Log homes may be site-built or pre-cut in a factory for delivery to the site. Some log home manufacturers can also customize their designs. Before designing or purchasing a manufactured log home, you need to consider the following factors for energy efficiency.
The R-Value of Wood
In a log home, the wood helps provide some insulation. Wood’s thermal resistance (or resistance to heat flow) is measured by its R-value. The higher the R-value, the more thermal resistance. The R-value for wood ranges between 1.41 per inch (2.54 cm) for most softwoods, and 0.71 for most hardwoods. Ignoring the benefits of the thermal mass, a 6-inch (15.24 cm) thick log wall would have a clear-wall (a wall without windows or doors) R-value of just over 8.
Compared to a conventional wood stud wall (31 D2 inches [8.89 cm] insulation, sheathing, wallboard, a total of about R-14), the log wall is apparently a vastly inferior insulation system. Based only on this, log walls do not satisfy most building codes’ energy standards. However, to what extent a log building interacts with its surroundings depends greatly on the climate. Because of the log’s heat-storage capability, its large mass may cause the walls to behave considerably better in some climates than in others. Logs act like “thermal batteries” and can, under the right circumstances, store heat during the day and gradually release it at night. This generally increases the apparent R-value of a log by 0.1 per inch of thickness in mild, sunny climates that have a substantial temperature swing from day to night. Such climates generally exist in the Earth’s temperate zones between the 15th and 40th parallels.
Minimizing Air Leakage in Log Homes
Log homes are susceptible to developing air leaks. Air-dried logs are still about 15% to 20% water when the house is assembled or constructed. As the logs dry over the next few years, the logs shrink. The contraction and expansion of the logs open gaps between the logs, creating air leaks, which cause drafts and high heating requirements. To minimize air leakage, logs should be seasoned (dried in a protected space) for at least six months before construction begins. These are the best woods to use to avoid this problem, in order of effectiveness:
Since most manufacturers and experienced builders know of this shrinkage and resulting air leakage problems, many will kiln-dry the logs prior to finish-shaping and installation. Some also recommend using plastic gaskets and caulking compounds to seal gaps. These seals require regular inspection and re-sealing when necessary. To be safe, always hire an InterNACHI inspector.
Controlling Moisture in Log Homes
Since trees absorb large amounts of water as they grow, the tree cells are also able to absorb water very readily after the wood has dried. For this reason, a log home is very hydroscopic—it can absorb water quickly. This promotes wood rot and insect infestation. It is strongly recommended that you protect the logs from any contact with any water or moisture. One moisture-control method is to use only waterproofed and insecticide-treated logs. Re-apply these treatments every few years for the life of the house. Generous roof overhangs, properly sized gutters, downspouts, and drainage plains around the house are also critical for moisture control.
Building Energy-Code Compliance for Log Homes
Because log homes don’t have conventional wood-stud walls and insulation, they often don’t satisfy most building codes’ energy standards—usually, those involving required insulation R-values. However, several states—including Pennsylvania, Maine, and South Carolina—have exempted log-walled homes from normal energy compliance regulations. Others states, such as Washington, have approved “prescriptive packages” for various sizes of logs, but these may or may not make sense in terms of energy efficiency. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 90.2 standard contains a thermal mass provision that may make it easier to get approval in those states that base their codes on this standard. To find out the log building code standards for your state, contact your city or county building code officials. Your state energy office may be able to provide information on energy codes recommended or enforced in your state.