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Saving with shallow foundations.

Since well before World War II, builders in the South have used slab-on-grade construction as a simple, efficient way to build houses. But the costs of adequate frost protection deterred northern builders from doing the same. New ways to build shallow foundations now give northern builders a chance to share in the savings.

Added Insulation. Research on shallow foundation systems confirms that they work. Because the systems rely on foundation insulation installed horizontally, away from the bottom of a shallow stem wall (with footing, if required), they push the frost line away from the house's exterior, eliminating concern about typical frost line depths [ILLUSTRATION FOR FIGURE 1 OMITTED].

The problem has been, however, that in a shallow foundation, the foundation and slab edge corners require extra, thicker insulation than do the stem walls. The amount of insulation needed for each corner depends on the severity of the climate [ILLUSTRATION FOR FIGURE 2 OMITTED].

In moderate climates (2,500 heating degree days or fewer), no horizontal insulation is required. Recommended insulation R-values also vary by climate, from R-4.5 to R-10.1 for stem walls and from R-1.7 to R-13.1 for the horizontal insulation. (These R-values are based on a measure of winter severity called the Air Freezing Index.) Obviously, the additional insulation adds to construction costs, but the savings in excavation and concrete work more than compensate.

Demonstration homes. To assess the performance and cost savings shallow foundations provide, HUD contracted three independent builders to build demonstration projects in North Dakota, Iowa, and Vermont. The NAHB Research Center coordinated the projects; the results are published in a report tided "Substantiating Data and Information for Frost-Protected Shallow Foundations," along with the "Design Guide for Frost-Protected Shallow Foundations."

Each builder saved 1 to 4 percent of the house sales price without compromising quality or value. The North Dakota project, a one-story, four-unit townhouse structure with units priced at $60,000 saved more than $600 a unit in hard costs. The Iowa project, a one-story, single-family custom home priced at $125,000, showed savings of almost $5,000. And in Vermont, the builder saved almost $2,000 a unit on its two-story condominium duplex, whose units sold for $100,000.

Based on these findings, the advantages of shallow foundations are no longer limited to warm climates.

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Author:Johnson, Arthur
Date:Nov 1, 1996
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