Building a Cordwood House

How to build an energy efficient house from cordwood masonry, including fuel savings from earth sheltering, insulation and the floating slab.

| April/May 1995

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    Two Thousand Square Feet for $27,000 . . . and a $75 Fuel Bill!
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    The 16' screed board spans from the perimeter footings to the 7' diameter masonry stove foundation. This shows Insulation, mesh, and footings.
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    Rob at work on a cordwood masonry
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    The sand pad is compacted in three runs of about 6"" each.
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    Dry stack corner blocks (8"" x 8"" x 16"") like cordwood in the bermed wall.
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    Jaki Roy uses a stainless steel butter knife to smooth the mortar.
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    The sun-facing cordwood masonry wall made easy. First lay an insulated double bed of mortar on the slab.
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    Using a trowel, apply surface bonding cement to a dampened wall.
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    Then, tie the inner and outer beds together with log ends. Repeat.
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    The masonry stove is 5' in diameter downstairs and reduces to 4' upstairs. Flues crisscross through the masonry to increase conduction.
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    Place waterproofing membrane vertically on the bonded walls.
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    The octagonal post-and -beam frame cuts the radial rafter spans in half. The masonry stove acts as a load-bearing column at the center of the home.
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    The French drain is made of perforated flexible tubing.
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    Sixteen rafters go all the way to the stove; the others stop at the octagon.
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    We apply waterproofing membrane to the primed spruce deck.
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    After laying a layer of 6 -mil black polyethylene over the foam insulation, we install a 2"" crushed stone drainage layer, then straw, then 7' - 8"" of earth.
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    We place 4"" of extruded polystyrene on the top of the membrane, protecting it from the freeze-thaw cycling.
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    Diagram of the house.
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    Construction details
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Visitors shake their heads when I tell them that we are heating Earthwood, our 2,000-square-foot home near the Canadian border, for $75 this winter. It's hard to tell if they're really impressed or think I'm trying to pull the wool over their eyes. But I really get their interest when I explain further that the house maintains a steady temperature, summer and winter, with no "spikes" in the temperature curve. North Country folks are used to waking up to a chilled home on subzero winter mornings.

There's no magic involved, and no attempt at deceit. Earthwood's performance is the result of employing several design characteristics not usually combined in American homes: a round shape, earth sheltering, cordwood masonry, solar orientation, and a 23-ton wood-fired masonry stove. And the cost? About $27,000 total, including labor and materials. Grudgingly, I admit that $6,500 of this figure went toward hired help; otherwise, my wife Jaki and I did the work ourselves. And $5,000 of the $27,000 represents the value of materials donated by manufacturers for field-testing purposes, but I've included them to give a true idea of cost. In 1995, an Earthwood-type home can be built for a materials cost of about $15 per square foot.

A Round House

The other building species (birds, bees, beavers, etc.) know instinctively that a round house is the most economical to build and the easiest to heat. Backed by millions of years of experience, a course in geometry would be wasted on animal architects. Same with so-called primitive man. Where materials are scarce and time is at a premium, it does not occur to many tribes to build any shape other than round. A round house encloses 27.3% more space than the most efficient rectilinear house, which is square. But Americans don't even build square houses anymore. Typically, the American home is twice as long as it is wide, and a round house enjoys a space percentage gain of 43% compared to such a rectangle! The same amount of time, materials, and labor (particularly when building with masonry units) yields a home that has 43% more space. And, when heat loss through skin area is considered, it's approximately 43% easier to heat on a per-square-foot basis.

The outside diameter of Earthwood is 38'8". With 16" thick walls, the inner diameter of 36' yields a usable internal area of 1,018 square feet on each of the two stories, just over 2,000 square feet total.



Earth Sheltering

But a 43% savings, while substantial, doesn't account for a $75 fuel bill. Our house is built near Plattsburgh, N.Y., with a heating season of 8,500 degree days. As we're 1.200 feet higher than Plattsburgh, we experience 9,000 degree days. (Compare Washington, D.C., for example, at 4,000 annual degree days or Memphis at 3,000). But, by earth sheltering the home, we can effectively change the "climate" just outside the fabric of the building. Let's say its -20°F outside. At 6' of depth, the earth is about 40° in winter, so our ambient temperature just outside the house fabric—again, think of climate—is 60° more favorable than a home built above grade. In effect, going 6' down is the equivalent of moving the home nearly 1,000 miles south!

Our south side is not earth sheltered at all, to take advantage of solar gain and to maximize light; but the earth berm on the north of the two-story home is 13' deep, so we have an average of 6' of earth sheltering around the home, plus an 8"-deep earth roof. Fuel savings are compounded.

Ralph_11
7/8/2007 5:09:14 AM

Hello, I am living in Belguim and I am trying to find resources to build and eco friendly home for cheap, I would like to spend around 60,000 for work, materials and all. I was hoping someone out there might have some information for me I would like to start building soon but I dont even know where to begin. Please if you know of any architects or firms in my area please let me know. I do apprieciate you time and thank you for your help. Lorennmemphis@yahoo.com




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