Research Report on Energy Efficiency of Log Home Buildings


Research Report on Energy Efficiency of Log Home Buildings

Credit: Bitterroot Valley Log & Timber
2036 US Hwy 93 N. - Victor, MT 59875  (406) 642-3091


Margaret Lowe, in her article entitled "Myths and Truths of Log Home Ownership" addresses the efficiency of Log Homes.

She wrote: "Myth #1: Log homes are not energy-efficient: FALSE, in capital letters. Early in this decade, the nation's model energy code finally recognized what the log home industry had claimed for 20 years -- that a log wall's thermal mass makes it as energy-efficient as a well-insulated wall. This claim wasn't acknowledged during all those years because thermal mass is difficult to quantify. Log home owners had the heating bills to prove it was true. but the Department of Energy and national code officials needed more than empirical evidence. So for 13 years the Log Homes Council ( a division of the National Association of Home Builders in Washington, DC) gathered scientific statistics from independent research projects to substantiate its claim.

"At the heart of the debate were R-Values, the measure of heat transfer through materials. When the energy crisis struck in the mid 1970's, state and federal governments had to quickly develop new energy-performance standards for residential construction and all the building materials used in that construction. Since the situation was a crisis, and the R-value methodology already existed, it became the standard, no questions asked.

"R-value measures a material's resistance of heat from one side to another. Log's have a relatively low resistance to heat transfer. In fact, they actually absorb and store heat in their cellular structure. This puts them at a serious disadvantage in the cold weather states. Producers had to overbuild their houses in order to meet total R-value requirements. This not only drove up construction costs, it also created a lot of confusion.

"The opposite of R-value, thermal mass, measures a material's capacity to absorb, store, and slowly release heat over time when temperatures drop. Logs do this very well because of their cellular structure, bulk, and thickness. The problem, was proving it.

"An early breakthrough came in studies conducted by the National Institute of Standards for HUD in 1981-82. The studies proved thermal mass does significantly reduce heating and cooling loads in moderate climates. however, energy experts continued to question the value of thermal mass during the winter months in northern climates where heat is a constant need (or during summer months in southern climates where cooling is in need) and thermostat settings are opposite outdoor temperatures.

"Two more recent studies, both conducted in cold climate states, proved the log home industry was right.

"In 1990, an independent testing agency, Advanced Certified Thermography, conducted a study for the Energy Division of the Minnesota Department of Public Service. Its focus was heat loss through air leakage, which was assumed to be a special problem with log homes because of their many joints. The study found the industry's improved joint construction and its use of expanded foam sealants and gaskets at joints and corner intersections had substantially reduced air-infiltration rates. The study concluded air leakage in a well-built, modern log home is not due to its log walls. In fact, in the 23 homes studied, it found air leakage occurs in the same places it occurs in conventional frame homes: at the top of cathedral ceilings, around window and door frames, and along the tops of walls where they join the roof.

"A second study, conducted in 1991 for the Log Homes Council by the National Association of Home Builders (NAHB) National Research Center, discovered the thermal mass of log walls does significantly reduce energy use for heating in cold climates. It based its conclusions on a comparison of actual energy use in eight log homes to the actual energy use of eight well-insulated frame houses during one winter. The 16 homes were evenly divided between upstate New York and Montana. The study also compared the homes' actual energy use to their predicted energy consumption. The results led to the conclusion that log homes were as energy-efficient as well-insulated frame houses. What is especially significant about this study is that the average R-value of the log walls was 44 percent lower than the average R-value of the frame walls. Obviously, the thermal mass performance of log walls is an advantage to log home owners."

According to an article by Peter M Hart of New England Log Homes: "the oft-cited R-value (resistance) factor is meant to be a measure of heating efficiency but actually, that concept is both inaccurate and misleading. As an example, an illusion is created to convey to the homebuyer that if three inches of fiberglass is good, then six inches is twice as good and naturally, 12 inches is four times better.

"NOT SO! After a certain amount of insulation has reached optimum, the overage of insulation is waste and does not justify the additional cost.

"To arrive at the R-value of a particular type of insulation, heat is passed through the material by conduction, under fixed temperatures and only when the material is completely dry. No consideration is given to convection, radiation loss, solar input, heat and storage capabilities and the influence of moisture.

"For instance, a test of fiberglass sample rated R-13 when dry is reduced to R-8.3 when conditions reach a moisture content of 1.5% -- a loss of 40% insulation efficiency."

These R-value standards were determined under unrealistic conditions due to the fact that we all live in a humidity laden atmosphere. This humidity level is compounded in the home environment.