Energy Efficiency and Log Homes
by Tom Gorman, PhD, University of Idaho

	Dr. Tom Gorman is an Associate Professor of Forest Products at the University of Idaho. Before coming to Idaho in 1987, Tom was a Research Engineer at the USDA Forest Products Laboratory in Madison, Wisconsin. He holds graduate degrees in Wood Products Engineering from S.U.N.Y. College of Forestry. Much of his research while at the University of Idaho has been related to log homes, including strength of laminated logs, kiln drying large timbers, and, most recently, the thermal performance of log walls, a study conducted for the Log Home Council of the National Association of Home Builders.

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The energy efficiency of log homes is a perplexing issue that has not been completely resolved. One hears stories of the old-timers' trap-line cabin, always drafty and cold. Even today, building officials often rely solely on R-value comparisons as a way to evaluate construction methods, and log wall performance is expected to be less than that of a framed wall insulated with fiberglass. But when you talk to those who live in modern, well-built log homes, you consistently hear about comfortable homes, with low fuel bills. What's going on, and why can't we get our hands around this issue?

Assuming that you are interested in building a log home that achieves its maximum thermal performance, the following topics address some of the major areas that you can usually control and will significantly impact energy efficiency.

Air Infiltration (leaks)

Heat loss in a "leaky" house occurs the most when the outdoor temperature is lowest. This is based on the principle of convective heat flow, or "the chimney effect", which causes heat to rise. The colder the temperature outside, the greater the tendency for this to occur. Leaks in the walls or logs will allow the heat to escape and as heated air is lost, cold replacement air enters the lower part of the house around windows, doors and any leaks in the walls, causing uncomfortable cold spots and drafts.

While this seems to be a simple issue, some areas of a house do not get sealed properly. Where log walls meet roof lines is one place where major heat loss can occur, and is often overlooked during construction. Recessed lights, chimneys, stove pipes, plumbing vents, and other breaks in the ceiling should all be identified as potential leaks and the area around them sealed.

The way that your logs are sealed is important, too. Air tight caulking or gaskets between logs and at the corners is effective. Checks in solid logs, which occur as the logs dry, can form a path for cold air to enter the house from the outside, especially when ends are exposed at the outside corners.

The argument that a house needs to "breathe" is one I do not buy. Problems such as high humidity or inadequate fresh air can be solved with mechanical fans in bathrooms and kitchens, or mechanical fresh air inlets. Better to have control over the air flow than let the house ventilate naturally -- this only results in the biggest air exchange when it is coldest outside.

Siting

Hopefully, you will have a spectacular view to enjoy from your home. With luck the view will be to the south, or reasonably so. Windows on the south side can allow solar radiation to enter the house during the day and provide warmth. There's even a bonus for log homes: the high-mass log walls (and perhaps a stone fireplace) will absorb some of the sun's heat and remain warm into the evening. Large windows on the north, however, will cause considerable heat loss, even with today's high-tech windows. A well-designed roof overhang will prevent overheating from the summer's sun, but will still allow the low winter sun's rays to enter the windows. For more information on getting a home to take advantage of the sun, the wind, and the topography, see the article, "Siting a House" in the February/March 1995 issue (No. 93) of Fine Homebilding Magazine.

Log Insulating Values

No one area regarding energy performance of log homes is less understood than that of heat transfer through log walls. Over the years, building officials have taken a simplified approach to specifying insulating values for walls. Unfortunately, the simplified approach does not apply to walls with high mass, such as logs. Most of my research with log homes has been focused on how the "mass-effect" effects energy performance. I've monitored a test house throughout a full year, conducted simulation tests with a sophisticated computer program, and read as many of other researchers' reports as I could find. High-mass materials have the ability to store heat during the day. As nighttime temperatures drop, the heat is there to reduce the need for supplemental heating. On the other hand, once high-mass walls get cold, it takes a while to heat them back up again. Still, log walls perform better overall than a light-frame wall with the same R-value.

The R-value of a material is a measure of how much heat will be conducted under "steady-state" conditions, or constant indoor/outdoor conditions. Steady-state conditions rarely occur outdoors, so R-value alone is not a good measure for high-mass walls.

Here in the northern U.S., many building jurisdictions require R-19 insulation levels in the walls. Typically, 5 1/2-inches of fiberglass is used to achieve this. But does it? Tests have shown that the overall performance of a wall insulated with fiberglass falls short of its rated value, for a variety of reasons. Still, this seems to be the measure by which log walls are compared. The R-value of dry wood is about l 1/2 per inch of thickness, so a 6 inch log wall has an R-value of about 9. This is why log walls are often penalized when R-19 walls are required by the building code.

Though the mass effect does exist with log walls, it only adds the equivalent of R-1 to R-5 to a wall, depending on siting, climate, wall thickness, and wood species. Consequently, the mass effect does not fully explain the low fuel bills enjoyed by so many log home owners. I suspect that the high-mass interiors, coupled with good solar gain, is adding more to these homes than many realize.

Other Insulation Issues

Though it is difficult to achieve high R-value walls when building with logs, other insulation strategies can be brought in to create a house that is very energy efficient. A study conducted in Idaho a few years ago demonstrated that log homes in cold climates can perform as well as stick-built homes with R-19 walls when various combinations of "upgrades" are incorporated. For example, by reducing air infiltration to below standard levels in R-19 stick-built homes, overall heat loss is comparable. Likewise, upgrades in floor or ceiling insulation, improved windows, or siting to take advantage of solar gain can also have similar results. There are no simplified guidelines, however. Each home must be assessed to determine the best strategy to follow. The best solution may not add prohibitively high, or any, cost at all to the home.

Code Requirements

Oftentimes local building regulations dictate minimum R-values for walls. When log walls are desired by a prospective homeowner, this can create a difficult situation. However, most codes allow an alternative method for demonstrating that a home will meet overall energy performance criteria.

Rather than the "component performance" approach, which spells out the insulation value for each component in a home, a "system performance" approach may be the most appropriate method for log homes. That is what was done in the Idaho study mentioned in the previous section. Many states recognize this approach, though it often takes a computer program to conduct the analysis.

Those states that take energy performance seriously will often assist with the system performance computer analysis of a home. Unfortunately, the mass-effect is usually not considered in the simplified software they commonly use. While this may not present any problems, some marginal house designs may not be approved if the mass effect credits are not incorporated. The computer program that I have used in my research is particularly appropriate, since it models energy loss for the complete home and incorporates the mass effect for walls.

If you are dealing with a building jurisdiction that dictates a specific R-value for the exterior walls, ask if your design can be analyzed as a system. The analysis may be provided by the state's energy office. This could help you develop a strategy that allows you build, despite the apparent code barrier. Should that approach fail, an energy consultant's analysis of your design may be necessary to gain code approval.

When building any home, one should hire a competent builder, use a good design, and ensure that quality materials are used. By following these rules during the construction of your log home, you can expect good performance in the final product. A well-built log home should be comfortable, with low fuel bills . The success stories are easily found. Find out what was done right and use them as a guide for your own home.