National Bureau of Standards Test Confirm Energy Conserving
"Thermal Mass Effect" for Heavy (Log) 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. Energy consumption of each structure was precisely recorded by NBS technicians during this entire period.

Test Results

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.  


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