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Log homes may be handmade on site or pre-cut in a factory for delivery to the site. Pre-cut log home kits have been produced since 1923. Some log home manufacturers can also customize their designs. Wall thickness' range from 6-16 inches (15.2-40.6 cm). However, even though such thickness sounds impressive and the log industry enthusiastically promotes the energy efficiency of log buildings there is considerable dispute as to their energy efficiency. The dispute originates from two points: The R-value of the wood and how tightly the logs fit together.
The R-Value of Wood
An R-value (Btu/ft2/hour/oF) is the rating of a material's resistance to heat flow. The R-value for wood ranges between 1.41 per inch (2.54 cm) for most softwoods to 0.71 for most hardwoods. Ignoring the benefits of the thermal mass, a six inch (15.24 cm) thick log wall would have a clearwall (a wall with no windows or doors) R-value of just over 8. Compared to a conventional wood stud wall [3? inches (8.89 cm) insulation, sheathing, wallboard, a total of about R-14] the log wall is apparently a far inferior insulation system. Based only on this, log walls do not satisfy most building code energy standards. However, to what extent a log building interacts with it's surroundings depends greatly on the climate. Because of the log's heat storage capability it's 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.
Log houses are susceptible to developing air leaks. Air-dried logs are still about 15%-20% water when the house is assembled. As the logs dry over the next few years, the logs shrink. The contraction (and expansion - see below) of the logs opens up gaps between the logs, creating air leaks and causing drafts and high heating requirements.
To minimize problems like these, logs should be seasoned (dried in a protected space) for at least six months before construction begins. The best woods to use to avoid this problem, in order of effectiveness, are cedar, spruce, pine, fir, and larch. Since most manufacturers and experienced builders know of these 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 resealing when necessary.
Since trees absorb large amounts of water as they grow, the tree cells are also able absorb water very readily after the wood has dried. For this reason a log building is very hydroscopic (logs absorb water quickly.) This promotes wood rot and insect infestation. It is strongly advised to protect the logs from contact with any water. One concept is to only build with logs that have had a water proofing-insecticide treatment and applying additional treatments every few years for the life of the house. Generous roof overhangs, properly sized gutters and down spouts, and drainage plains around the house are critical to making the building last.
Several states, including Pennsylvania, Maine, and South Carolina, have exempted log-walled homes from normal energy compliance regulations. Others, such as Washington, have approved "prescriptive packages" for various sizes of logs. 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. The referrals below are additional sources of information.
As with any structure, properly designed passive solar methods can lower energy use and help you gain approval for a log building that would not otherwise comply with your state energy codes. Factors to consider include:
- Size, type and placement of windows;
- Orientation of the building;
- Airtightness of the structure;
- Size and type of logs used;
- Amounts of attic and floor insulation;
- Heat storage mass inside the building;
- Local climate conditions
Consulting a passive solar architect may be worthwhile since the proper sizing of the sun exposed windows is crucial to the efficient performance. Some designers suggest incorporating thermal storage such as masonry floors or walls, to absorb solar energy and increase the thermal mass effect. Some log home manufacturers offer solar log homes, or are able to custom-build them.
Credits: US Department of Energy (http://www.eere.energy.gov/consumerinfo/factsheets/ca8.html)