Foundation

In general, if a basement is built as part of the foundation, it should be designed as livable space. This reduces environmental impacts for several reasons. A basement that is utilized as livable space makes the greatest use of considerable investments in time and material. A livable basement also reduces the need to construct additional above ground space and potentially reduces future environmental impacts associated with additions. Finally, a basement is naturally cooler in the summer and warmer in the winter than above ground space. Livable basements reduce the energy use and cost of homes measured on a per square foot basis. A livable basement requires special attention to improving the indoor environmental quality of the space. Adequate access to light, ventilation, and egress must be incorporated with carefully constructed moisture and soil gas management systems. (See the IAQ section below for more.)

Options and Analysis

alternatives foundation cost ($/sf floor area) global warming potential (lbs of CO2/sf floor area) additional LCA measures
(per sf floor area)
IAQ practice
slab with stem wall (864 sf floor area) $20.88 16.4 energy consumption (Btu)90737 betterstandard
solid waste (lbs) 7.02
air pollution index 1.35
water pollution index 0.0023
frost-protected shallow foundation (864 sf floor area) $16.69 10.9 energy consumption (Btu)67600 betterstandard
solid waste (lbs) 3.69
air pollution index 0.87
water pollution index 0.0023
crawl space (864 sf floor area) $28.1518.1 energy consumption (Btu)131063 goodstandard
solid waste (lbs) 6.78
air pollution index 1.79
water pollution index 0.0023
garden basement, unfinished (1728 sf floor area)* $16.4511.0 energy consumption (Btu)79970 typicalstandard
solid waste (lbs) 4.29
air pollution index 1.03
water pollution index 0.0035
poured concrete basement, unfinished (1728 sf floor area)* $17.3814.0 energy consumption (Btu)89780 typicalstandard
solid waste (lbs) 5.71
air pollution index 1.20
water pollution index 0.0029
concrete masonry unit (cmu) basement, unfinished (1728 sf floor area)* $19.3413.6 energy consumption (Btu)$19.34 typicalstandard
solid waste (lbs) 5.36
air pollution index 1.10
water pollution index 0.0029
insulated concrete form (ICF) basement, unfinished (1728 sf floor area)* $15.7819.5 energy consumption (Btu)108468 bettertraining required
solid waste (lbs) 4.38
air pollution index 1.57
water pollution index 0.0029
permanent wood foundation basement, unfinished (1728 sf floor area)* $14.3014.9 energy consumption (Btu)$14.30 typicaltraining required
solid waste (lbs) 3.51
air pollution index 1.92
water pollution index 0.0041

* The floor area includes the main floor and the unfinished basement. Other than adding the basement, the house size remains the same in all respects. Costs and LCA measures are divided by 1728sf for foundations with basements and 864sf for foundations without basements.
Cost information is based on Means Cost Works 2007. LCA information is based on Athena EIE 3.0.3

Cost
The frost-protected shallow foundation is the least expensive foundation option. It can be 20% less expensive than slab-on-grade with a stem wall. Wood foundations are the least expensive of the full foundation options. Based on cost data for the Twin Cities (Means 2007), poured concrete systems have an edge over CMU; they are about 10% less expensive. Cost of poured concrete and CMU construction varies widely by region - check with local contractors before choosing one over the other based solely on cost. Cost of ICFs varies by manufacturer and wall thickness, but may be faster to construct and cheaper than poured concrete or 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 because it is located below grade where ground temperatures 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. Overall energy costs per square foot of habitable space decrease when a basement is built and finished as livable space. See the energy section on the basement page. Frost-protected shallow foundations are generally better insulated than slab-on-grade foundations with stem walls.

Durability
If properly constructed, insulated, drained, and waterproofed, all foundation types can be durable and long lasting. However, most foundation types have critical issues that must be addressed to ensure durability. For example, wood foundations are particularly susceptible to damage from moisture. Accordingly, extra attention should be paid to moisture management. This includes proper slope and drainage away from the exterior walls, backfilling with porous materials like gravel or sand, careful installation of high-quality waterproofing materials, and careful installation of interior air barriers and vapor retarders. Frost-protected shallow foundations can easily be damaged by careless excavation or use of heavy equipment near the perimeter of the house. This can misplace or puncture the insulation skirt that projects horizontally from the foundation, leading to frost heave and damaged foundation. Garden basement foundations are not as deep as full-height basement foundations and must be planned carefully so that egress windows do not result in frost penetration below the foundation. A full height basement is less prone to frost penetration because it typically extends 6-8 feet below grade, compared to a garden basement's typical 4-5 foot depth. Crawl space foundations are highly susceptible to damage from moisture condensation. Water vapor rising from the ground or escaping through the house envelope, combined with cold air temperatures in the crawl space, can lead to condensation and corresponding mold and indoor air quality issues. Constructing an insulated crawl space and conditioning the space as part of the home's interior volume can address these issues.

