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Log Homes & the Great R-Value Myth

By Clint Gray

One of the most confusing issues a log home buyer faces concerns the energy efficiency of log homes. All too often, questions are met with an oversimplified opinion or confusing technical jargon. Even when inquiring of building professionals, few seem to have answers concerning log home energy efficiency.

Most of the confusion stems from a lack of easily obtainable scientific data, unfamiliarity with the subject, and the seemingly complex nature of the question. Although it doesn’t take an engineering degree to understand energy efficiency issues, a basic understanding of energy dynamics goes a long way towards clearing up the confusion.

Drawing a correlation between a log home and a conventional home helps bring focus to the issues. What is it exactly that makes a log home different than a conventional home? A log home is a conventional home with solid timber exterior walls - period! All of the other components used to build a log home (including the foundation, floor system, interior partitions, windows doors, plumbing electrical, HVAC and roof system) are comprised of conventional materials, built in conventional fashion by conventional tradesmen.

Energy efficiency in conventional construction is easy to rate since it is always measured by the the R-value system. R-values are easy to establish because insulation manufacturers provide a rating with each batt of fiberglass insulation that is put into a conventionally framed wall or roof system. Put an R-13 batt of insulation in a frame wall, bingo, an R-13 rated wall. The basic difference between a log home and a conventional home lies in the exterior wall system. That's why all the controversy surrounding log home energy efficiency concerns how exterior log walls are rated.

In its simpliest terms, R-value measures a materials resistance to the transfer of heat from one side to the other. This system of rating seems to work fairly well for the conventional insulation materials it was designed to measure. But how well suited is it for wood, and does the way you go about rating that log wall reveal the truth about its ability to provide energy efficiency? When tested in laboratory conditions, using the R-value system, logs score an average rating of only about R-1 per inch of wood. This poor rating is due to the fact that the cellular structure of wood actually absorbs and stores heat. This characteristic puts logs at a great disadvantage when tested by the R-value system.

The R-value rating system came into widespread use when the energy crisis arrived in the 1970's. The federal government put local code enforcement agencies under compulsion to impose a system of energy standards upon the construction industry. To assure compliance, a method of measuring a building's energy performance was needed. Since the R-value system already existed, was easy to apply, and the situation was viewed in terms of a national emergency, it was quickly adopted as the new standard for energy measurement. The imposition of these new regulations meant that many new log homes would no longer be in compliance with code. In many areas, log home buyers were denied mortgages or building permits based upon the log homes' low R-value. A direct result of this system was that many manufacturers, builders, and home owners were forced to overbuild their log homes to meet total R-value requirements. This situation had the effect of driving up construction costs without any measurable benefits or added livability for the homeowner.

Long prior to the 1970's, log homes had enjoyed a reputation as being energy efficient. Countless log home owners had reported finding their energy bills equal to or lower than those of their neighbors living in conventional homes. The Log Home industry had long recognized an unseen energy efficient facet of log walls known as thermal mass. Thermal mass is a material's ability to absorb, store and release heat back into a structure over time. Thermal mass reduces energy consumption by releasing stored heat back into the building as the outside temperture drops.

Throughout the late 1970's and into the 1980's the log home industry lobbyed to get recognition of the thermal mass effect of log walls. Recognizing the R-value sytem as a limited form of energy measurement, their strategy centered around a call for energy testing and hearings to challenge the sole importance of R-values in establishing energy code requirements.

In 1981-82, the National Institute of Standards¹. conducted a series of tests at its facility outside of Washington DC for the Dept. of Housing and Urban Development. The tests were conducted over the course of seven months using six 20' X 20' buildings that were identical except for the construction of the exterior walls, one of which was log. The test houses were maintained at the same temperature levels throughout the test period and the energy consumption was closely monitored.

