Energy Consumption of Log Homes – A report of tests conducted by the National Bureau of Standards
Log Homes and Energy Efficiency. – A report by the US Department of Energy
For those of you who might question the energy efficiency of log Homes, the above links should be comforting. However, I have long believed that the best proof lies in the utility bills of Log home owners.
Following is an unsolicited comment: Thank you Julie. "I have lived in a Log cabin in Jackson Hole several years ago and now have been living in another for the last 2 years in the mountains of northern California. We don't have a heating system, so the woodstove is the heat source. It stays quite comfortable winter and summer. The wood decor makes the decorating aspect fun. The wood has a warm feeling. The house in Wyoming was amazingly warm in the frigid temps. I would definitely live in a Log home again and build one if the opportunity presented itself." -----------------------------
And another unsolicited comment:Thank you Bob. My wife and I bought a Log home 7 years ago and would not go back to a stick home. log Homes are very warm in the winter and cool in the summer. We live in Sask. Canada where the winter months will reach -40f and the summer months will reach +40f. Right now we are looking at putting an addition on to our home .
----------------------------- During 2005, the gas and electric bills for the B&H model "Sandy's Joy" (Link will open in a new tab) totaled $3,049.53 - slightly more than $250 per month. This 2300 square foot (plus the heated and cooled basement) model Log home is located outside Fredericksburg, VA. ----------------------------------
The following testimonial was received via email on September 2, 2006: I live in a Log home in Ocala Florida where it reaches 100 degrees f in the summer. I have lived here for 11 years and my electric bill would average 125 dollars a year for the first 10 years. However the last year we have seen rate increases which brought it up to an average of 190 per year. My home has a basement which has central heat and air included in the 2500 hundred square feet. I have a lot of friends that pay a considerable amount more than me with less square footage. Tricia
Do you you have experience (good or bad) with the energy cost of log Homes?
If you are willing to share this experience,
send email > firstname.lastname@example.org.
National Bureau of Standards Test Confirm Energy Conserving “Thermal Mass Effect” for Heavy (l) Walls in Residential Construction
Summary of Test Findings:
A study was conducted by the National Bureau of Standards (NBS) for the Department of Housing and Urban Development (HUD) and the Department of Energy (DOE) to determine the effects of thermal mass (the bulk of solid wood Log walls, or brick and block walls) on a building’s energy consumption. For the test, six 20’x20′ test buildings were built on the grounds of the National Bureau of Standards, 20 miles north of Washington, DC, in the fall of 1980. Each structure was identical except for construction of its exterior walls. The buildings were maintained at the same temperature levels throughout the 28-week test period between 1981 and 1982. NBS technicians precisely recorded energy
consumption of each structure during this entire period.
- During the three-week spring heating period, the Log building used 46% less heating energy than the insulated wood frame building.
- During the eleven-week summer cooling period, the Log building used 24% less cooling energy than the insulated wood frame building.
- During the fourteen-week winter heating period, the Log building and the insulated wood frame building used virtually the same amounts of heating energy.
The National Bureau of Standards technicians conducting the test calculated the R-value of the Log building, which was constructed with a 7″ solid square log, at a nominal R-10. It rates the insulated wood frame building, with its 2’x4′ wall and 3-1/2″ of fiberglass insulation, at a nominal R-12, thus giving the wood frame structure a 17% higher R-value. Yet during the entire 28 week, three season test cycle, both buildings used virtually identical amounts of energy. This led the National Bureau of Standards to conclude that the thermal mass of Log walls is an energy-conserving feature in residential construction.
NBS Tests Confirm Energy-Conserving “Thermal Mass Effect” of Log Walls Full Report:
In the first extensive field testing of its kind, researchers at the Commerce Department’s National Bureau of Standards (NBS) have confirmed that walls of heavyweight construction (such as those built with solid wood ls, concrete block or brick) exhibit an energy conserving “mass effect” in residential buildings during the summer and the intermediate heating season representative of fall or spring in a moderate climate. However, no mass effect was observed during the winter heating season.
According to NBS researchers, these extensive field tests should help resolve a controversy over whether residences having heavyweight walls consume less energy for space heating and cooling than buildings having lightweight walls of equivalent thermal resistance.
The National Bureau of Standards research team found that the heavyweight walls (including building number 5, the Log structure) “did exhibit a thermal mass effect and thus save significant amounts of energy both in the summer cooling season and the intermediate heating season representative of fall or spring in this (Washington, DC) area.”