Life Cycle Analysis (measures are given per square foot of floor area)

Energy consumption
Purely from an embodied energy standpoint, shallow frost-protected and slab-on-grade foundations have better environmental performance because they use less material. On a square foot basis, however, the greater amount of material for a basement can be offset by the increase in useable space. In addition, basements and garden basements require less energy to heat and cool, making them more efficient than floor space above grade. ICFs typically have greater R-values than poured concrete and CMU foundation walls. This allows ICF's to recoup their initially high embodied energy and global warming pollution with energy savings over the lifetime of the foundation. Wood foundations, although considered a renewable resource, typically rely on large-diameter timber with a growth cycle of 35 to 120 years. This is unfavorable for homes designed for projected life spans of 50 years. In addition, wood used for below-grade applications must be treated with chemicals to withstand high moisture levels and potential for insect and fungus infestation. The toxic chemicals applied and the treatment process itself dramatically increase the embodied energy of the wood.

Solid Waste
In general, foundation types that use less concrete generate less solid waste. Insulated concrete forms use less concrete than poured concrete walls, while also reducing discarded formwork. Frost-protected shallow foundations also use less concrete (and potentially formwork) than slabs with stem walls. Garden basements use half the concrete of a full-height concrete basement wall, while wood foundations use the least concrete of any option. These foundation types all generate relatively small amounts of solid waste.

Pollutants Generated in Production
Wood for below grade applications requires special treatment with toxic chemicals that are applied under high pressure. This is why treated wood foundations generate the highest levels of air and water pollution. Concrete also generates a considerable amount of pollutants during its energy-intensive manufacture. Foundation types that minimize concrete use, such as frost-protected shallow foundations, emit less pollution.

Indoor Air Quality
Indoor air quality in basements is a particular concern because the space is located below grade where it is more susceptible to water, humidity, and soil gases. Providing and maintaining a healthy indoor environment in the basement requires special attention to IAQ strategies for design, construction, and occupation. Design and construction strategies include water-managed foundations that keep rain away from the foundation wall perimeter (gutters, overhangs, downspouts, and proper grading). Other strategies provide drainage away from the sub-grade foundation walls (drain pipes, porous backfill, dampproofing, or waterproofing). In addition, soil gas ventilation systems should be combined with proper foundation design and construction techniques to limit possible admission of radon, water vapor, herbicides, methane, and other toxins. Wood foundations create another concern for indoor air quality because the toxic chemicals used to treat the wood have been found to off-gas and leach into the surrounding soil. Concrete foundations do not off-gas and are considered inert after curing.

Materials
Frost-protected shallow foundations use the least concrete of the systems considered for non-basement construction. If a crawl space is desired, use wood from FSC certified forests for the floor joists and decking. If additional floor space is desired, the garden basement option provides just as much space as a full-height concrete basement while using 40% less concrete. Full basement options that use the least amount of concrete are the insulated concrete form and the CMU. Leave-in-place formwork creates a life-cycle advantage for the ICF system over the cast-in-place concrete wall because formwork is not discarded after use.

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 air or water. ICFs reduce the recyclability of both the concrete and insulation material because the concrete bonds to the insulation.

Practice
While there are unique installation considerations for each foundation type, insulated concrete forms and frost-protected shallow foundations are the only foundation systems that may need additional installation training.

Other Resources

Publications

Web Sites

1 "Cement from CO2: A Concrete Cure for Global Warming?" Scientific American, August 2008.

2 Ernst Worrell, Lynn Price, C. Hendricks, L. Ozawa Meida. "Annual Review of Energy and Environment" Vol 26, 2001. Lawrence Berkeley National Laboratory.