The Thermal Efficiency of Cordwood Walls

An energy consultant looks at how cordwood walls stack up against conventional structures in terms of thermal efficiency.

| January/February 1983

Cordwood construction certainly isn't a stranger to the pages of MOTHER EARTH NEWS, and regular readers are probably already well aware of how inexpensive and easy to build cordwood walls can be. However, not many folks really know whether these log and mortar surfaces are effective — as compared with conventional structures — in controlling heat flow. Of course, the walls are thick (at least 6" and often more than 12 "); it would be safe to assume that they provide some insulative value, but there's a good bit of calculation involved in determining just how well stackwall measures up to studwall in terms of heat-holding potential.

Well, I've done that piece of research, and discovered that a poorly insulated cordwood wall (no matter how economical it may be to construct) would — over its lifetime — cost its owner thousands of dollars in fuel bills. In fact, under some circumstances a potential homeowner might be better off putting money in a standard, fully insulated house to begin with!

Now don't get me wrong. I'm not trying to discourage any future stackwallers (or irritate any current enthusiasts). I just want to point out a few possible shortcomings in their thermal efficiency, and show how these can be surmounted. In truth, I'm fully convinced that, if enough thought is given to proper wall construction, this building technique can still be one of the best available alternatives to expensive traditional construction.

The Three Keys

The thermal efficiency of a stackwood wall depends on three variables (we won't concern ourselves with direct air leakage): thickness, composition (that is, the ratio of wood to mortar), and the amount (if any) of added insulation.

Obviously, the thicker the wall is, the better it'll be at keeping the heat in a house from escaping. Of course, practicality limits the length of logs used (door frames and windows have to fit in, right?), and most cordwood walls range from 6" to 24" wide, depending on climate and the availability (and affordability) of building materials.

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.

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