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How Efficient Are Log Homes

National Bureau of Standards Test 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. 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. 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 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) area."
 

The Use of R-Values
Most state, provincial 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 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. 

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, 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.

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 11% 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 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 and is entitled "A Field Study of the Effect on Wall Mass on the Heating and Cooling Loads of Residential Buildings." 

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.

Log Homes and Energy Efficiency

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 provincial or state energy office. 

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;
    insulation levels;
    heat storage mass inside the building; and
    the local climate.


The Energy Efficiency of Log Homes

by Bill Kolida

Bill Kolida is a North American log home regulatory specialist. In the 90's, 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 his involvement in home building, there are two truths he has come to learn:

1. People who own log homes love them, and do not complain about energy efficiency problems

2. People who live in conventional frame housing wished they owned a log home.

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 cavity.
    the ability of the wall to block air transfer from inside to out.
    the ability of the wall system to store heat and radiate it back later.