National Bureau Of Standards Tests
"Thermal Mass Effect"
For Heavy (Log) Walls In
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. NBS technicians precisely
recorded energy consumption of each structure during this entire
• 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 Rvalue.
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.
NBS Tests Confirm Energy-Conserving "Thermal Mass Effect" of
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)
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
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.
As with all such test procedures, these tests 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 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.
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
Construction Details of Walls
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.
An un-insulated wood frame home, nominal R-4 (without mass) with
same detail as above, but without the fiberglass insulation.
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.
An un-insulated masonry home, nominal R-5 (with exterior mass) with
8" block, furring strips, vapor barrier, 1/2" gypsum drywall, and no
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.
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
Interior surfaces were painted off-white. Exterior surfaces of buildings
1,2 and 4 were painted approximately the same color as the exterior
face brick of buildings 3 and 6.
Four double-hung, insulating glass (double pane) windows, with
exterior storm windows, two in south facing wall, two in north facing
wall. Total window area was 43.8 sq. ft. or 11% floor area.
One insulated metal door on east wall. Total door area was 19.5 sq. ft.
Ceiling & Roof System.
Each test building contained a pitched roof with an attic space
ventilated with soffit and gable vents. The ventilation opening was
consistent with the HUD Minimum Property Standards. Eleven inches
of fiberglass blanket insulation (R-34) was installed over the ceiling of
each test building.
The edges of the Concrete slab-on-grade floors were insulated with 1"
thick polystyrene insulation at both the inner and outer surfaces of the
Each test building was equipped with a centrally located 4.1 kW
electric forced air heating plant equipped with a 13,000 Btu/h split
vapor-compression air conditioning system.
Technical Report Available
A complete technical presentation of this study was prepared by D.M.
Burch, W.E. Remmert, D.F. Krintz, and C.S. Barnes of the National
Bureau of Standards, Washington, DC, in June, 1982, and is entitled
"A Field Study of the Effect on Wall Mass on the Heating and Cooling
Loads of Residential Buildings." This study was presented before the
"Thermal Mass Effects in Buildings" seminar held in Knoxville,
Tennessee, on June 2-3, 1982, Oakridge National Laboratory,
Copies of this report and other studies are available by writing to: US
Department of Commerce, National Bureau of Standards, Center for
Building Technology, Building 226, Room B114, Gaithersburg, MD
The log building used by the National Bureau of Standards for this
energy conservation study was donated and erected by members of
the Log Home Council. Since the inception of the Log Homes Council in
1977, well over a quarter of a million dollars have been spent on
research and testing projects related to the log home industry.
Members of the Council have voluntarily contributed tens of thousands
of hours of their time to accomplish these tasks for the benefit of the
industry and the builders and owners of log homes. On January 1,
1982, the Log Homes Council affiliated with the National Association of
Home Builders as part of the Building Systems Councils. In July 1985,
the Council membership expanded due to a merger with the North
American Log Builders Association. All members of the Council are also
individual members of the National Association of Home Builders and
through their dues support the many worthwhile activities of the
NAHB. The Log Homes Council is a non-profit, voluntary membership
organization representing some sixty manufacturers of log homes.
A research report published by the Log Homes Council of the National Association of
Home Builders, 1201 15th Street, NW, Washington, DC 20005 -- (800) 368-5242 ext. 576
Barbara K. Martin, Executive Director