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 between 1981 and 1982. Energy consumption of each structure was precisely recorded by NBS technicians during this entire period.
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 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 Washington, DC).
Construction Details of Walls
An insulated wood frame home, nominal R-12 (without mass) with 5/8″ exterior wood siding, 2×4″ 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 insulation.
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 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 state energy office. You can also contact the U. S. Department of Energy’s Building Standards Hotline: (800) 270-CODE (2633)
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 efficiency problems.
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 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.
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 reasons:
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 inch.
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 years!
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, landscaping, etc.)
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 Canada).