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.

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, 2x4″ 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 logs 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.

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