Log home manufacturers can also customize their designs. 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. 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. 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.