Energy efficiency became an issue during the energy crisis of the early 1970s.  State and federal governments quickly developed new energy standards for residential construction.  To assure compliance, officials needed a way to quantify the energy performance of all residential building materials.  Since the situation was a crisis and R-value methodology existed, it became the standard  expression of the ability of insulated products to retain heat.  The problem is that R ratings benefit insulation products but don’t reflect actual performance.

     R-value measures a material’s resistance to the transfer of heat from one side to another.  Logs have a relatively low resistance to heat transfer.  In fact, they actually absorb and store heat in their cellular structure.  This put logs at a serious disadvantage in the cold winter states.  It forced log-home producers to overbuild their homes, especially their floor, window and roof systems, to meet total R-value requirements.  This drove up construction costs without any measurable benefit to buyers.

     Instead of R values, log homes rely on the time-honored principle of thermal mass.  Thermal mass is a material’s capacity to absorb, store and slowly release heat over time.  Logs do this well.  They have thermal mass because of their cellular structure, bulk and thickness.  What’s more, this thermal mass provides significant energy-savings benefits because it releases heat back into the house when temperatures drop.

     While the claim is true, it wasn’t officially acknowledged, in part because thermal mass is difficult to quantify.  Log-home owners had the home heating bills to prove it was true, but Department of Energy and code officials needed more than empirical evidence.  So, the Log Homes Council, a division of the National Association of Home Builders, spent years gathering scientific evidence from independent research projects to verify the effectiveness of thermal mass. 

     Early studies of log homes proved thermal mass properties significantly reduce heating and cooling loads in moderate climates.  The National Institute of Standards conducted the most important of these studies for the Department of Housing and Urban Development in 1981-82.  However, energy experts continued to question the value of thermal mass during the winter months in northern climates.  They doubted its benefits when heat is needed constantly and thermostat settings are opposite outdoor temperature.

     At last, two studies, both conducted in cold-climate states, demonstrated the energy efficiency of thermal mass in colder climates.  In 1990, an independent testing agency, Advanced Certified Thermography, conducted a study for the Energy Division of the Minnesota Department of Public Service.  It focused on heat loss through air leakage, assumed to be a problem with log walls because of their many joints.  The study found that the producers of log homes have substantially reduced air-infiltration rates.  It credited this reduction to improved joint construction and the use of expanded foam sealants and gaskets on all joints and corner intersections.

     Leakage in the study’s 23 test homes occurred in the same places as it does in frame houses: at the peak of the cathedral ceilings, around window and door frames and along the tops of walls.  The study concluded that air leakage in well-built, modern log homes is not due to their log walls.

     The NAHB’S Research Center conducted the second study for the Log Homes Council in 1991.  It showed that the thermal mass of log walls does significantly reduce energy use for heating in cold climates.  It drew its conclusion by comparing the actual energy use of eight log homes to the actual energy uses of eight well-insulated frame houses during one winter.  The number of houses was 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 the frame houses. 

     What is especially significant is that the log walls’ average R-values were 44 percent lower than those of the frame walls.  Clearly, the thermal mass performance of log walls is an advantage to log-home owners.  They don’t have to invest in additional building materials to meet codes that do not improve the livability of their homes. This conclusion also makes life easier for log-home producers.  They no longer have to fight energy codes based on R-values.

     Yes, wood doesn’t stack up well against manufactured insulation when judged by R rating alone.  Thanks to thermal mass, however, solid wood equals or exceeds the performance of insulation.

     One final point about R values.  A thin sheet of insulation material, no matter how high the R value, provides little load-bearing support.  Look at some of the homes being built with matchsticks wrapped in Tyvek, and the first thought that comes to mind is “tar-paper shack.”  Logs provide insulation plus support.  There’s something reassuring about having walls that are 8 inches thick.

     The place to boost insulation is the roof since, as everyone knows, warm air rises.  This fact explains why more and more log homes are using foam-core insulated panels for roofs.  These panels allow for the high open ceilings so popular in today’s log homes, while providing very high insulation ratings.  Some panels exceed R-30.  By insulting your roof and making sure that it, your windows and doors fit snugly against your logs, you will ensure energy efficiency that will exceed that of conventional homes-plus provide the romance and reassurance of solid wood

     The desire to establish a reasonable fire rating for solid wood walls included a search for existing methodology that could be logically applied.  The research effort uncovered formulas for determining fire resistance for beams and columns published by the American Institute of Timber Construction, American Forest and Paper Association, CABO National Evaluation Service and adopted into each of the building codes.

     Given that the standard only provides guidelines for columns [exposed on four sides] and beams [exposed on three sides], the approach to calculate a fire rating for a log wall began with the conservative equation for beams   Based on the research data on timbers and fore testing, the Log Homes Council contends that the fire-resistive ratings of solid wood walls can be determined on the basis of the minimum thickness of the wall.  As the log wall thickness increases, so does the fire rating of the log wall assembly. 

     The effort of the Log Hoes Council to gain recognition of solid wood walls as a fire-resistive construction culminated in 1996 with the acceptance of log wall construction in the Urban-Wildland Interface Fire Code of the ICC.  This code defines log-wall construction as a type of construction in which exterior wall are constructed of solid-wood members and where the smallest horizontal dimension of each solid wood member is at least 6 inches. 

     Over the years, there have been many reports of fires that have burned inside and outside of log buildings without destroying the building’s structural integrity, illustrating the fire resistive nature of solid wood walls.  It is a combination of the  insulating response of the charred wood at the surface with the slow rate at which flame will spread along the wood surface, and the fact that there are no concealed cavities in a log wall through which the fire may travel[ultimate fire-blocking!].  Combined with the selection of beam and deck second floor and roof options often incorporated into log buildings, log structures are a top choice for endurance and integrity in a fire.

     The unique aspect of most log homes is that the logs themselves are what make up the structural soundness of the building.  Because of this, most solid wood walls can be expected to satisfy one-hour fire-resistant ratings while greater wall thickness can result in even longer ratings.  So, you see, log homes not only are not energy inefficient and flammable, but they also are more efficient and less likely to suffer structural fire damage than conventionally built homes.  The evidence is impressive, but as anyone who owns a log home will testify, science is no match for actually living in one of these safe, snug and beautiful homes.