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Research Report on Energy Efficiency
of Log Buildings
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, D.C., 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 twenty-eight 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 (see chart A).
·
During the eleven-week, summer cooling, period, the log building
used 24% less cooling energy than the insulated wood frame
building (see chart B).
·
During the fourteen-week, winter heating period, the log building
and the insulated wood frame building used virtually the same
amounts of heating energy (see chart C).
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
2x4 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.
LHC'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 Homes Council
(predecessor to NALHC). 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 a part of the Home
Manufacturers Councils. In July, 1985 the Council membership
expanded due to a merger with the North American Log Builders
Association, and the name was changed to the North American Log
Homes Council. 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
North American Log Homes Council is a non-profit, voluntary
membership organization representing some sixty manufacturers of
log homes.
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 (the Washington, D.C.) 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
Home Manufacturers 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 Home Manufacturers
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 a 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 occurs during the day
when the cooling system 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 generally occurs 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 windows
were kept 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, D.C. 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.
CONCLUSION
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, D.C.).
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 R14 (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 R12 (with
interior mass) with 4" brick, 3-1/2" loose fill perilite
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.
Windows:
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 1 1% floor area.
Doors:
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 footing.
Heating/Cooling Equipment:
Each test building was equipped
with a centrally located 4.1 kWh 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, D.C. 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 the report and other studies are available by writing
to: U.S. Department of Commerce, National Bureau of Standards,
Center for Building Technology, Building 226, Room Bl 14,
Gaithersburg, MD 20899.
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