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FINAL REPORT
EVALUATION OF LOG HOMES' HEATING ENERGY PERFORMANCE IN
NORTHERN CLIMATES
Prepared for
National Association of Home Builders North American Log Homes Council
l5th and M Streets, NW
Washington, D.C. 20005
by
NAHB Research Center 400 Prince George's Boulevard Upper Marlboro, MD
20772-8731
Project No.: 2132
April 5, 1991
ACKNOWLEDGEMENTS
This report was prepared by the NAHB Research Center (Research Center) for the North
American Log Homes Council (NAI HC) of the National Association of Home Builders
(NAHB). The principal investigator was David H. Saunders with technical review by Donald F.
Luebs and Frank S. Luciani, P.E. Field assistance was provided by Raj S. Siloand. Tracer gas
analysis was provided by Dr. Ban Huat-Song of the Research Center's AIMS Laboratory. Special
acknowledgement is due w David Reed of the NAI-HC Energy Committee, and James Birdsong
and Barbara Martin of the NAHB Building Systems Council for their assistance. Appreciation is
extended to all NALHC members who helped locate homes and, of course, to all home owners
who provided the homes for this project.
EXECUTIVE SUMMARY
The purpose of this study was to perform a preliminary evaluation of the hating energy
performance of log homes in northern climates by comparing actual energy consumption of each
log home to its predicted energy use and to the predicted energy use of similar conventionally
insulated wood-framed homes.
Conceptual barriers based on the complexity of prescribing thermal mass factors complicate
compliance with energy requirements for log homes. Energy codes in northern climates often
consider the thermal mass effect of solid wood wall construction as negligible during the heating
season.
Previous studies have concluded that reductions in heating attributable to wall mass were
significant when heating was required for only pan of a day. However, when heating was
required continuously, the heating load became a function of steady-state R-value, and no mass
effect was measurable during the hearing season. Two relevant questions were suggested: Do log
homes approach a steady-state condition over a typical heating season? and, if not, Do log homes
alternate between dynamic and steady-state conditions over a typical heating season sufficiently
to provide net energy benefits attributable to wall mass?
Four (4) log homes were chosen in the northwest Montana region and four (4) log homes where
chosen in the upper New York State region. Log thickness ranged from 4.0 to 7.17 inches.
Building characteristic data were collected during site visits including air leakage measurements,
utility/fuel consumption data, and building plans.
The results of the study suggest the following:
Analysis of fuel consumption data suggests the log homes are at least as efficient as
conventionally insulated counterparts.
The above efficiency is achieved even though the average R-value of the log walls is
44% less than the average R-value of the conventionally insulated walls.
Annual healing energy based on measured fuel consumption data compared well with
computer simulation with individual differences ranging from -16% to +13%.
Air leakage measurements indicated that the homes have reasonably low estimated air
infiltration rates ranging from 0.33 to 0.66 air changes per hour (ACH).
Log homes may alternate between steady-state and dynamic conditions sufficiently to
provide thermal mass benefits over a heating season.
Future research is recommended focusing on comparisons between actual energy consumption of
log homes and actual energy consumption of conventionally insulated homes.
INTRODUCTION
This study was undertaken by the NAHB Research Center (Research Center) for the Log Homes
Council to evaluate the heating energy performance of log homes. Significant challenges exist
for the log home building industry in documenting the thermal mass benefits and the energy
efficiency of log homes in northern climates. Previous studies have shown that thermal mass can
reduce heating loads if the building experiences fluctuating net heat gains and losses; that
thermal mass delays heat loss due to outdoor temperature drops; and that thermal mass can
reduce heating equipment cycling (Gustinis and Robertson, 1984). However, the extent of such
benefits in log homes in predominantly heating climates has not been well documented.
PURPOSE
The purpose of this study was to perform an evaluation of the heating energy performance of log
homes in northern climates by comparing actual energy consumption of each log home to its
predicted energy use and to predicted energy use of similar conventionally insulated woodframed homes.
Two relevant questions concerning log homes in northern climates are suggested here: Do log
homes approach a steady-state condition over a typical heating season? and, if not, do log homes
alternate between dynamic and steady-state conditions over a typical heating season sufficiently
to provide net benefits attributable to thermal-mass?
