Log Homes and Energy Efficiency Facts

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.

Air Leakage

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.

Water Problems

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.

Code Compliance

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.

Getting Approval

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.

Information Sources

The following are sources of information on the log home building industry.

International Logbuilders' Association
Email: info@logassociation.org

Log Home Builder's Association of North America

Log Homes Council
National Association of Home Builders

Bibliography

The following articles and publications contain additional information on log homes.

Articles

"Air Leakage of Log Homes," Home Energy, (8:6) p. 40, November/December 1991.

"Airtightness of Log Homes," J. Nisson, Energy Design Update, (10:5) p. 9, May 1991.

"A Beginner's Guide to Log Homes," E. Beal, Countryside & Small Stock Journal, (78:3) p. 38, May 1994.

"Fairness in Log Homes," D. Reed, Energy Design Update, (9:12) pp. 5-6, December 1990.

"Finding Air Leakage in Log Homes—A Few Surprises," J. Nisson, Energy Design Update, (9:10) p. 6, October 1990.

"Log Home Beauty Is Compatible with Energy-Smart Building Technology," Good Cents, (1:4) pp. 22-24, April 1991.

"The (Non?) Advantage of Thermal Mass in Log Homes," J. Nisson, Energy Design Update, (13:9) p. 8, September 1993.

"R-Values of Log Walls," R. Kadulski, Solplan Review, (No. 15) pp. 8-9, June/July 1987.

"Raw Talent," J. Fleet, Custom Builder, (10:1) pp. 20-26, January/February 1995.

"Rustic Grandeur: Some Log Homes Are Much More Than Cabins," Fine Homebuilding, (No. 104) pp. 102-3, August/September 1996.

Publications

The Builder's Experience: Thirty Steps to a Complete Log Home, G. Flech, TAB Books, Inc., Blue Ridge Summit, PA, 1988. Out of print.

Building a Log House in Alaska, Cooperative Extension Service, University of Alaska, 1982. 90 pp. Available from Cooperative Extension Service, Distribution Center, University of Alaska Fairbanks,

Building with Logs, B. A. Mackie, Log House Publishing Co., 1997. 128 pp. ISBN 0-920270-16-6.

The Log Home Book, C. Teipner-Thiede and A. Thiede, 1993. 224 pp.

The Owner-Built Log House: Living in Harmony with Your Environment, B. A. Mackie, Firefly Books, 2001. 232 pp. ISBN 1552975487.

The Thermal Performance and Air Leakage Characteristics of Six Log Homes in Idaho; RCDP Cycle 3, C. Roos et al., Bonneville Power Administration, 1993. 58 pp. Available from the National Technical Information Service (NTIS), Email: info@ntis.gov. NTIS Order Number DE94000943.

This fact sheet was reviewed for accuracy in March 2003.

NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof.