. Examples include: water, earth, stone/masonry and clay. Only wood has significant R-value (insulating properties) as well as significant thermal mass (energy storage).Think of your log wall as a battery that can store heat and release it later. This makes wood a very special building material, considering that it is renewable, attractive and well established as a building material.

When considering what size and type of logs to use for a log home, keep in mind that more mass makes for more energy storage. Further, thermal mass depends upon the species and density of wood, which is directly related to moisture content. Generally speaking, a lower moisture content results in a lower density which translates to a higher insulating value. By selecting a species like Eastern White Pine, which has a relatively low density, and then kiln drying your logs to bring the moisture content down you can create a log wall with insulating and energy storage capabilities that are superior to any other type of mass wall.

Recent studies conducted by the Log Homes Council have shown that when it comes to overall thermal performance (not just the R-value), a properly designed and constructed log wall will perform just as well, and probably even better than a comparable stud-framed wall. With a complete understanding of how a log wall works to keep you comfortable, you can make an educated decision about what size and type of log is right for your home.