BACKGROUND
Energy codes and standards are beginning to recognize the benefits of high mass construction.
However, conceptual barriers revolving around the complexity of prescribing thermal mass
factors complicate compliance with energy requirements for log homes. For example, energy
codes in northern climates often consider the thermal mass effect of solid wood wall construction
as negligible during the heating season. This has resulted in energy efficiency requirements that
are greater for homes built of solid wood wall construction than for homes built of insulated
wood-frame construction with increased requirements typically placed on other building
components such as more efficient windows or higher ceiling insulation levels.
The perception of negligible benefit of wall mass in heating climates is based partly on a thermal
mass research program in the early 1980s sponsored by the United States Department of Energy
(DOE). The objective of that program was to expand our knowledge of the effect of thermal mass
in building envelopes in reducing heating and cooling energy requirements of buildings. One
study in this program, conducted by the National Institute of Standards and Technology, formerly
the National Bureau of Standards, concluded that no measured thermal mass effect of log walls
was observed during the winter heating season when space heating was provided each hour.
However, when heating was required for only pan of a day, significant reductions in heating were
attributed to wall mass (Burch et aL 1984). A second study, conducted by the New Mexico
Energy Research and Development Institute, and known as the Southwest Thermal Mass Study,
also concluded that if heating is required continuously, then the heating load is a function of
steady-state R-value only (Gustinis and Robertson, 1984).
These conclusions were based on theory and corroborated with measured data. They state that
the mass of a building material in itself does not affect the heating energy use in buildings; and
that other factors are involved. These factors include the dynamic response of building mass to
diurnal changes in outdoor temperature and insulation, interior fluctuations in temperature and
internal heat gains, and to some extent occupant behavior.
Another conclusion of the Southwest Thermal Mass Study was that lightweight wood-flamed
insulated buildings that respond rapidly to outdoor temperature changes and to changing internal
heat load are more likely to float above a thermostat steeping and, therefore, would have larger
heat loss due to the higher indoor-outdoor temperature difference. Whereas, high mass buildings
such as log homes can attenuate heat transfer induced by outdoor temperature changes and can
absorb excess energy introduced in the interior from solar and internal gains (Gustinis and
Robertson, 1984). In other words, a building with little mass will result in temperature excursions
above the thermostat setpoint more readily and, therefore, will use more energy than a building
with similar R-value but incorporating interior or envelope mass.
The DOE Thermal Mass Program has provided fundamental understanding of building mass
effects. On the one hand, the information from this program can be used to reasonably
characterize the energy conserving effects of building mass in energy codes and standards. Or the
other hand, the program has tended to confuse the issue of whether building mass is a net benefit
in predominately heating climates.
METHODOLOGY
The following four tasks were used to accomplish the objective:
Four homes were selected in each of two regions, one in western Montana and the other in
upstate New York;
Required data were obtained for each home including building characteristics, previous energy
consumption records and air leakage measurements;
Actual and predicted energy use of each log home and the predicted energy use of similar woodframe insulated houses that comply with local energy codes were determined; and
Results of the log homes and wood-frame insulated homes were analyzed and compared.
Log Home Selection
Over fifty log home manufacturers and distributors were contacted for help in locating homes to
participate in the study. Of these contacts, four homes were chosen in the Montana area, two
each from two different manufacturers, and four homes were chosen in the New York region
from three different manufacturers {one manufacturer was the same as for two of the homes in
Montana). This provided eight homes for the study representing four different manufacturers.
The criteria for selecting homes for the study were as follows:
1. Homes must not have a wood stove;
2. Homes should be occupied during the heating season;
3. Homes must have at least one full year of existing utility dam; and
4. Homes must be accessible to project personnel during the study.
Site visits were made to each of the eight study homes during the month of October. Field survey
forms were used to record complete building characteristic data; building plans were obtained,
reviewed and checked with actual construction; photographs were taken; utility bills were
obtained; and air leakage measurements were conducted using a blower door assembly on all
homes with the exception of one (due to field difficulties) which had an air infiltration rate
measured using perfluorocarbon tracer gases from the Research Center's Air Infiltration
Measurement Service (AIMS). Based on these measurements, estimated natural infiltration rates
were derived for the computer simulations.
