A Foundation Supports a House While Holding Back Moisture
A slab can be built on a level site with little excavation and a modest amount of materials. With the right insulation details, a slab can be a useful thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. — especially when using in-floor radiant heat.
Crawl spaces are not living spaces, but they can be great places to run ducts, pipes, and wires. They're most valuable when included as conditioned parts of a house — especially to prevent heat loss from HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment.
Basements can provide a lot of living space cheaply. Good water-management and insulation/air-sealing strategies will keep them warm and dry.
Pier foundations support loads at a number of distinct points rather than continuously. Compared to basements or slabs, piers require much less material and excavation, reducing demands on and off the site. Because of their small footprint, they need to sit on stable ground.
Foundations should be well insulated and keep water at bay
Depending on where you're building, a conventional foundation could be made from poured concrete, brick, block, or stone. Because the foundation is buffered by the earth, temperature swings are less extreme than at the roof, but moisture issues are very important: Every drop of water that hits the roof and walls runs off to the ground.
Footing drains help a foundation stave off groundwater. And foundations also have to disrupt capillarity, the tendency of concrete walls and floors to wick water inside. A foundation is a bad place to cut corners because problems are expensive and complicated to fix after a house is finished.
Except in the mildest climates, insulation should always be installed under slabs, at slab perimeters, and at basement and crawl space walls.
Foundation preferences differ around the country.
Slab foundations work well with radiant floor heating. They can also be a finished floor, which saves a lot of money. Slabs work well in areas that have high water tables and, in frost-protected versions, can be an inexpensive way to get out of the ground in cold areas as well.
Crawl spaces are said to offer none of the benefits of a basement and all of the drawbacks, but they can be built to provide warm and dry storage space. Some people choose them in order to save money on excavation and concrete costs up-front. A sealed and conditioned crawl space can be a good place to put mechanical equipment. Building codes often require that unconditioned crawl spaces be vented to the outside, which can bring in excessive moisture and cause health and durability issues.
Basements provide extra potential living space if detailed well enough to stay dry. For the extra cost of digging a little deeper and building the foundation a little taller, you can double a home's living space. Too often basements are an afterthought — not completely inside or outside the thermal envelope and prone to air leakage and moisture problems. Well-detailed basements are warm, dry, and healthy.
Pier foundations are good choices for homes on remote or steep sites. Because they use the least amount of concrete and cause least amount of site disturbance, pier foundations have the smallest environmental impact.
Soil is the forgotten part of a foundation
"...the soil a foundation rests on is part of the foundation system; it's a building material, just like the 2 by 4 studs that frame the house." Says engineer Robert Felton. It's definitely green to follow Felton's practical advice on how soils are the bedrock of durable foundations. Felton's rules of thumb for evaluating, compacting, and replacing soils are nicely covered in the Fine Homebuilding article, "Soil: The Other Half of the Foundation" (see FURTHER RESOURCES below).
DEEPER SHADES OF GREEN
Fly ash can replace Portland cement in a concrete mix
Portland cement is energy intensive to make, and produces about 6% of the world’s carbon dioxide emissions. The CO2 comes from both the manufacturing process and the chemical process of calcining limestone.
One way to reduce the environmental burdens of concrete is to use fly ash (or other pozzolans, such as blast-furnace slag) instead of Portland cement. Fly ash is a waste product from coal-fired power plants.
Portland cement accounts for 12% of most concrete, replacing it with 15% to 25% Type C or Type F fly ash are fairly common, and mixes up to 60% are sometimes used.
Having a tough time convincing your building inspector?
Ask local or regional departments of transportation what fly ash mixes they have tested and support. This can help the building department (and the concrete contractor) get on board.
Tip: Look for a concrete supplier who is familiar with fly ash mixes
A seasoned fly ash mixer can give valuable advice about differences in installation and performance. Concrete block makers also are using industrial byproducts such as slag in their products. The types and grades of fly ash and blast-furnace slag can vary widely in quality and quantity. Some of these materials come from manufacturing or energy production activities here in the United States, but increasingly these materials are being imported from overseas because of higher demand. In this case, the offset may be debatable.
Fine Homebuilding: Soil:The Other Half of the Foundation
- Charles Lockhart
- Chuck Lockhart
May 22, 2010 10:22 AM ET
May 22, 2010 8:40 AM ET