The National Bureau of Standards conducted a study to determine the relationship of a structure’s thermal mass to its energy consumption. The structures compared were a 7” solid wood log (nominal R-10) and an insulated wood frame with 3 ½” fiberglass insulation (nominal R-12). The results were:
Spring heating period - log structure used 46% less heating energy
Summer cooling period – log structure used 24% less cooling energy
Winter heating period – both structures used same amount of heating energy
The conclusion was that the ‘thermal mass’ of logs is an energy conserving feature.
Despite a lower R value, logs are more energy efficient than the wood frame/insulation structure. So what exactly is the R value? It is a measure of a material’s ability to impede heat flow. What is thermal mass? It is the capacity of a material to store thermal energy for extended periods, i.e., ‘heat capacity’ rather than heat impedance. Material with high thermal mass can absorb daytime heat and release it during the night. It serves to ‘flatten out’ the daily temperature fluctuations.