Courtesy of Scott Widick
An example of how thermography can be used to pinpoint areas of energy loss. Dark areas are colder. The vertical rectangular gradient scale to the right side of the photo shows the approximate temperatures in the photo.
THERMAL PERFORMANCE
Although an understanding of the thermal performance of log homes is not crucial to the ability to capably inspect them, inspectors will sometimes be asked about this aspect. An inspector’s ability to successfully answer general questions about the energy performance of log homes will instill confidence in clients and help build inspector credilbility.
R-VALUES
The R-value of a material is a measurement of its resistance to heat flow across its thickness. A thick section of a particular material will have a greater R-value than a thinner section of the same material. R-values can also be giver per inch of thickness in order to more easily compare the thermal performance of different materials.
The R-value per inch of thickness of wood is not high compared to thermal insulations typically installed in conventional homes. Depending on the wood species and growth conditions, the R-value of wood ranges from R-1 to R-1.5. Typical R-values for commonly-used conventional thermal insulation types are R-3 to R-4.
THERMAL MASS
The R–value of a home may not be an accurate indication of how well a home will actually perform. In addition to resisting heat flow, logs in walls also provide thermal mass which has the ability to store and release heat. How effectiveness thermal mass is at lowering heating and cooling costs depends on the size of the logs, the wood species and the climate in which the home is located, but thermal mass can reduce annual heating or cooling costs considerably.
Generally speaking, the denser a log is, the greater thermal mass it provides. Thermal mass provides fewer savings in extremely hot or cold climates. In most climates across the U. S., especially those in which log homes are popular, thermal mass will provide benefits in reducing heating and cooling costs.
The moisture content of the logs will also have some effect on the energy performance of logs.
LOG VS CONVENTIONAL
A log home built form 7-inch solid wood walls might have an Value of R-9. In most climates in the U.S., a conventional home would have to be insulated to R-13 to R-15 to provide comparable annual heating and cooling performance.
This is assuming that the log home and a conventional home have been built using similar designs, attic insulation, HVAC systems and windows. (1)
ENERGY PERFORMANCE
The overall energy performance of log homes will vary with individual homes. The efficiency with which a log home uses energy depends on the following:
- Overall tightness of the building envelope
- Areas insulated and the levels to which they’re insulated.
- Thermal properties of the wall logs.
- Thermal properties of windows
- Efficiency of heating and cooling equipment
- Efficiency of electrical appliances
- HVAC configurations
ENERGY AUDITS
Clients wanting to gain a more accurate understanding of the energy-efficiency of a home and to find areas for improvement should consider having an energy audit performed. In performing an energy audit, in addition to considering the items listed above, an auditor may examine energy bills spanning the preceding year and may use blower door testing and/or thermal imaging to pinpoint areas of energy loss.
Sources for More Information
(1) Howard, B.D., 2001, “Log Homes Thermal Mass and Energy Efficiency: Assessment of Energy Efficiency Calculations and Ratings of Log Homes Compared to Other Residential Wall Structural Systems”. Prepared by Building Environmental Science and Technology, Edgewater MD, USA, Contractor report the NAHB Log Homes Council.
