I am planning to build a log house, using only the logs as the exterior wall and without additional insulation or finishing Log-house kit manufacturers claim that logs are equal or superior in insulating qualities to conventional construction with 3 1/2 inches of insulation. In view of escalating energy costs, can you provide any comparison between the insulating qualities of log construction and 3 1/2 inches of insulation in conventionally built wall? G. D. Fassler Cherokee, Iowa
With a 3 1/2-inch fiber-glass insulation batt, you have an R Factor of about 11, but no mass. But with the logs you do have mass. What mass can do for you is equalize the temperature fluctuations from outside to inside.
It's very hard to tell in a log cabin if the mass in dominant enough to make it perform as well as an R-11 fiber-glass wall.
A good rule of thumb on the R value for wood is 1.25 per inch of thickness. So if the widest point of a log is 8 inches, the most you would have is R-10 at that point. Don't forget, however, that as the logs narrow down, the R value drops off signifficantly. Thus, with logs alone it seems hard to expect the cabin to perform as well as a frame-batt system in a conventional dwelling.
As the wood dries out, chinks and holes will develop and air infiltration can become a factor. An interior finish, of course, will help this problem.
Have you thought of using an insulation board on the inside walls? Styrofoambrand insulation, for example, made by the Dow Chemical Comapany, has an R value of about 5 to the inch.
Check with the manufactures of competing products for the R value of their line.