Do Log Homes Meet Code?

HOME CLINIC: Does the state energy code allow me to build a log home? What is the insulation value of a 14-inch log? J.S., Enumclaw

DEAR J.S.: Yes, the Washington State Energy Code allows log homes.

The code includes a Prescriptive Path, commonly referred to as a cookbook approach, for showing that log homes are in compliance. The required building "ingredients" are listed: prescribed window, floor and ceiling R-values, and when followed the house is deemed to comply with the code. Using a prescriptive path is the easiest compliance method because no energy calculations or analyses are needed.

However, here's some background. The insulation value of a 14-inch log is based on the average thickness of the overall log wall. The wall thickness will be 14 inches at the middle or widest part of the log, but less where two logs meet. We developed a chart to determine an average log wall thickness. We found an average thickness of 10 inches if we assume the wall is 8 inches wide where the logs meet.

To calculate the insulation value of the log, we multiplied the R-value of the material by its thickness. The R-value of fir, pine, or cedar is 1.25 per inch and when multiplied by the average thickness (10 inches by 1.25) we see your wall will have an insulation value of R-12.5. This may seem low when compared to a conventional R-19 wall, but the house will be more energy efficient than it appears. Log homes contain more thermal mass than a typical framed house. Solid wood walls have a greater capacity to store heat, which can lessen your heating load.

The Log Home Prescriptive Paths are based on average log thickness - you don't have to calculate an R-value for the logs. To use a prescriptive path, you first determine what climate zone the house will be built in (Western Washington is Climate Zone 1) and then what fuel will be used. Heating fuel types are split into two categories: electric resistance or other fuels (natural gas, oil, propane or heat pumps).

For example, if you built your house in King County and used a heat pump for heating, the following would apply: ceiling R-38; floor R-19 and glass U-.65.

Maximum amount of glazing (window/skylight area) equals 21 percent of the floor area.

For a free copy of the Log Home Prescriptive Paths or Average Log Thickness Chart, contact the Energy Hotline at 1-800-962-9731.

By the Education and Information Network of the Washington State Energy Office. This information can be made available for people with disabilities; please call 1-800-962-9731 (Voice and TDD).