Reprinted with permission from Back Home Fall 1992
Hindsight how-to from a cordwood
and earth-shelter pioneer.
By Rob Roy
Too often, the stories of alternative houses reach print before the structures are fully complete. I'll have to confess that more than once I've understated the test of time, having been so eager to share new ideas with the world that doing so "right now" blinded me to the importance of the "years later" follow-up.
Happily, the saga of "Earthwood"our round, earth-sheltered, cordwood-masonry home in upstate New Yorkisn't that kind of story. As I write this, Jaki and I are celebrating our tenth anniversary of moving into a finished house, which in the interim has even been through some minor remodeling.
Earthwood is 2,300 square feet of home built not ten miles from the Canadian border. We can see Montreal's skyline from our windplant tower. Here, with our 9,000-degree-day climate, the need for energy efficiency is a given. Yet when we set out to build this, our third cordwood-masonry dwelling, we wanted to consider things other than simply shelter and fuel-things such as gardening, food preservation, home industry, recreation, aesthetics, and spiritual comfort.
The thought of a round house may not sit well with many lending institutions ...or even some spouses. Yet in nature the most efficient nest shapein terms of energy conservation and economy of labor and materialsis round.
A round structure with
the same perimeter as a square one encloses 27 percent more area; when compared
to a rectangle (in which most of us live), the area rises to 43 percent. No
wonder birds and beavers build their homes so energetically; they can't wait
to move in!
To owner-builders, the economy of a round shape is clear, especially with masonry walls, which do not require extensive use of sheet goods. When it comes to heating and cooling, the advantages are similar: the round house uses less skin area to enclose a given space, so less energy is lost. And the wind moves around it more easily, too, with less of a chilling effect. If you doubt this, think of the last time you saw a square igloo.
The cost of building Earthwood, about $12 per square foot, reflects this economyaided further by the use of low-cost cordwood-masonry walls, a technique that uses short logs stacked much like firewood, held together with a special mortar blend and insulated at the center with a variety of materials.
A common misconception about earth-sheltered housing is that earth is good insulation. This is not true; settled earth is poor insulation. However, earth is a very good capacitor. Its mass takes the swings out of both daily and seasonal temperature curves.
At 8' of depth, the coldest earth temperature in northern New York is about 40°F, around March 1. So building a house in the earth is like building in a warmer winter climate. The ambient conditions of an earth-sheltered house in our area approximate that of an above-grade house in coastal South Carolina. In summer the cooling advantages are similar. Earthwood is surrounded by 65°F soil in August, so the home is naturally cool without air-conditioning.
Of course, we get questions: Is it dark? ...Do you feel "closed-in"? ...Does it leak? The answers are: Nope, not yet, and not any more! Using plenty of windows for ambient light is the key to satisfying the first two questions. As for the last, we did have a small leak in the bedroom about five years ago, caused by ice-damming under the soil-retaining timbers along the edge of the roof. We repaired the damage and shimmed up the timbers 1" or so to let the water drain off.
There have been no problems since, and I now follow the technique of underground guru Malcolm Wells who uses moss instead of railroad ties to retain the earth. It provides better drainage and looks nicer too.
The most important single
benefit of the earth at Earthwood is that the house remains at a consistent,
comfortable temperature. Remember, there's an awful lot of thermal mass that
has to change temperature in order for the home to warm up or cool down. Only
from the south does the house look like a two-story structure; the earth-berming
is concentrated at the northern half. Forty percent of the cylinder is earth-bermed,
and the roof is covered completely.
On the roof we use just enough soil to maintain the green cover (grass, wildflowers, whatever)about 7" where we live. Beyond that depth additional insulation is better achieved with an extra inch of extruded polystyrene rather than another foot of earth, simply because such a heavy structure is needed to support the load. Soaking wet soil weighs 120 pounds per cubic foot, and at that rate it doesn't take much before the size and cost of structural support becomes prohibitive. Even with 7" of earth, we need nearly three dozen 5" X 10" pine rafters and have to keep our spans down to 8'.
