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This article explains the effects of log home thermal mass on heating and cooling costs. We give R-values for solid log walls, we compare solid log walls to log-slab-sided wood frame wall construction, and we cite expert research on thermal mass in log-constructed buildings.
This series of articles provides information on the heating & cooling characteristics of solid log home construction. We include illustrations of log structures from several very different areas and climates in both the United States and Norway. Our page top photo shows a modern solid log home in Pennsylvania.
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R-Values for wooden log walls given by the U.S. DOE are in error except for square log walls. D-logs and round logs that are given a nominal log thickness, say 6" logs are calculated by DOE as having an R-value of just over 8. This is incorrect for non-square logs because the cross section of the log is 6" only at the log's widest point.
A correct assessment of the R-value of a wooden log wall needs to be calculated based on the average wall thickness, considering the variation in thicknesses over the curvature of the logs. Therefore the DOE's value is on the "high" end of the R-value of a log wall.
Slab-sided log homes that use conventional stud framing for walls will have about the same R-value as other wood-framed buildings of similar construction, plus the added value of the average thickness of the slab siding.
Air leaks in log homes (or in any home) will have a significant, possibly dominant effect on the home's heating and cooling costs. See Minimizing Air Leakage in Log Homes.
Our photo (left) shows an older solid-log home in Pennsylvania under winter conditions. Notice the absence of snow on the roof and the large icicles? Where is the most heat loss in this log home? What are the effects of the thermal mass of this solid log home on its heating (or summer cooling) costs?
The effects on heating efficiency and costs of the thermal mass of log walls on a solid log home not a new question, though the debate continues.
A widely-cited study of interest to log home owners and builders studying the energy characteristics of log buildings was completed in 1982: A Field Study of the Effect on Wall Mass on the Heating and Cooling Loads of Residential Buildings, D.M. Burch, W.E. Remmert, D.F. Krintz, and C.S. Barnes, National Bureau of Standards.
The study was undertaken in response to rising energy costs and an ongoing search for ways to improve building energy efficiency in the 1980's - conditions that are appropriate in today's climate of rising energy costs as well. The study also was undertaken to test the results of non-empirical computer modeling of the effects of thermal mass on building energy consumption. Those computer modeling results had indicated that the effects of thermal mass on energy consumption would be small. From the document abstract:
The two bar graphs below, also from this study, show the effects of thermal mass on the test chamber intermediate heating and summer cooling season loads. Buildings 1-6 are listed left-to-right in these charts in order 1, 3, 5, 6, 2, 4 and are identified as:
As we point out at Warnings below, readers should not assume that these results translate directly to a real home with interior partitions and furnishings.
[Click to enlarge any image]
Additional excerpts from the original article include:
What this means in less scientific language is that in an amount that varies depending on their thermal mass, the walls of a home, such as a log home or a solid masonry home, will reduce the extent of temperature swings inside the building during intermediate heating seasons and during the cooling season. Another way to view this observation is that energy is stored in the thermal mass of the walls and returned to the interior as heat during the intermediate heating season, or during the cooling season, the thermal mass of walls may absorb some interior heat, cooling the building interior. At least, for a while.
The 1982 thermal mass study continues to point out that
The authors conducting this Field Study of the Effect on Wall Mass on the Heating and Cooling Loads of Residential Buildings ("The Log Home Report") included careful consideration of the effects of solar gain, air infiltration or exfiltration, and similar considerations that would otherwise have confounded the study results, especially when comparing such different building wall construction methods as wood frame, masonry, and solid logs.
And sound mathematical techniques and instrumentation were used in the study methodology. But the study results do not translate directly to actual furnished residential structures, as the authors explained in some cautionary notices:
Watch out: The study, referred to by some sources as "the Log Home Report" also includes important cautions that should not be ignored:
We interpret the study and the author's cautions to mean that while there is an effect of building thermal mass on building heating or cooling loads during some seasons (intermediate heating season and summer cooling season), the actual effects of thermal wall mass likely to be observed in a real, furnished, residential structure, will be different, and possibly significantly less for normal buildings.
OPINION: the benefits of increased thermal mass on building energy costs has been discussed and is demonstrated in energy-efficient building designs such as passive solar heating, but to state the portion of energy savings attributed to thermal wall mass alone in all structures requires careful study.
We discuss the effects of thermal mass in energy-efficient building design at PASSIVE SOLAR HEAT PERFORMANCE, at PASSIVE SOLAR HOME, LOW COST, and in another example at BLOCKBED RADIANT FLOORS - SOLAR DESIGN. For a further understanding of the role of thermal mass in energy efficient buildings, see the thermal mass links provided below.
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Frequently Asked Questions (FAQs) about log home wall thermal mass versus log wall R-values
Questions & answers or comments about the R-values and heating or cooling characteristics of log construction: log home energy efficiency, thermal mass, and the effects of thermal mass on heating and cooling costs.
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