The Use of R-Values:
Most state and local building codes require specific “R-Values,” or thermal resistance values, for the walls, ceilings, and floors of houses. The R-Values in these codes vary with geographical location and climate considerations. The Building Systems Councils’ technical staff and other industry professionals have often challenged the exclusive reliance on R-Values alone to rate the energy efficiency of a wall’s building materials while ignoring the thermal mass effect inherent in heavyweight (l) walls. R-Values are recognized by most professionals to be a reliable indication of the thermal performance of a material–under conditions of constant interior and exterior temperatures. The Building Systems Councils’ technical staff argues that these are not the conditions that exist in the “real world,” where outdoor
temperatures vary widely during a typical day-night cycle. To obtain a true rating of building’s thermal efficiency in these conditions, building codes must also consider the “mass effect” of heavyweight (l) walls.
What Is “Mass Effect”?
According to NBS researchers, “the mass effect relates to the phenomenon in which heat transfer through the walls of a building is delayed by the high heat (retention) capacity of the wall mass. Consequently, the demand for heating or cooling energy to maintain indoor temperature may, under some circumstances, be pushed back until a time when wall heat transfer and equipment operating conditions are most favorable.” This heat retention phenomenon is also referred to as “thermal capacitance” or time lag–the resistance of a material (such as solid wood walls) over time to allow a change in temperature to go from one side to the other.
How Mass Saves Energy:
NBS researchers explained the energy saving effect of mass during the summer cooling season this way: “In an insulated wood frame building, which is considered to have low mass, the maximum wall heat gain rate during this season is operating most often and working the hardest. In a heavy walled building (such as the Log building), however, the heat transfer lag means the maximum wall heat gain rate general during the cool night period when the cooling plant is operating least often or not at all. Consequently, the cooling energy requirement is reduced.”
The NBS test showed that the Log structure performed better than the insulated wood building in the intermediate heating season and the summer cooling season; however, there was no appreciable difference during the winter heating season. During the winter heating season, no effect of mass was noted since all insulated buildings and the Log building required comparable amounts of heating energy each hour to maintain their predetermined indoor temperatures.
As with all such test procedures, these tests have their own limitations, according to NBS, and therefore these factors should be considered in using the results. The structures had no partition walls or furniture; items which would tend to give the wood frame structures some of the mass effect. Also, the buildings were closed at all times, and the buildings were constructed to maximize the mass effect attributable to the walls. Also, the results are very climate dependent, and results relate to the moderate climate found in the Washington, DC, area.
Future tests to be carried out on the six buildings will address some of these limitations by installing partition walls and opening windows when appropriate. Moreover, a recently developed NBS computer model that predicts the energy consumption for multi-room structures will be validated and subsequently used to extend the NBS test results to other locations and climates around the country.
The Building Systems Councils is gratified that its long struggle to gain recognition for the importance of “thermal-mass” has been confirmed by these tests and that the energy efficiency of log Homes has been proven. The Council is presently participating in a similar testing program being conducted by the Oak Ridge National Testing laboratory in Albuquerque, New Mexico, and hopes to add the results of those tests to this material in an effort to gain acceptance of “thermal mass effect” in building codes throughout the country. We further await the results of future tests to be performed by the NBS at this test site and the results of the NBS computer-modeling program.
- Technical Information
- Description of Test Buildings
Six 20′ wide and 20′ long one room test buildings with a 7-1/2″ high ceiling were constructed outdoors at the National Bureau of Standards facility located in Gaithersburg, Maryland (20 miles north of Washington, DC).
- Construction Details of Walls
- Building #1 – An insulated wood frame home, nominal R-12 (without mass) with 5/8″ exterior wood siding, 2×4″ stud wall, 3-1/2″ fiberglass insulation, plastic vapor barrier, and 1/2″ gypsum drywall.
- Building #2 – An un-insulated wood frame home, nominal R-4 (without mass) with same detail as above, but without the fiberglass insulation.
Building #3 – An insulated masonry home, nominal R-14 (with exterior mass) with 4″ brick, 4″ block, 2″ polystyrene insulation, plastic vapor barrier, furring strips and 1/2″ gypsum drywall.
- Building #4 – An un-insulated masonry home, nominal R-5 (with exterior mass) with 8″ block, furring strips, vapor barrier, 1/2″ gypsum drywall, and no polystyrene insulation.
- Building #5 – A Log home, nominal R-10 (with inherent mass) with 7″ solid square wood ls with tongue and groove mating system, no additional insulation, no vapor barrier, and no interior drywall.
