The Energy Efficiency of Log Homes
by Bill Kolida - June 1999
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).
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.