Thanks to books and magazine articles, cordwood masonry is gaining acceptance as a legitimate, long-life building technique. Many cordwood homes around the country are built with bank financing, though the majority are still constructed by owner-builders on a pay-as-you-go basis-the best way, in my view, if you aim to own your house, not rent it from your banker.
Earthwood was our third house to make use of cordwood walls, the techniques having been refined on the first two efforts. Yet those walls at Log End Cottage and Log End Cave, built 12 to 15 years ago, have gracefully weathered on the exterior and haven't changed inside since the homes were completed.
At Earthwood, though, we went with larger 16"-thick cedar walls and used a much heavier mortar matrix, since the cordwood walls themselves are self-supporting. (The Log End houses had external post-and-beam frames.) Both the inner and outer mortar joints are 4" to 5" wide and average 1 1/2" thick. This lends strength, beauty, and exceptional thermal mass to the mortar joints.
Our insulating method is worth mentioning because it has worked so well. We add about two or three spadefuls of lime to a wheelbarrow of sawdust and work it over with a hoe until the sawdust is of a uniform light color. As we build, we pour this insulation between the inner and outer mortar joints with a small spouted bucket.
Nowhere in the wall is heat conducted directly from the inner to the outer mortar joint. The treated sawdust is economical, environmentally friendly, and has been found (in testing at Penn State) to be equal to fiberglass as insulation. I think it's better, because if it gets wet for any reason, it sets up like beadboard instead of mattingand stayingdown, like fiberglass. If it stays dry, it remains as loose fill insulation, much like vermiculite ...which, by the way, is another viable option.
Our mortar mix is 9 parts sand, 3 parts sawdust, 3 parts Type-S (builder's) lime, and 2 parts Portland cement. If masonry cement is used, go with 3 parts cement and 2 parts lime instead. The sawdust (softwood is best) comes from a sawmill, is passed through a 1/2" mesh screen, and is soaked in a drum overnight before being used. This wet sawdust helps to retard the set, preventing mortar shrinkage.
The cordwood walls are a great success. They're warmspiritually and thermallyand beautiful. We haven't grown tired of them and still enjoy building new cordwood walls whenever we get the chance. In fact, we've built a small, round, lakeside summer cottage using 12" cedar log-ends and a geodesic dome.
Time: The Real Test
Looking back over the past decade, I'd have to say that we easily met our energy conservation goals. We use 3 1/4 full cords of dry hardwood each year for heating and for much of our winter cooking and domestic hot water needsincluding a sauna. A typical wood-heated home in this region would be expected to use between six and 12 full cords annually.
Some of the credit for this is due to the fact that we burn almost half our wood in a very efficient 23-ton stone masonry stove, the central pillar of the house. But there's no denying that the home's circular shape, earth covering, and cordwood construction add significantly to the outcome.
Earthwood, by the way, is not plugged in to commercial power. The house gets all its electrical energy from wind, solar, and "son" power. . . the last being 16-year-old Rohan, whose mountain bike is mounted to a commercially available generator and bracket (Real Goods, 966 Mazzoni St., Ukiah, CA 95482). Although this works very wellRohan can steadily equal the output of two 50-watt photovoltaic panelswe never seem to run out of power from our 1,000-watt Sencenbaugh windplant and our six Siemens M75 PV panels.
For recreation we've included a game room, pandering to my long-time fascination with billiards. There's a solar room/greenhouse, where plants can be started in proximity to the raised-bed gardens. And we've made plenty of space for our home industries, such as canning, beer making, and maple sugaring, and for the needs of our business, Earthwood Building School.
Earthwood feels good to us. Who wouldn't feel secure in a home with an almost womblike quality and a constant temperature? The cordwood walls have the varied relief of pointed stonework, with the Warm colors and textures of wood. The massive timbers impart a sense of strength. And there's a definite satisfaction in having done something by ourselves on this large a scale.
Would we do anything differently?
Nothing majorbut then, it took us three tries to get exactly what we
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