In colder climates, keeping heating costs as low as possible is an important consideration for those planning to build a new home. While there are many factors that influence heating costs, most often prospective log home buyers express concern about the insulating quality (R factor) of the walls. However, there is data that suggests that the R factor of the walls may be one of the least important factors to consider. For example..
- Tests have shown that at least 97% of heating costs are not related to wall construction.
- A log wall can reduce cold air infiltration, which can account for up to 1/3 of of heat loss in a frame home, by 40% or more.
- In a cold climate, a frame building will theoretically require 15% more energy to heat than an equivalent log building.
- Only 1% added moisture will reduce the R factor of fiberglass or cellulose insulation by 50%.
There are five major contributors to the cost of heating a frame home (see chart below); The walls are the least important source of heat loss. With the measured efficiency of many furnaces at 50%, most of the energy used to heat a home effectively goes up the chimney. This means that the first priority should be the selection of a high efficiency furnace or heating system. Next in importance is air infiltration, much of which is the result of essential venting. The remaining infiltration can be reduced significantly, as is indicated below. Log walls reduce infiltration of cold air. When researchers at the National Bureau of Standards measured air infiltration of buildings that were identical except for the wall construction, they found that the log wall was significantly better than frame or masonry block walls. When this data is combined with the data above, it is obvious that a log wall had the potential to significantly reduce one of the major contributors to heating costs. While the NBS tests did not identify the exact factors which made a log wall superior, common sense suggests there will be less infiltration if there is a high quality sealing of the interface between log surfaces. Wall mass reduces energy consumption. Dr. F. Arumi-Noe’ of the school of Architecture at the University of Texas has developed a computer model that demonstrates heating costs based on time-dependent (dynamic) factors, thus including the effect of the mass (or thermal inertia) of a wall. As Dr. Arumi-Noe’ states, “Consideration of the thermal inertia of walls takes into account the fact the walls neither heat up nor cool down instantaneously…These effects..result always in a net reduction of energy consumption…” The results of this model, when applied to a home in the Ottawa, Ontario are, result in a log home using 15% less energy than a frame home over a 60 year life cycle. What is the real R-factor of a frame wall? The R factor of a material is measured under ideal conditions in a laboratory. There is no guarantee that the actual R factor after installation will not be significantly less. In addition to exposure to moisture during shipment and on-site storage, dust saturation, compacting, material settling and construction defects (unintended empty spaces) also contribute to breakdown of insulation value. So it is quite likely that the actual R factor of the insulation used in a frame wall is substantially less than the theoretical value.
As quoted in The Energy Economics and Thermal Performance of Log Houses, by Doris Muir and Paul Osborne, Muir Publishing Co. LTD, 1982. The material contained herein is/has been largely extracted from this report, copies of which can be obtained from the Log Home Guide Information Center, 164 Middle Creek Road, Cosby, TN 37722.