For nearly 30 years, the log-home industry has invested heavily in the concept of thermal mass to explain the warmth and comfort experienced by log-home owners. Thermal mass (the ability of a material to absorb, store and later release heat) has blended into the energy codes, and those codes continue to change in a political environment.
Log-home owners have proclaimed the warmth and comfort of their homes for as long as anyone can remember. An Internet search revealed a report comparing 17th-century life in Colonial America that indicated Swedish settlers in Delaware had a healthy, snug lifestyle in their log buildings, while the Dutch and English suffered illnesses in their framed dwellings.
Granted, these log homes were smaller in size and had fewer windows and doors than today’s log homes, but warmth and comfort have also been noted in the larger lodges and outbuildings of national parks built in the 1900s.
Environmental awareness in the 1960s, coupled with the Oil Embargo of the 1970s, changed building codes and enforcement of those codes forever. Evolving energy-conservation standards were a factor in the formation of the Log Homes Council, now a member of the Building Systems Councils of the National Association of Home Builders. These standards focused on limiting heat loss from buildings by prescribing minimum insulation levels, an approach that continues today. The standards emerged as the Model Energy Code (MEC) in 1983. That’s the same year that the National Bureau of Standards (NBS) tests documented the effect of thermal mass by comparing the performance of six buildings whose only difference was the wall assembly (wood-frame with and without insulation in the cavity; masonry without any insulation, insulated on the interior and insulated on the exterior; and a log wall).
The tests demonstrated that thermal mass does have a tempering effect on the enclosed air space, moderating change in the interior as outside temperatures change. The report shows that the 7-inch-thick log wall outperformed the 2-by-4, R-11 insulated wall. It also demonstrated that when insulation was added to the mass wall assembly, it was most beneficial when added to the exterior.
Based on the testing, it became evident that solid-wood walls could be analyzed in terms of both their static R-value and thermal mass. The interesting thing is that a wood species that has a good R-value is one that has less density (lower weight, less wood fiber and more trapped air); hence, it takes a thicker wall to establish thermal mass. And the inverse is true, that a dense wood species will demonstrate thermal mass with a thinner wall, but its R-value is not as high. With R-value controlling the energy code requirements, a new “dynamic R” measure was needed to represent log-wall performance.
Much more detail on the new energy codes was published in the magazine. The energy-code changes seem to be getting closer to what log-home owners have been saying. All log homes have warmth from natural resistance to heat loss, radiant comfort from thermal mass and further reduction of energy consumption by low air-infiltration rates.
Rob Pickett of Rob Pickett and Associates (www.robpickettandassoc.com), a housing consulting firm in Hartland, Vermont, is a specialist in log and timber building systems and a past president of the Log Homes Council. Rob is the business manager of TimberLogic LLC (www.timberlogic.com) and oversees thermal analysis (REScheck), construction documents and code compliance. Submit questions for this column to firstname.lastname@example.org.