- Building #6 – An insulated masonry home, nominal R-12 (with interior mass) with 4″ brick, 3-1/2″ loose fill perlite insulation, 8″ block and 1/2″ interior plaster walls.
- Interior/Exterior Surfaces
Interior surfaces were painted off-white. Exterior surfaces of buildings 1,2 and 4 were painted approximately the same color as the exterior face brick of buildings 3 and 6.
Four double-hung, insulating glass (double pane) windows, with exterior storm windows, two in south facing wall, two in north facing wall. Total window area was 43.8 sq. ft. or 11% floor area.
One insulated metal door on east wall. Total door area was 19.5 sq. ft.
- Ceiling and Roof System
Each test building contained a pitched roof with an attic space ventilated with soffit and gable vents. The ventilation opening was
consistent with the HUD Minimum Property Standards. Eleven inches of fiberglass blanket insulation (R-34) was installed over the ceiling
of each test building.
- Floor System
The edges of the Concrete slab-on-grade floors were insulated with 1″ thick polystyrene insulation at both the inner and outer surfaces of the footing.
- Heating/Cooling Equipment
Each test building was equipped with a centrally located 4.1 kW electric forced air heating plant equipped with a 13,000 Btu/h split vapor-compression air conditioning system.
- Description of Test Buildings
Technical Report Available
A complete technical presentation of this study was prepared by D.M. Burch, W.E. Remmert, D.F. Krintz, and C.S. Barnes of the National Bureau of Standards, Washington, DC, in June, 1982, and is entitled “A Field Study of the Effect on Wall Mass on the Heating and Cooling loads of Residential Buildings.” This study was presented before the “Thermal Mass Effects in Buildings” seminar held in Knoxville, Tennessee, on June 2-3, 1982, Oakridge National laboratory, Oakridge, Tennessee.
Copies of this report and other studies are available by writing to: US Department of Commerce, National Bureau of Standards, Center for Building Technology, Building 226, Room B114, Gaithersburg, MD 20899.
The Log building used by the National Bureau of Standards for this energy conservation study was donated and erected by members of the Log Home Council. Since the inception of the log Homes Council in 1977, well over a quarter of a million dollars have been spent on research and testing projects related to the Log home industry.
Members of the Council have voluntarily contributed tens of thousands of hours of their time to accomplish these tasks for the benefit of the industry and the builders and owners of log Homes. On January 1, 1982, the log Homes Council affiliated with the National Association of Home Builders as part of the Building Systems Councils. In July 1985, the Council membership expanded due to a merger with the North American Log Builders Association. All members of the Council are also individual members of the National Association of Home Builders and through their dues support the many worthwhile activities of the NAHB. The log Homes Council is a non-profit, voluntary membership organization representing some sixty manufacturers of log Homes.
A research report published by the log Homes Council of the National Association of Home Builders, 1201 15th Street, NW, Washington, DC 20005 — (800) 368-5242 ext. 576 Barbara K. Martin, Executive Director
U.S. Department of Energy – Energy Efficiency and Renewable Energy
log Homes and Energy Efficiency
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 ls 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 l’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 ls are still about 15%-20% water when the house is assembled. As the ls dry over the next few years, the ls shrink. The contraction (and expansion – see below) of the ls opens up gaps between the ls, creating air leaks and causing drafts and high heating requirements.
To minimize problems like these, ls 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 ls 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 ls 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 l-walled homes from normal energy compliance regulations. Others, such as Washington, have approved “prescriptive packages” for various sizes of ls. 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;
Air tightness of the structure;
Size and type of ls 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.
The following are sources of information on the Log home building industry. After visiting any of the three links below, use your browser’s back button to return here.
- International Log builders’ Association
- Email: email@example.com
- Log Home Builder’s Association of North America
- log Homes Council (of The National Association of Home Builders)
Bibliography (NOTE – THESE CAN BE LINKED TO AMAZON)
The following articles and publications contain additional information on log Homes.
“Air leakage of log Homes,” Home Energy, (8:6) p. 40, November/December 1991.
- “Air tightness of log Homes,” J. Nisson, Energy Design Update, (10:5) p. 9, May 1991.
- “A Beginner’s Guide to log Homes,” E. Beal, Countryside and 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.
- “l Home Beauty Is Compatible with Energy-Smart Building Technoly,” 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.
- 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 ls, 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: firstname.lastname@example.org. NTIS Order Number DE94000943.
This fact sheet was reviewed for accuracy in March 2003.
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.
U.S. Department of Energy
Content last Updated: 06/17/2004