Analysis of Actual Energy Use
Actual energy consumption of the log homes was analyzed by the Princeton Scorekeeping
Method [PRISM) which requires at least one year of fuel consumption data and historical avenge
outdoor temperature data from a nearby weather station. Missoula, MT, and Albany, NY,
weather station data were used in this study to provide weather adjusted indices for heating
called the "Normalized Annual Consumption" (NAC). The NAC index provides a measure of
what energy consumption would be during a year of typical weather conditions. The reliability of
PRISM estimates of the NAC have been reported to be within 3 percent of the estimate (Fels et
al. 1986).
Analysis of Predicted Energy Use
Energy predictions were made with a computer building energy program called CALPAS3~.
Inputs to this program consist of building characteristic data such as areas of walls, windows,
roofs and floors; thermal properties such as heat loss coefficients CLT-values), and heat
capacities and conductivities of materials; infiltration estimates; and estimates of healing system
efficiency. The output of CALPAS3 is an annual prediction of heating fuel consumption along
with monthly summaries of fuel consumption estimates and interior temperatures. Simulations of
energy consumption over extended periods such as daily and monthly, have been shown to be
well matched to measured data, while detailed occupant behavior would be necessary to predict
actual hourly performance (Atkinson et a/. 1981).
Thermal Properties of Log Walls
Input data for the CALPAS3 program were obtained from the site visits and building plans.
Actual insulation levels were verified during the site visit or through discussion with the builder.
R-values of the logs were calculated by the method discussed by TenWolde et al. (1988).
Equation 1 is a linear relationship among thermal conductivity, k, in Btu-in/hr-ft/sq-°F, dry
density, D, in lb/ft3 and moisture content (MC), M, in percent.
k = (D/1000)(21.55 + 0.4514M) + 0.1292
(1)
The thermal conductivity of each of the three log species represented by this sample of eight
homes was calculated at a MC of 12 percent and axe shown in Table 1. The resistivity, 1L is the
inverse of the conductivity, k.
Average log thickness was determined by a simplified geometric method described by Collister
(1990). These thicknesses were used to determine overall R-value of the log.
LOG HOME SITES
The four (4) log homes chosen for the Montana region (approximately 8000 HDD) are located in
Frenchtown, Hamilton, and Livingston, Montana, and in Wapiti, Wyoming. The four (4) log
homes chosen for the New York region (approximately 7000 HDD) are located in Evans Mills
and Milton, New York and in Huntington and Southampton, Massachusetts.
The Frenchtown home, designated MT1, is approximately eight years old and has 3416 ft2 of
conditioned floor space, including a fully conditioned basement It has 984 ft2 of exterior walls
constructed of 4x8 inch flat-cut Western Red Cedar logs and 594 ft2 of interior log wall
partitions. Total glazing area is 595 ft2, with 426 ft2 in a south orientation. The heating system is
fueled by propane gas with an estimated system efficiency of 70 percent Records for one year of
gas use were available for this house. Figure 1 shows the south side of this home.
The Hamilton home (MT2) shown in Figure 2. is three years old and has 2119 ft2 of conditioned
floor space. It has 1487 ft2 of exterior walk constructed of 4x8 inch Western Red Cedar logs fiatcut on all sides. There is 538 sq ft of glazing area, a sunspace on the eastside and 303 sq ft of
glazing that is oriented 25° west of south. The house is built over a crawlspace and contains no
significant internal mass. The home is heated with natural gas with an estimated system
efficiency of 85 percent Two years of utility data were available for this house.
The Livingston home (MT3) shown in Figure 3, is one year old and is constructed of Lodgepole
pine rounded on two sides with an average thickness of 7.17 inches. The home has 3715 ft2 of
conditioned floor area including a fully conditioned basement. It has 1343 ft2 of exterior log
walls and 84 ft2 of interior log wall partitions. There is 493 ft2 of glazing area, with 269 ft2
facing south. The house is heated with propane gas with a system efficiency estimated at 90
percent. One year of fuel consumption data was available for this house.
