Modern kit log home on the Susquehanna in PA Log Home R-Values
Thermal Mass, & Wall Mass Effect on the Energy Costs & R-Values of Log Homes

  • R-VALUES & THERMAL MASS in LOG HOMES - CONTENTS: R-Values of Log Homes & Solid Log Walls. Thermal Wall Mass Study in Log, Masonry, & Wood Frame Homes, 1982. Energy Efficiency of Log Homes. Effects of solid log construction thermal mass on log home heating and cooling costs. Field Study of the Effect on Wall Mass on the Heating and Cooling Loads of Residential Buildings
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Log Home R-Values & Thermal Mass:

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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Heating, Cooling, and Insulation R-Values & Characteristics of Log Homes

Log home in Pennsylvania (C) Daniel FriedmanR-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.

[Click to enlarge any image]

Our photo 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?

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.


Log Home Thermal Mass Effects on Heating, Cooling, & Comfort

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, [PDF] 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:

Six test buildings were extensively instrumented for measuring heating and cooling loads, wall heat transmission, and indoor temperature and humidity. During these measurements, the effect of wall mass on heating and cooling loads was observed. these buildings were exposed to a winter heating season, an intermediate heating season, and a summer cooling season.

The test buildings were 20 x 20 ft. (6.1 x 6.1 m) one room buildings constructed at Gaithersburg, MD. These buildings had the same floor plan and orientation. They were identical, except for the wall construction, which was as follows: insulated lightweight wood frame; un insulated lightweight wood frame; insulated masonry with outside mass; uninsulated masonry; log; and insulated masonry with inside mass;

The insulated buildings, including the log building were designed to have walls of approximately equivalent steady-state thermal resistance; the uninsulated buildings were also designed to have walls of approximately equivalent steady-state thermal resistance.

No reductions in heating energy attributable to wall mass were observed during the winter heating season, when the buildings typically did not float (i.e. some energy was applied each hour). However, during the intermediate heating season and the summer heating season, when the buildings floated during a portion of the day (i.e. no heating or cooling load occurred during a portion of the day and the indoor temperature rose above, or fell below the indoor set temperature), significant reductions in load attributable to wall mass were observed. Wall mass was observed to have a larger effect when it was placed inside the wall insulation as opposed to outside the wall insulation.

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:

  1. insulated lightweight wood frame wall structure
  2. un insulated lightweight wood frame wall structure
  3. insulated masonry wall with outside mass
  4. uninsulated masonry wall
  5. log wall construction
  6. insulated masonry wall with inside mass

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.

Wall Mass Effects on Cooling Load (C)

[Click to enlarge any image]

Additional excerpts from the original article include:

Energy consumption for residential space heating and space cooling represents about 12 percent of the total energy required in the United states. With the advent of fuel shortages which have produced spiraling energy costs, much attention has been focused on strategies for reducing energy consumption in residential buildings.

A strategy, which is the subject of this paper, deals with the effect of wall mass on the heating and cooling loads of residential buildings.

The effect of wall mass may be illustrated by considering a residential building exposed to an outdoor condition for which the outdoor temperature approaches the balance point (float zone) for the building. If the heating/cooling plant is turned off, the indoor temperature will fluctuate in response to the outdoor diurnal temperature variation. The building envelope will provide a reduction in the amplitude of the diurnal outdoor temperature wave form.

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.

Wall Mass Effects on Cooling Load (C)

The 1982 thermal mass study continues to point out that

... the amplitude reduction will be considerably greater for the masonry [or solid log] residence than for the wood-frame residence, owing to the large heat capacity of the masonry [or log] material.

Therefore, if high and low thermostat set points are established for space cooling and space heating, the masonry [or log] building will have considerably smaller indoor temperature excursions above and below the high and low set points, thereby causing its heating and cooling energy [consumption] to be smaller than that for the wood-frame residence.

Warnings about The Log Home Report Conclusions

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:

The effect of thermal wall mass has been shown to be climate dependent. [Study references 1, 2, 3, and 5]. Therefore, the test results of the [1982] study should not be directly extended to other climates.

Perhaps it is equally important is the fact that the test results should not be extended to a real house situation for the following reasons.

  1. The test buildings were one-room test cells which did not contain interior partition walls and interior furnishings. The addition of interior partition walls and interior furnishings would have added considerable interior mass which would have affected the observed results; and
  2. Heat transmission through the walls of the test buildings was a larger part of the overall envelope heat transfer compared to a typical house due to high thermal; resistance in other components of the building envelopes (i.e. the ceilings contained R-34 (R-6.0 m2.K/W) glass-fiber insulation, the windows contained triple-glazing, the floor slabs were insulated over the top with R-11.2 (R-1.97 m2.K/W) polystyrene insulation, and the air infiltration rates were quite small.

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.


Continue reading at ENERGY EFFICIENCY of LOG HOMES where we introduce the R-value of solid wood, log home air leaks, and the thermal mass of log homes, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.


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