For years, the log home industry has known that a log wall's thermal mass makes it as energy efficient as a well-insulated frame wall but this has been very difficult to quantify. Log home owners had the home heating bills to prove it was true, but Department of Energy and code officials needed more than empirical evidence. Finally, the Log Homes Council, a part of the Building Systems Councils of the National Association of Home Builders (NAHB) has gathered enough scientific data from independent research. The nation's Model Energy Code now recognizes the energy-conservation benefits of thermal mass.

What this means for log home producers is that they no longer have to fight energy codes based on R-values. Now log home owners don't have to invest in additional building materials to meet codes that do nothing to improve their homes.

R-value measures a material's resistance to the transfer of heat from one side to the other. Logs actually absorb and store heat in their cellular structure. They have a relatively low resistance to heat transfer. This put logs at a disadvantage in cold weather states where homes were overbuilt to meet total R-value requirements. This added significant expense, especially in the floor, window and roof systems with no additional benefit.

In the 1970's during the energy crisis, new energy standards were developed by state and federal agencies for residential construction. During the crisis situation it was easiest to adopt the 'R' value system since it already existed. Since that time it has become virtually the only standard of measurement. The building materials industry has done everything possible to insure this.

The Log Home Council had two goals in mind. Thermal mass is a material's ability to absorb, store and then slowly re-release the heat over time. Demonstrating this was the first goal. Because of their cellular structure, bulk and thickness, logs accomplish this very well. The second goal was to show a time delay in the re-release at night when the temperatures had dropped.

In moderate climates early studies showed thermal mass significantly reduced the heating and cooling loads. The National Institute of Standards conducted the most important of these studies in 1981-82. However, the energy experts continued to question the value of thermal mass during the winter months in northern climates. Its' abilities were doubted when heat is needed constantly and thermostat settings are opposite outdoor temperatures.

In 1990, an independent testing agency answered this question to the benefit of the log home industry. Advanced Certified Thermography conducted a study for the Energy Division of the Minnesota Department of Public Service. It focused on heat loss through air leakage, assumed to be a problem with log walls because of their many joints. Improved joint construction and the use of expanded foam sealants and gaskets on all joints and corner intersections are credited with the changes. The study found the industry has reduced air infiltration rates in the past 15 years significantly. The leakage noted in the 23 test homes showed it to be in the same places as frame houses: at the peak of the cathedral ceilings, around window and door frames and along the tops of walls. The study concluded that air leakage in well-built, modern log homes is not due to their log walls.

During the NAHB's Research Center study conducted in 1991, it showed the thermal mass of log walls does significantly reduce energy use for heating in cold climates. It based its conclusion on a comparison of the actual energy use of eight log homes to the actual energy uses of eight well-insulated foam houses during one winter. The number of houses were evenly divided between upstate New York and Montana. The study also compared the homes' actual energy use to their predicted energy consumption. The results led to the conclusion that log homes were as energy efficient as the frame houses.

The Log Homes Council's energy committee chairman, Dave Carter said, "The significance is that the log walls' average R-values are 44 percent lower than the frame walls' average R-value. Clearly, we must conclude the thermal mass performance of log walls is an advantage to log home owners."