The test summary proved that the log home (rated as an R-10 wall) performed as well as the insulated wood frame house (R-12 rated wall) during the winter heating period. The log home consumed 24% less energy during the summer cooling period and 46% less during the transitional spring/fall heating period. It is important to note that the log home was rated at 17% less R-value than the frame house in this test.

These test results raise concerns over the integrity of fiberglass as an insulator. If the R-value rating system hides the inherent value of solid timber and its thermal mass effect, could it also cloak some inherent deficiencies in fiberglass? To determine the R-value of an insulator, heat is passed through the material by thermal conduction. This conduction is done using fixed temperatures when the material is completely dry. No consideration is given to real world exposure conditions such as convection, radiation loss, solar input, thermal mass capabilities, and, most importantly, the influence of moisture. When as little as 1% moisture content is added to fiberglass, its effective R-value drops by as much as 50%. In the real world, a home is bombarded with moisture from within and without both during the construction process and during habitation of the home. Fiberglass insulation is not nearly as effective as its ideal laboratory rating when exposed to the real world. Log walls on the other hand, with a closed cellular structure, have non-depreciating R-values and will retain thermal mass capabilities and energy efficiency throughout the life of the home.

The National Institute of Standard's testing scored a great victory for the Log Home industry and Log Home buyers by establishing the thermal mass capabilities of log homes. As a direct result, log homes were once again included in federally backed mortgage lending programs. In spite of these test results, energy experts continued to question the value of thermal mass during the lengthy winter months of nothern climates. Further concerns were raised over the question of whether or not the many joints in a log home's walls contributed to heat loss through air infiltration.

In 1990, Advanced Certified Thermography, an independent testing agency, conducted a study for the Energy Division of the Minnesota Dept. of Public Service². Using 23 log homes for testing, the study focused on the issue of heat loss through air infiltration. The results of the test concluded that air infiltration in log homes generally occurs in the same places it does in conventional homes, along the tops of walls, around window and door penetrations and at the peaks of the roofs. These tests concluded that air infiltration in modern log homes is NOT due to the joints in their log walls.

In 1991, the National Association of Home Builders Research Center conducted a study for the Log Home Council to determine whether or not the thermal mass of a log home's walls could in fact reduce the use of energy for heating in the lengthy winters of northern climates. Eight log homes and eight well insulated homes were evenly divided between upstate New York and Montana. The study concluded that the thermal mass of log walls significantly reduces energy consumption used for heating even in the lengthy winters of nothern climates. The results also concluded that log homes were as energy efficient as the super insulated frame homes used in the test even though the R-value of the log homes was rated 44% lower than the walls of the frame homes.

As a direct result of the two most recent studies, the Department of Energy and the Model Energy Code³ now recognize the benefits of thermal mass in log walls. Unfortunately, many log home builders and buyers still encounter resistence from code enforcement agencies who are unfamiliar with the results of these three tests, and who still require a log home to be rated strictly by the R-value system without proper consideration and allowances being made for the thermal mass effect of log walls. Hopefully, this article can provide assistance in sourcing the scientific data that has proven log homes to be an energy efficient form of housing that hold their own without having to be needlessly overbuilt to compensate for perceived deficiencies.

Clint Gray has built over a hundred custom homes thoughout the South. For the last 13 years he has specialized in the design and development of log home systems and is presently a Factory Representative for Sisson Log Homes, one of the South's largest producers of factory direct Log Home Systems.
Footnotes:
1.) Contact: Mr. Douglas Burch, National Institute of Standards & Technology, BR-B 320, Gaithersburg, MD, (301) 975-6433. Free publication.
2.) Contact: Minnesota Dept. of Public Service, Energy Div., Metro Square Bldg. Suite 200, 121 7th Place East, St. Paul, MN, 55101-2145. (612) 296-5175. Free publication.
3.) CABO Model Energy Code Book, 1995 edition. Contact: Southern Building Code Conference, 900 Montclair Rd. Birmingham, AL 35213. (205) 591-1853 $13.00 plus $3.00 SH.  [LHNZ]

          


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