THE THERMAL EFFICIENCY OF CORDWOOD WALLS
(Page 2 of 4)
January/February 1983
By Roger C. Gregoire
Before I go any further in this discussion, I'd better give a brief definition of a term that's essential in any discussion of thermal efficiency . . . R-value. Put simply, it's a measure of how resistant a given thickness of a particular material is to heat flow: the higher the R-value of any substance, the greater its ability to stop heat loss. Therefore, when you consider buying insulation, you want to make sure that you get the best R-value you can for your money. [EDITOR'S NOTE: For further information about the R-value and use of different kinds of insulation, check out the article on page 54 of MOTHER NO. 78.]
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The walls of a cordwood house, without additional insulation, consist of three mediums, each of which has a different ability to resist heat movement: mortar (which has an R-value of 0.20 per inch of thickness) . . . wood (with an average R-value of 1.0) . . . and air (the R-value of which depends upon a number of conditions).
It's the combination of materials that constitutes the second variable. If you build a 12"-thick wall using 50% wood and 50% mortar, for example, you can expect to lose about 34,875 BTU per square foot a year. (My calculations are based upon the climate of my hometown of Hartford, Connecticut, which has approximately 6,350 degree-days a year.) Now if you vary the ratio a bit-by stacking the logs tighter-and have, say, 60% wood to 40% mortar (giving your wall a slightly higher total R-value), you might stand to lose approximately 32,250 BTU . . . for a reduction in heat loss of 7-1/2%.
ON YOUR MARK, GET SET . . . INSULATE!
As you see, then, log width and wall makeup are important . . . but it's the third variable, the use (and placement) of additional insulation, that can turn a potential "icebox" into a real heat-saver!
Of course, you'll first have to decide where to put your insulation. And as I see it, you've got several options . . . three of which are detailed in Chart A. Schemes 2 and 3 are pretty conventional techniques, using logs that are all the same length. Both incorporate insulative air space between layers of mortar . . . the difference between them is that No. 2 has one large air space, while No. 3 uses two narrower gaps.
