National Bureau of Standards Test
Confirm Energy Conserving "Thermal Mass Effect" for Heavy (Log) Walls in
Summary of Test
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. Energy consumption of
each structure was precisely recorded by NBS technicians 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 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
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) 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.
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 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 polystyrene
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 plaster walls.
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 footing.
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: U.S.
Department of Commerce, National Bureau of Standards, Center for Building
Technology, Building 226, Room B114, Gaithersburg, MD 20899.
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
Log Homes and Energy
From the: Consumer Energy
Information Briefs at EREN. - Residential Building
Log homes may be hand-made on-site or pre-cut in a
factory for delivery to the site. Pre-cut log home kits have been produced
since 1923. Log home manufacturers can also customize their designs. Wall
thickness' range from 6-16 inches (152-406 millimeters [mm]). The log industry
enthusiastically promotes the energy efficiency of log buildings. While there
is general agreement on the aesthetic value of log homes, their energy
efficiency is disputed.
The conventional measure of a structure's energy efficiency is the R-value of
the building material. An R-value (ft2h °F/Btu) is the rating of a material's
resistance to heat flow. The R-values for logs differ according to the type of
wood, ranging from about 1.41 per inch (25.4 mm) for some softwoods to 0.71 for
certain hardwoods. For example, a 6-inch (152.4 mm) diameter log would rate R-8
or R-9 at best. Using conventional analysis, a wood stud wall with 3+ inches
(88.9 mm) of fiberglass insulation and sheathing, siding, and wallboard rates
about R-14 or R-15. On the basis of the R-value, log walls do not satisfy most
building code energy standards.
The R-value rating, however, does not take into account a log's heat storage
capability. Logs act as thermal mass, storing heat during the day and gradually
releasing it at night. A 1982 study conducted by the National Bureau of
Standards found that, in certain climates, this thermal mass effect compensated
for low R-values. The thermal mass effect is most significant in milder,
sunnier climates, such as the sunbelt region, where the outdoor temperature
frequently moves above and below the thermostat setpoint. Some states, such as
California, compute thermal mass effect and R-value together to determine
building code compliance.
Several states, including Pennsylvania, Maine, and South Carolina, have
exempted log-walled homes from normal energy compliance regulations. Others,
such as Washington state, have approved "prescriptive packages" for various
sizes of logs. The American Society of Heating, Refrigerating, and Air
Conditioning Engineers (ASHRAE) 90.2 standard contains a thermal mass provision
that may make it easier to get approval in other states that base their codes
on this standard. Computer simulations using thermal mass measurements and
regional weather data have demonstrated compliance in states such as New York.
To find out the log building code standards for your state, contact your local
city or county building code officials. If your local officials are unfamiliar
with log home standards, contact your state energy office. You can also contact
the U. S. Department of Energy's Building Standards Hotline: (800) 270-CODE
As with any structure, passive solar design methods
may also boost a log home's energy efficiency. Factors to consider include:
the type and placement of windows;
orientation of the building;
airtightness of the structure;
size and type of logs used;
heat storage mass inside the building; and
the local climate.
Consulting a passive solar architect or designer may be wise, since the proper
sizing of the south-facing glass is crucial to the efficient performance of a
log house. (If you live in the southern hemisphere, the glazing will face
north.) A concrete floor or some other heat storage material absorbs solar
energy. Some designers suggest placing a masonry wall, known as a Trombe wall,
directly behind the glass to increase the thermal mass effect. Adding a Trombe
wall requires extensive remodeling, unless their house already has a thick,
un-insulated south-facing wall. Many log home manufacturers offer solar log
homes, or are able to custom-build them.
A potential problem with log homes is cold air and moisture infiltration
through gaps between the logs. Manufacturers claim that kiln drying the logs
prior to finish shaping and installation reduces or eliminates these gaps. They
also recommend using plastic gaskets and caulking compounds to seal the walls.
These seals may fail if the logs warp, shrink, or rot. The best woods to use to
avoid this problem, in order of effectiveness, are cedar, spruce, pine, fir,
and larch. The logs should also be seasoned for at least six months.
The Energy Efficiency of Log Homes
by Bill Kolida
Bill Kolida is a North American log home regulatory specialist. In
1995, he represented the log home industry in Canada on the National Energy
Code. He has been responsible for developing the insulation performance
standard for two provincial building codes in Canada. He has also written a
paper for the National Assn. of Home Builders - Log Homes Council on log home
energy efficiency issues. In the past, he has worked for BC Hydro both as a
program manager and consultant on new home energy efficiency issues. He is also
a certified heating and ventilation system design specialist in Canada.
Currently, he sits on a standards development committee in Canada.
Over the years through my involvement in home building, there are two truths I
have come to learn:
1. People who own log homes love them, and do not complain about energy
2. People who live in conventional frame housing wished they owned a log home.
With this discussion, I wish to put to rest the many myths about energy
problems in log homes.
