Preferred techniques and materials
Several factors come into play when builders strive for a green approach to foundation, footer, and slab installations.
- Foundations, footers, and slabs should always be detailed to reduce thermal bridging as much as possible.
- Forming can account for a significant portion of the total cost of poured concrete. Essentially, things get built twice: once in forms and again in concrete. Plywood has been the mainstay of concrete-forming companies for many years, though some companies have invested in reusable, durable forms—a more resource-efficient solution that is still relatively labor-intensive. Reusable forms also require form-release agents, most of which are petrochemical-based and off-gas large amounts of volatile organic compounds (VOCs). Vegetable-based form-release oils are available.
- For large buildings and some sites with poor soil conditions, foundations are engineered to specific structural requirements and fairly extensive use of concrete may well be necessary. For certain applications, replacing up to 50 percent of the cement in a concrete mixture with fly ash from coal-burning power plants will reduce the environmental impact of producing the material, and this substitution can actually strengthen the concrete. Fine-ground blast-furnace slag from metal foundries can have properties similar to those of fly ash.
- For homes and smaller buildings, alternative foundation systems are available that can reduce concrete use and increase energy efficiency. Many such products consist of stay-in-place insulating concrete forms (ICFs) made of polystyrene foam or a cementitious matrix of recycled foam or recycled wood fibers. Expanded polystyrene (EPS) foam should be preferred until extruded polysterene (XPS) foam insulation becomes available without ozone-depleting HCFCs (hydrochlorofluorocarbons, a class of chemicals that have been used to replace chlorofluorocarbons, or CFCs, which cause even more ozone depletion than HCFCs). Some EPS foam products have an integral borate treatment, which helps keep damaging insects out of the foam. The brominated flame retardants used in most EPS foam have health and environmental risks that are generating significant concern.
- Precast concrete foundation walls are available in some areas. They use less concrete than site-cast foundations and are designed to accommodate interior insulation.
- The depth of a foundation wall (and thus the amount of material required) can be reduced by raising the frost line, generally by placing foam insulation horizontally (usually extending about 4 feet) around the foundation. These “frost-protected shallow foundations” can save money and materials where crawl-space or slab-on-grade foundations are used in cold climates. “Rubble trench” foundations are another option—one that was favored by Frank Lloyd Wright. Pier foundations, which can reduce excavation requirements and concrete use significantly, may also be an appropriate choice.
- Non-asphalt-based dampproofing reduces the risk of introducing chemicals into local aquifers and VOCs into the building, and they can last longer.
Other alternative products
In some parts of the country, rigid mineral wool panels are available that help insulate foundation walls while also providing effective drainage. Mineral wool typically includes iron-ore slag—a pre-consumer waste product. And recycled aggregate or crushed glass can be used as aggregate in the concrete or as backfill for foundation drains.
Foundations can also be designed with termite shields or be backfilled with special termite-proof sand, so that toxic soil-treatments are not required. Pesticides commonly used around foundations introduce hazardous chemicals to the environment and must be periodically reapplied.
BIBLIOGRAPHY ITEMS
Building Science for Building Enclosures
by John Straube et al.
(2005)