Fact Sheet »

Description

The foundation provides a variety of functions, including structural support for the house; prevention of moisture, soil gases, radon, and ground water from entering the living spaces; retention of heat in winter, and - if properly constructed - additional livable space for the homeowners. Quality construction and planning are essential for an efficient, durable, and healthy foundation. The choice of a basement versus no basement and issues of livability and health must be weighed in selecting a foundation type, see House: Basement for a discussion on basements versus on-grade construction. In our region, foundations may be constructed from a variety of materials and systems, including poured concrete, concrete masonry units (CMU), insulated concrete forms (ICF) and wood. The following comparative analysis identifies the relative economic, energy, and environmental implications of eight different foundation systems: slab on grade with stem wall, shallow frost protected, crawl space, garden basement, poured concrete basement, wood basement, concrete block basement, and Insulated concrete forms.

Recommendations

Each foundation type has distinct economic, environmental, and construction advantages and disadvantages. A frost protected shallow foundation is the least expensive, most material efficient option for non-basement construction. For a balance of cost, material efficiency and improved indoor environmental quality when a livable basement is desirable, a garden basement with a cast in place concrete wall is recommended. Priorities and trade-offs will need to be considered to determine the most appropriate foundation.

Foundation Alternatives

Criteria Summaries

Cost: The frost protected shallow foundation is the least expensive of the non-construction construction systems explored. It is typically 10-15% less than slab-on-grade with a stem wall and crawl space foundation. Wood foundations are the least expensive of the full foundation options. Based on cost data for the Twin Cities (Means 2004), poured concrete systems have an edge over CMU (about 15% less expensive). Cost of poured concrete and CMU construction varies widely by region, check with local contractors in your area before choosing one over the other based solely on cost. Cost of ICFs varies by manufacturer and wall thickness, but is comparable to poured concrete and CMU construction below grade.

Energy: If properly constructed and insulated, the choice of foundation does not significantly impact the thermal performance of the house located above grade. However, basements do provide additional space that has a natural thermal advantage since it is located below grade where ground temperatures below frost depth are relatively stable. Since basements are earth-tempered, they retain heat in the winter and are cool in the summer, which can reduce heating and cooling loads in basement spaces when compared to above-ground spaces. Overall energy costs per square foot of habitable space decreases when a basement is built and finished as livable space. (See the energy section in House: Basement)

Materials: Frost protected shallow foundations use the least concrete of the systems considered for non-basement construction. The slab-on-grade with stem wall uses a greater amount of concrete than the crawl space, which has a wood floor rather than a concrete floor. This can make the crawl space more resource efficient than the slab-on-grade if the wood floor is constructed with manufactured wood products or wood from certified forests. The garden basement option uses 40% less concrete and nearly 20% less steel when compared with a full height concrete basement. The above grade portion of the wall will most likely use a wood assembly, which has less environmental impact than the concrete wall. (See Assemblies: Wall) Full basement options with the least concrete are the insulated concrete forms and the CMU. Lack of form work does have a material advantage for the ICF system over the cast-in-place concrete wall, which utilizes materials during construction that are disposed of after repeated use.

IAQ: In comparing foundations options, indoor air quality issues in basements are a particular concern since the space is located below grade where it is more susceptible to water, moisture, and soil gases which can negatively impact air quality and habitability. Providing and maintaining a healthy indoor environment in the basement includes strategies for design, construction, and occupation. Design and construction strategies include water-managed foundations that keep rain away from the foundation wall perimeter (including strategies such as gutters, overhangs, downspouts, and grading that slopes away from the foundation). Other strategies provide groundwater drainage away from the sub-grade foundation walls (drain pipes, porous backfill, dampproofing, or waterproofing). In addition, soil gas construction methods and ventilation systems should be used to control possible admission of radon, water vapor, herbicides, methane, or other toxins.

Life Cycle Thinking:

• Energy consumption (especially non-renewable, fossil fuel energy): Purely from the standpoint of use of material, shallow frost protected, slab on grade or slab/stemwall systems have a better environmental performance because they use less material. On a square foot basis, however, the higher amount of material for a basement can be offset by the increase in useable space. Insulated concrete forms, which use less concrete than typical poured concrete walls, include insulation. Wood foundations, although considered a renewable resource, rely on first growth timber with a life cycle of 35 to 120 years. This weights wood heavily toward higher life cycle impacts for residences with projected life times of 35 to 50 years. If the products can be made from lower grade wood materials, grown and harvested from certified forests, the overall impacts are reduced.
• Pollutants generated in production: Wood products need additional additives to protect them from insect infestation, moisture penetration and fire. These additives add negative impacts to the life cycle assessment of wood, and begin to move the comparison of wood and concrete products to be more equal.
• Potential for out-gassing in the building: The chemicals used to treat wood used in foundations may leech out into the surrounding ground.
• Durability of the product: If properly constructed and waterproofed, all foundation types can be durable and long lasting. The lifecycle cost analysis of durability includes: 1) standard durability for slab-on-grade with footings, poured concrete foundation, and CMU; 2) good durability for slab-on-grade with shallow frost footings, garden basement, and wood basement foundation; and 3) better for insulated concrete forms (ICF).
• Potential for future recycling: Concrete has the best program for recycling - it is used for fill or as aggregate in lower grade mixes. Wood foundations, because they are chemically-treated, have little use as recycled material. They cannot be used for landscape mulch and must be landfilled or burned, releasing limited toxins into the surrounding air or water. ICFs, because the insulation adheres to the concrete, reduce the ability of both materials to be effectively recycled.

Practice: While there are unique installation considerations for each foundation type, insulated concrete forms (ICF) are the only foundation system that may need additional installation training, since it is a less common system.

Environmental Context

A basement is inherently more resource consumptive than a slab or crawl space since it is simply larger and uses more materials. Poured concrete or concrete block are the most common construction materials for foundations. Cement is the principal ingredient in concrete. Producing one ton of cement results in the emission of approximately one ton of CO2, created by fuel combustion and the calcination of raw materials. Cement manufacturing is a source of greenhouse gas emissions, accounting for approximately 7% to 8% of CO2 globally.

Citations
Mehta, P.K. "Role of Pozzolanic & Cementitious By-Products in Sustainable Development of the Concrete Industry"

Neitzert, F. et al. Canada's Greenhouse Gas Inventory: 1997 Emissions and Removals with Trends). (Eco-Smart Concrete