Comparing Heating Performance between Conventional Frame and Log Homes
The big question asked by log home consumers is "How Energy Efficient are Log
Homes?" In 1991, the Research Centre at the North American Home Builders'
Association, conducted a study on the energy efficiency of log homes entitled
Evaluation of Log Homes ' Heating Performance in Northern Climates. For their
study, they examined the heating performance of conventional frame homes with
R-19 batts in New York State, to homes with 4-inch western red cedar walls in
the same region.
The study showed that the two wall systems provided the same benefits of energy
efficiency. A number of companies have built similar western red cedar homes in
Northern British Columbia, Saskatchewan and Alaska with similar results.
R-factor is not the only issue.
The amount of energy used to heat any home involves more than
just the R-value of the wall system. It also involves:
the tightness of fit and dryness of insulation in the wall
the ability of the wall to block air transfer from inside to
the ability of the wall system to store heat and radiate it
Log Walls are Tight Insulators
Wood is an insulator. In each log wall, there are millions of tiny air
pockets which insulate home owners from the elements. As an insulation system,
log walls can be far more effective at blocking heat transfer for the following
The effectiveness of insulation depends on how well it fits the cavity. Batt
insulation will sag over time, creating cold air paths for heat transfer.
Insulation can also be damaged by interior condensation penetrating ineffective
or damaged vapour barriers, or by failure of external water screens. In either
case, the effective R-value of the wall cavity will diminish over time. These
problems do not occur in log walls. Air barriers play an important role in
keeping heated air inside the house. If constructed properly, log wall systems
are more effective air barriers than the polyethylene sheeting found in
conventional housing. Air tightness tests on Canadian rectangular-milled log
homes have out-performed conventional housing for years. All solid objects have
the ability to retain heat and radiate it at a later time. This thermal mass
property will reduce utility bills, and is one of the reasons why many log home
owners have experienced lower heating bills with their new home. For log home
energy efficiency, your best choice is western red cedar. Based on thermal
efficiency standards listed in ASHRAE handbooks, it has the highest R-value per
Thicker Wall Systems Are Not Worth Their Investment
As part of developing a national energy code for Canada in 1995,
the Canada Codes Centre of the National Research Council sponsored a study on
log home energy efficiency entitled: Construction Report for Solid Wood Walls
in Houses - Final Report. This study reviewed the cost of building a variety of
round and profiled log wall systems in every Canadian province. The results of
this study showed that for the coldest Canadian region and the most expensive
heating fuel, a four-inch thick wall system was the most energy efficient log
wall system. There is no energy pay back in going to a thicker wall system.
In 1996, I studied this issue for the North American Home Builders' Log Homes
Council. For an average 1,600 square-foot home with pine logs, it would cost
about $4,700 to go from 6-inch to 8-inch thick logs. (Note: In 1999 dollars, no
less than $US 6500.) If you built with western red cedar or oak, the cost would
be higher. No matter where you built this home in North America, you would not
save enough energy to get your money back—even if you owned this home for 30
As a homeowner, you have to make a very serious decision about how to spend
money wisely. It is not wise to spend $6500 on something with no payback. You
would be better off to take the money and spend it on maintenance-free
materials (metal roof, metal clad windows, tile floor upgrade in high traffic
areas, etc), or on features that will enhance the market value of your house
(more or bigger windows, Jacuzzis, higher quality kitchen cabinets,
If you choose a thicker wall, do so because you like the look. Not because it's
a wise investment in energy.
Are You Worried About Freezing to Death In Your Log Home?
Ironically, most people in the U. S. are more concerned about heating and
cooling problems than folks in Canada. Hard to understand why? The bigger
problems should occur in northern climates where the heating needs of a
structure will be the greatest. But they don't - not even with a western red
cedar wall system.
Below is a table showing a variety of heating degree day (HDD) readings across
North America. HDD readings are used by weather services to determine how often
you will heat your home. The higher the HDD, the more often you will have to
turn on the heat to keep warm.
Ft. Nelson, B. C. is the home of the lodge for Stone Mountain Safaris. An 8700
square foot hunting lodge heated primarily by two wood stoves and a backup
propane furnace. The tight lock between logs and the thermal mass of wood keep
people warm even on the coldest days.
Heating Problems in Log Homes
If there are heating problems in log homes, they are no different than the
problems found in conventional housing. These problems have nothing to do with
the wall system, but with the design or installation of the heating system. If
the heating system is undersized, improperly installed, or the thermostat is
not effective, there will be heating problems and potentially high heating
bills. Many heating contractors have not been formally trained to do their
work. The heating system in any new home should be designed and installed by
certified contractors (ACCA contractors in the U. S. - HRAI contractors in