National Bureau of Standards 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 logs, 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 (log) 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 (log) 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.

Test Limitations

As with all such test procedures, these test 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

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.

Click here for the "Technical Details" of the study