R-Value of Log Walls
Wood rates about R-1 per inch, but air-sealing is also an issue with log buildings. July 13, 2006
Question
What is the R-factor in a red pine log? I operate a sawmill that makes log cabin logs and have been asked this question by building inspectors. I read that you should multiply the width of the log by 1.5 to determine the R-factor. So if you have a 10" log, the R-factor would be 15. Also, does the R-factor increase if the log is kiln dried compared to air dried?
Forum Responses
(Sawing and Drying Forum)
From contributor D:
Wood has an R factor of about 1. It would not make any difference how it is dried. It varies with species, but red pine should be right about there.
From contributor L:
To add to the above reply: dried wood has an average R-value of 1/inch of wood. So your 10 inch timber once dried will have about an R-10 value.
From contributor S:
But log walls have an attribute called thermal mass. Adobe and trombe walls (sp) also have thermal mass, as do large containers of water. Why don't tall water towers freeze solid in the north? They act as a heat/cold reservoir moderating fluctuations in temperature because of stored energy. From what I've read, R values are not really an accurate way to measure thermal masses and are only one factor that needs measured.
From contributor D:
You are right in a way about the mass idea. When the sun hits the outside of a wood wall, it has to heat the wall and thus, there is a time delay from the time the sun hits the wall until the inside sees the effect. But it will see the effect. Since there is a daily fluctuation in sun and temperature, it is not an important issue for a kiln. It is more of an important issue in heating and air conditioning where you have a much different effect from an R8 window than you do from an R8 wall and you consider that when sizing an air conditioning unit. But the net result over the day is not a big enough deal to worry about.
Build a tight kiln and insulate to at least R30. That saves heat but, just as important, prevents condensation from forming, which will destroy the building over time.
From Professor Gene Wengert, forum technical advisor:
Using a multiplier of 1.0 is a conservative approach to determining the R-factor. But the answer is a bit misleading, as there is often a great deal of leakage at the joints. (It is similar to have a well insulated house, but then leaving the windows open.) It is advisable to suggest that the interior walls of a log home also be framed with 2x4s conventionally and insulation added to provide better heat insulation overall. This is especially true for the bathrooms and kitchens. It is important to insulate the floors and ceilings as well, especially in bathrooms and kitchens. Certainly, the drier the log, the better the insulation value (the larger the R-value). This is likely why some people suggest using a 1.5 multiplier, as that would be closer for very dry wood.