Average log thickness was determined by a simplified geometric method described by Collister
(1990). These thicknesses were used to determine overall R-value of the log.
The Wapiti. WY home (MT4) shown in Figure 4 is four years old and is built over a crawlspace.
It uses electric heat and has 1442 ft2 of conditioned floor space. The house has 1210 ft2 of
exterior wails constructed of Western Red Cedar logs that are rounded on two sides with an
avenge thickness of 7.17 inches, and has 40 ft2 of interior log walls. It has 230 ft2 of glazing,
with g5 ft2 facing south. Two years of utility data were available for this house.
The Evans Mills home (NY1) shown in Figure 5 is four years old. It is all electric, and has 2159
ft2 of conditioned floor area including a conditioned basement. The house has 1367 ft2 of
exterior walls constructed of Western Red Cedar logs with an average thickness of 5.7 inches.
Total glazing area is of 180 ft2. There are no interior mass partitions. Two years of data were
available for this house.
The Southampton. Massachusetts, home (NY2) shown in Figure 6 is three years old. It has 2859
ft2 of conditioned floor area and part of the basement is also conditioned. The house has 1232 ft2
of exterior walls constructed of Eastern White Pine logs with an avenge thickness of 7.17 inches
and 161 ft2 of interior log wall. Glazing area totals 336 ft2. The house is heated with oil-fired
forced hot water with a system efficiency estimated at 80 percent. Two years of fuel use records
were available.
The Huntington. Massachusetts home (NY3) shown in Figure 7 has been occupied for one year.
It has 1159 ft2 of conditioned floor space. The basement is not conditioned. It has 1261 ft: of
exterior walls constructed of Eastern White Pine logs with average thickness of 7.17 inches and
296 ft2 of interior log partitions. There is 330 ft' of glazing area. The house is heated with oilfired forced hot water with an estimated system efficiency of 80 percent. One year of fuel dam
was available for this house.
The Milton home (NY4) shown in Figure 8 has been occupied for one year. It has 1932 ft: of
conditioned space over an unconditioned basement. It has 1500 ft2 of exterior walls constructed
of Eastern White Pine logs with an average thickness of 5.25 inches and has an interior log
partition area of 56 ft2. Total glazing area is 264 ft2. The house is heated with oil-fired forced
hot water with an estimated system efficiency of 76 percent. One year of fuel data was available
for this house.
RESULTS
Air Leakage Measurements
A prevalent concern with log homes is that the many joints in the log walls may result in higher
air infiltration rates compared to conventional homes. However, a recent study performed for the
Minnesota Department of Public Service, which surveyed the air leakage of 23 log homes using
infrared thermography and a blower door, found that most leakage was located at cathedral
ceilings, window and door frames, and the tops of the walls (Klossner, 1990).
Research Center staff performed fan depressurization tests on seven of the eight homes in this
study in accordance with ASTM E779-1987 "Method for Determining Air Leakage by Fan
Pressurization." The results of the air leakage measurements made on the seven homes indicate
relatively low air change values as shown in Table 2. House MT1 did not have a blower door test
performed on it because of a large open flue of a free standing fireplace. As a result, this home
had its natural air infiltration measured with the Research Center's AIMS system. This
measurement provided a measured air change rate of 0.18 for the one week period which was
adjusted for heating season avenge outdoor temperatures to an estimated average air change rate
of 0.35.
The estimated natural air infiltration rate for all homes ranged from 0.33 to 0.66 air changes per
hour (ACH) with an average of 0.43 ACH. The leakage ratios in Table 2 may be used to compare
the houses to one another. The leakage ratio is the estimated leakage area at 4 Pa per 100 ft2 of
conditioned surface area, which includes basement, wails if the basement is conditioned. The
data suggest that infiltration rates typical for conventionally insulated homes are readily
attainable in log home construction.
TABLE 2
RESULTS OF AIR LEAKAGE MEASUREMENTS
Site
ID
NY 1
NY2
N'Y3
NY4
MT1
MT2
MT3
MT4
Leakage Area (sq-in)
at 4 Pascals (Pa)
117
178
199
105
N/A
117
223
83
Leakage Ratio 4 Pa
(sq-in/100 sq-ft)
2.6
2.7
4.4
2.1
N/A
2.1
3.2
2.0
Air Infiltration Rate
(ACH) at 50 Pa
5.7.
