THE THERMAL EFFICIENCY OF CORDWOOD WALLS

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Scheme 1 is perhaps the most timeconsuming method of cordwood construction, requiring two separate walls built of short lengths of wood, held together by just enough full-length spacer logs to assure structural stability.

If you plan to rely on air spaces alone to improve your walls' energy efficiency, small gaps will provide better insulation than large ones. This fact becomes evident when we compare the performance of the three configurations shown in Chart A with that of a solid cordwood wall: Scheme 3 — with its two narrow spaces — achieves a 12% reduction in heat loss . . . whereas Schemes 1 and 2 actually contribute to heat loss, adding 1% and 4% respectively. However, by filling the gaps with insulating material (even sawdust, which you ought to have on hand if you're cutting your own wood), you can increase the R-value of all three walls.

For a better idea of which materials work best, look at Chart B. The bar graph not only illustrates how effective two different insulants are when used in the three cordwood construction schemes I've described, but also compares those options with standard 2 X 4 and 2 X 6 studwalls. Notice that the best cordwood-plus-sawdust assembly compares poorly with a fiberglass-filled 2 X 6 studwall . . . but if you use cellulose (which has a per-inch Rvalue of as much as 3.7) in stackwood wall No. 1, the result will be as efficient as the fully insulated frame wall!

Of course, you could build with longer logs or leave larger gaps for insulation than those illustrated, and thus produce more energyefficient walls . . . but doing so would also up your initial construction cost. It's all a matter of how much wood (and money) you have available.

THE BOTTOM LINE

Your main consideration will be the cash you can save in the long run by building a cordwood house rather than a standard structure. In order to supply that information, I'd like to examine the life-cycle costing of the various cordwood schemes I've already described, as compared with those of traditional insulated and uninsulated 2 X 4 and 2 X 6 frame dwellings.

Life-cycle costing, as you may know, is the process of estimating how much cash something will consume over its useful lifetime (in this case, I've assumed a very conservative structure life of 50 years). In Chart C, then, I've estimated the cost of one square foot of stackwood wall in comparison with a square foot of traditional wall . . . when certain expenses are taken into account. The costs I've chosen to include are initial price . . . maintenance over the years (a standard house will need two coats of paint every eight years, while some portion of the mortar in cordwood will have to be replaced every 15 years or so) . . . and fuel (oil) for heating.

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