Log Homes and Energy Efficiency
From the: Consumer Energy Information Briefs at EREN. – Residential Building Log homes may be hand-made on-site or pre-cut in a factory for delivery to the site. Pre-cut log home kits have been produced since 1923. Log home manufacturers can also customize their designs. Wall thickness’ range from 6-16 inches (152-406 millimeters [mm]). The log industry enthusiastically promotes the energy efficiency of log buildings. While there is general agreement on the aesthetic value of log homes, their energy efficiency is disputed. The conventional measure of a structure’s energy efficiency is the R-value of the building material. An R-value (ft2h °F/Btu) is the rating of a material’s resistance to heat flow. The R-values for logs differ according to the type of wood, ranging from about 1.41 per inch (25.4 mm) for some softwoods to 0.71 for certain hardwoods. For example, a 6-inch (152.4 mm) diameter log would rate R-8 or R-9 at best. Using conventional analysis, a wood stud wall with 3+ inches (88.9 mm) of fiberglass insulation and sheathing, siding, and wallboard rates about R-14 or R-15. On the basis of the R-value, log walls do not satisfy most building code energy standards. The R-value rating, however, does not take into account a log’s heat storage capability. Logs act as thermal mass, storing heat during the day and gradually releasing it at night. A 1982 study conducted by the National Bureau of Standards found that, in certain climates, this thermal mass effect compensated for low R-values. The thermal mass effect is most significant in milder, sunnier climates, such as the sunbelt region, where the outdoor temperature frequently moves above and below the thermostat setpoint. Some states, such as California, compute thermal mass effect and R-value together to determine building code compliance. Several states, including Pennsylvania, Maine, and South Carolina, have exempted log-walled homes from normal energy compliance regulations. Others, such as Washington state, have approved “prescriptive packages” for various sizes of logs. 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 other states that base their codes on this standard. Computer simulations using thermal mass measurements and regional weather data have demonstrated compliance in states such as New York. To find out the log building code standards for your state, contact your local city or county building code officials. If your local officials are unfamiliar with log home standards, contact your state energy office. You can also contact the U. S. Department of Energy’s Building Standards Hotline: (800) 270-CODE (2633) As with any structure, passive solar design methods may also boost a log home’s energy efficiency. Factors to consider include:
- the type and placement of windows;
- orientation of the building;
- airtightness of the structure;
- size and type of logs used;
- insulation levels;
- heat storage mass inside the building; and
- the local climate.
Consulting a passive solar architect or designer may be wise, since the proper sizing of the south-facing glass is crucial to the efficient performance of a log house. (If you live in the southern hemisphere, the glazing will face north.) A concrete floor or some other heat storage material absorbs solar energy. Some designers suggest placing a masonry wall, known as a Trombe wall, directly behind the glass to increase the thermal mass effect. Adding a Trombe wall requires extensive remodeling, unless their house already has a thick, un-insulated south-facing wall. Many log home manufacturers offer solar log homes, or are able to custom-build them. A potential problem with log homes is cold air and moisture infiltration through gaps between the logs. Manufacturers claim that kiln drying the logs prior to finish shaping and installation reduces or eliminates these gaps. They also recommend using plastic gaskets and caulking compounds to seal the walls. These seals may fail if the logs warp, shrink, or rot. The best woods to use to avoid this problem, in order of effectiveness, are cedar, spruce, pine, fir, and larch. The logs should also be seasoned for at least six months.
The Energy Efficiency of Log Homes
by Bill Kolida June 1999 Bill Kolida is a North American log home regulatory specialist. In 1995, he represented the log home industry in Canada on the National Energy Code. He has been responsible for developing the insulation performance standard for two provincial building codes in Canada. He has also written a paper for the National Assn. of Home Builders – Log Homes Council on log home energy efficiency issues. In the past, he has worked for BC Hydro both as a program manager and consultant on new home energy efficiency issues. He is also a certified heating and ventilation system design specialist in Canada. Currently, he sits on a standards development committee in Canada. Over the years through my involvement in home building, there are two truths I have come to learn: 1. People who own log homes love them, and do not complain about energy efficiency problems. 2. People who live in conventional frame housing wished they owned a log home. With this discussion, I wish to put to rest the many myths about energy problems in log homes. Comparing Heating Performance between Conventional Frame and Log Homes The big question asked by log home consumers is “How Energy Efficient are Log Homes?” In 1991, the Research Centre at the North American Home Builders’ Association, conducted a study on the energy efficiency of log homes entitled Evaluation of Log Homes ‘ Heating Performance in Northern Climates. For their study, they examined the heating performance of conventional frame homes with R-19 batts in New York State, to homes with 4-inch western red cedar walls in the same region. The study showed that the two wall systems provided the same benefits of energy efficiency. A number of companies have built similar western red cedar homes in Northern British Columbia, Saskatchewan and Alaska with similar results.
R-factor is not the only issue.
The amount of energy used to heat any home involves more than just the R-value of the wall system. It also involves:
- the tightness of fit and dryness of insulation in the wall cavity.
- the ability of the wall to block air transfer from inside to out.
- the ability of the wall system to store heat and radiate it back later.