8.1
10.5
6.3
N/A
6.2
9.6
6.8
Estimated Natural Air
Infiltration Rate (ACH)
.33
.40
.66
.40
.35
.39
.55
.34
Based on Research Center AIMS results and adjusted for heating season weather conditions
Energy Analysis Results
Energy predictions were made for each log home, as constructed, and for conventional wood-framed homes
of the same designs. Insulation levels in the conventional comparison homes complied with the standards of
the New York State (NYS) Energy Code. Table 3 shows the thermal properties of each log home as
calculated by Equations 1 through 4. Table 4 shows the U-values and R-values for the log homes and for the
conventionally insulated wood-framed homes complying with the NYS Energy Code. ASHRAE Handbook
(1989) values were used for building components other than logs. These values were used as inputs to the
CALPAS3 computer program.
The compliance of the conventional comparison homes was determined by building component
performance. For example, in a climate zone of 7000 HDD, the NYS Energy Code states that the wall
component for a non-electric heated home must have an overall U-value not exceeding .16 Btu/hr-fr2-°F,
while the U-value of an electric heated home must not exceed .11 Btu/hr-ft2-°F. The overall U-value
includes all opaque walls, glazing and doors. Therefore, a home with large glazing area will have a lower Uvalue (higher R-value) requirement for its opaque wall section compared to the same home with less glazing
area.
Also, homes that axe electrically heated must have lower U-values for their opaque wall sections compared
to homes of same design that use non-electric heal Three of the eight comparisons in Table 4 have
essentially equal roof and foundation insulation levels as their counterpart. The other five comparisons
shown in Table 4 indicate differences in values for the roof and foundation, with the largest difference for
foundations. All homes in New York must have foundation R-values of at least R-11. NY2 and NY3 in
Massachusetts did not have to insulate their foundation during construction. The NY4 home was ownerbuilt and is scheduled to have foundation insulation soon.
Table 5 shows the CALPAS3 energy predictions based on the thermal properties listed in Tables 3 and 4.
The predicted energy of the three log homes with equal roof and foundation insulation levels as their
counterpart (NY1, MT1 and MT2) compares well with the predicted energy of their conventionally
insulated counterpart. The different levels of foundation insulation appear to be a major factor in the cases
where the energy prediction for the conventional home is less than the prediction for the log home. These
differences in U-values between the building components of the log homes and their conventional
comparison homes confound the comparison of the energy predictions shown in Table 5.
TABLE 5
LOG HOME ENERGY PREDICTION RESULT
COMPARED TO CONVENTIONAL HOME ENERGY PREDICTION
Heating Energy Prediction (MBtu/yr.)
Conventionally
Site
NY1
NY2
NY3
NY4
MT1
MT2
MT3
MT4
Log Home
44,969
120,010
79,650
110,517
133,939
113,804
134,059
34,780
Insulate Home
49,484
96.995
60,151
83,759
133,703
122,707
129,082
33,166
A more appropriate comparison was made with equivalent component insulation levels, except
for wall components built of logs. Insulation levels in the roof, ceiling, and foundation of the
conventional homes were set to the same levels as exist in the log homes. This provides a basic
comparison between the log walls and wood-framed walls with insulation levels that comply
with the NYS Energy Code. The comparative wall insulation levels are shown in Table 6. The
large difference in R-values between the Montana log homes and their conventional counterparts
is primarily due to the large glazing areas which, for code compliance, necessitate a lower Uvalue for opaque wall sections. In addition, MT4 is heated by electric resistance which requires
lower opaque wall U-values to comply with code.
TABLE 6
INSULATION VALUES FOR LOG WALLS IN LOG HOMES AND WOOD-FRAMED WALLS IN
CONVENTIONAL HOMES
Log
U-value
R-value
Conventional
U-value
R-value
Percent Difference
in R-value Log to
Site
(Btu/hr-ft2-°F) (hr-ft2-oF/Btu)
NY 1
NY2
NY3
NY4
MTI
MT2
MT3
MT4
.11
.096
.096
.128
.151
.15 1
.108
.0895
(Btu/hr. ft2-°F.)
