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National Bureau Of Standards Tests

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. NBS technicians precisely

recorded energy consumption of each structure during this entire


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 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

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)


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 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

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.

Interior/Exterior Surfaces

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.

Floor System

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


Heating/Cooling Equipment

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,

Oakridge, Tennessee.

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


BSC's Participation

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

Energy Efficiency