How to Lay a Foundation for a Passive Solar Home

In this installment of a series on building a passive solar home, the forming, pouring, and laying of the footings, foundation, and floor slab are discussed.

| September/October 1983

All too many of us have found that building an energy-efficient home seems to be a dream, a fantasy that's kept just out of reach by escalating prices and high interest rates that contribute to the high cost of building a home. Of course, MOTHER has long been exploring ways of breaking this vicious circle of waste, and one inventor of solutions—whose work we've shown you before (starting back in 1981, in an article on earth-sheltered architecture)—is architect Angus W. Macdonald. Angus has developed a number of housing designs that apply low-cost building techniques to passive solar, earth-tempered homes, and he's agreed to relate, in a series of articles that will span at least six issues of MOTHER, much of what he's learned about planning and building such structures. The series will follow the actual construction of one of the architect's standard designs—Sun Cottage—and will include photos and illustrations of each step. 

In our last issue, we covered budgeting and cost control, and in this installment, we'll discuss how to lay a foundation, including the forming, pouring, and laying of the footings, foundation, and floor slab.

The most critical factor in the construction of any building is proper measurement, because errors made at this point will be very costly later on if (for example) manufactured items—such as doors, windows, and cabinetry—fail to fit into the spaces allotted for them. What's more, when building a dwelling like the Sun Cottage, in which a structural kit must fit into a masonry shell, it's even more important to pay careful attention to the tape measure. So don't rush ahead with trenching or pouring concrete until you're absolutely sure of the dimensions you've laid out on the ground!

While you're at it, you should also double-check your home-to-be's location and orientation, to make certain that you've allowed room for services, access, and future expansion (and to insure that you're not encroaching on any setback requirements your lot may have). Make sure, too, that your floor elevation will allow for proper waste-pipe slope to your septic tank or sewer connection. And remember, the excavation depth itself will be eight inches below floor level, and the bottom must be as flat as possible, while sloping slightly toward the front to prevent standing water when (not if) a rainstorm hits during construction.

Most contractors "line out" (that's building lingo for laying out with a string or line) a structure by placing "batter boards" (explained later) at the points where the corners will be. On an earth shelter, however, the top of the footings will be some distance below grade, so you'll need to excavate an extra 2' width right around the house area. This "border" will allow room for batter boards, provide a work area for waterproofing operations, and assure that you'll have adequate space for the foundation drain system.

Although the excavation itself demands only fairly rough measurements, when it comes to lining out the footings, you'll have to muster all the accuracy you can. A builder's level (wielded by someone who knows how to use it) will provide the best results, but you can get by with some wood scraps, mason's twine, a carpenter's spirit level, and a 100-foot measuring tape.

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