9.1
10.4
10.4
7.8
6.6
6.6
9.3
11.2
(hr-ft2-ft2/Btu)
.072
.077
.09
.077
.052
.074
.052
.042
Conventional (%)
13.9
13.0
11.1
13.0
19.2
13.6
19.2
23.8
35
20
6
40
66
52
52
53
Previous computer simulations by other researchers have indicated significant benefits of thermal
mass. Results presented here support those earlier findings. As shown in Table 7 and illustrated
in Figure 9, the CALPAS3 simulations indicated less energy consumption for seven of the eight
log homes than for the conventionally insulated homes despite large differences in conventional
wall insulation levels. The data indicate that MT1 will require approximately equal amounts of
heating energy as its conventional counterpart even with a wall R-value level 66 percent less than
its conventional counterpart (R-6.6 versus R-19.2). Not shown in the tables is that the
conventional counterpart's overall UA-value (Btu/hr-°F) is 15 percent less than the overall UA
for MT1. This suggests that the log home, with 15 percent less overall insulation, can heat its space with an
equal amount of fuel as the conventional home.
TABLE 7
ENERGY PREDICTIONS FOR
EQUIVALENT INSULATION LEVELS
EXCEPT WALL COMPONENTS COMPARED TO NORMALIZED ANNUAL CONSUMPTION (NAC)
Site
NYI
NY2
NY3
NY4
MT1
MT2
M'I3
MT4
Annual Heating
Energy Prediction
(MBtu)
Log Home
Conventionally
Insulated Home
44,969
120,010
79,650
110,5 17
133,939
113,800
134,059
34,780
48.574
127,073
88,046
115,133
133,211
122,602
136,833
36,760
NAC
Heating
(MBtu)
BPI
(Btu/ft2- HDD)
Log Home
49,724
120,480
69,800
111,300
154,066
100, 145
115,530
39,545
3.3
6.0
8.6
8.2
5.6
6.1
3.9
3.4
Thousand Btu
Table 7 also indicates the NAC estimates based on utility/fuel data and the building performance index
(BPI) which is a normalization of the NAC to heating degree days and conditioned floor area. The
difference in the NAC to energy predictions of the log homes ranges from -16 percent to +13 percent.' The
BPI index allows homes to be compared to each other, lower BPI indices indicate a more efficient
home. The two homes with the highest BPI have unconditioned basements, which reduces the conditioned
floor area, and NY3 had the highest infiltration estimate.
CONCLUSIONS
The energy efficiency of eight log homes in this study compares well with conventionally
insulated wood-framed homes. Computer simulations indicated that the log homes use equal or
less heating energy as their conventionally insulated counterparts and that log mass is a
significant benefit. Analysis of fuel consumption data resulted in normalized annual heating
consumption estimates that are at least as efficient as conventionally insulated counterparts.
These results suggest that log homes may alternate between steady-state and dynamic conditions
and would not, therefore, only approach a steady-state condition over a heating season. This
implies that a log home would provide energy storage benefits over a heating season. Homes in
the Montana region had large glazing areas oriented to the south, which increase the solar gains
to the interior. These gains can be absorbed by interior and exterior log walls, which can have a
significant affect on the dynamic behavior of the thermal mass. Homes in the New York region
had glazing areas more typical of the average new home, yet may still have significant internal
gains to affect the dynamic response of the thermal mass.
RECOMMENDATIONS FOR FUTURE RESEARCH
This study on eight log homes in two northern regions is necessarily limited. The log home
industry should pursue research to document the actual energy consumption of log homes as
compared to the actual energy consumption of conventional wood-frame homes. PP, JSM can
provide reliable estimates of normal annual consumption, and can be renormalized to heating
degree days and conditioned floor area for cross comparisons. Long-term air infiltration
measurements can be made by the Research Center's AIMS Laboratory to provide avenge heating
season air change rates. One-time measurements of heating system efficiencies would allow
loads attributable to the building envelope to be separated from the estimated NAC, The criterion
for excluding wood stoves from log homes could be relaxed to allow more representative log
homes in future studies, although the NAC estimates for wood-burning homes may be less well
determined.