- Although all logs have good thermal properties, some are better than others. Thermal performance is a combined measure of both the heat conductivity ("R" value) and thermal diffusivity, sometimes called thermal mass effect. Less dense woods, such as cedars and pines, have higher "R" values (make better insulators), but lower thermal mass (heat storage and transfer delay). High density woods, such as the oaks, and hickories, make poorer insulators (lower "R" value) but are better in heat storage/delay. The best woods are those that have the best combination of the two measures: moderately dense woods such as some pines, Western Larch, and Douglas Fir. Large diameter logs have better thermal performance than small diameters.

The energy efficiency of a home is determined by so-called R-values (resistance to thermal flow) of building segments such as walls, floors, roof, windows, etc., and by the air tightness of the building envelope.
In log homes (and other heavy structures) there is alsothe heat mass that influences the heat-holding abilityof the structure, thus influencing heating bills.
Let’s start with the basics. Log homes are made of wood (well, parts of the walls are anyway).
The cross section of the log under the microscope appears like a bundle of straws. Those “straws” are filled with air when the moisture content of the wood is lower than so-called fiber saturation point (somewhere around 28 percent).
It is common knowledge that air is a very good thermal insulator, and the more layers of air in the system, the better its insulating properties. Therefore,
wood seems like an ideal material to separate the warm and cozy interior of your house from the freezing winter conditions outside, right?
Bigger is not always better except in log walls, where the thicker wall provides better thermal performance. At least the R-value (thermal resistance)
of the wall will be higher. By the way, even in this case the larger the number, the better the wall should perform

The Major factor contributing to the high R-value of wood is the microscopic makeup of the wood itself. Air is a very good insulator. Gases like air do not transfer heat very well because their molecules are far apart from one another and greatly extend the time necessary for convective and conductive flow to occur. Dry wood happens to be composed of porous cells that hold air after the moisture has been removed.

Thermal Resistivity of Various Softwood Species Used in Log Construction