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Foundations

A foundation in the Northeast has a lot of jobs to do: it keeps sensitive parts of the structure protected from the ground, it provides a sturdy and level surface on which to build the structure, it keeps the house above it from sinking, by spreading its weight like a snow-shoe in a snowy field, and it prevents the house from rising, due to frost-heave, when water below the building freezes, expands, and pushes the building up. We will be discussing the differences in how different foundations achieve these goals below.

Standard Concrete Foundation: Frost heaving occurs when water that is under the house walls freezes and expands, lifting everything above it. The most common way to prevent this result is to fill the space under the house walls down to the depth at which water will not freeze (the frost line) with concrete; this depth varies greatly and is affected by a variety of variables, such as local climate and snow coverage. Since the concrete is continuous to a depth where the ground does not freeze — effectively displacing any water from collecting below the structure — it is held stable in the soil. The concrete walls need only go down as far as frost depth; such walls are called frost walls and can be used with crawl spaces or earthen floor systems. The advantages of concrete are that it is very strong in compression (and when properly reinforced, in tension) — particularly important when resisting soil pressure to create underground rooms, it is not easily damaged by water, and it is readily accessible in most parts of the country, from hiring a truck to pour to sub-contracting out the entire foundation construction. The trouble with cement: Standard concrete mixes are made of gravel, sand, and cement. The cement is the binder that holds the mix together and makes it act like a rock. In fact, cement is made from rock. Limestone is heated to very high temperatures to make it, and therein lies the problem. It takes a good deal of fossil fuel to cook the limestone, depleting resources and creating carbon dioxide. Furthermore, the process of changing limestone to cement also releases carbon dioxide, further contributing to the greenhouse effect. Worldwide, cement production accounts for about 10% of total greenhouse gas emissions. The less cement we can use in our foundations, the better. [back to top]

High Fly Ash Concrete: Fly ash, a by-product from coal-burning power plants, can be added to concrete mixes to make them stronger. Because of this added strength, the amount of cement in the mix can be reduced. While many concrete companies routinely add a small amount of fly ash to their mixes, it is possible to specify a high fly ash content in order to save significant amounts of cement, and thus reduce the amount of climate changing emissions related to the concrete. High fly ash concrete does behave differently when it is poured, so be sure to discuss it with your concrete contractor. In general, high fly ash concrete is fine for frost walls and grade beams; it is trickier to finish as a floor slab.
[resources: Making Better Concrete, by Bruce King, www.ecobuildnetwork.org] [back to top]

Limecrete: Limecrete is very similar to concrete, except that instead of Portland cement as a binder, it uses hydraulic lime. Lime and Portland cement are both derived from limestone, but making building-grade lime requires less fossil fuels, and outside of transportation and other external carbon loads, it does not contribute further carbon dioxide to the atmosphere because the carbon dioxide that is driven off to make the lime is reabsorbed when it cures. Limecrete is softer than concrete, so a floor made from it may show patterns of use over the years.
[resources: www.strawbalefutures.org.uk] [back to top]

Piers: Pier foundations are often prized for being an affordable foundation system with minimal impact required. Piers of treated wood, concrete, or other structural material are buried in the ground below frost-line, connected to a footer that helps spread the structural load evenly onto the ground and resist uplift of the pier from frost heaving. Pier foundations are designed to support the structure of the building, and are placed strategically under point loads in the building, such as posts in a post-and-beam structure, or to carry beams that span the piers and support the floor. Although piers require less materials and excavation than standard concrete walls, they carry the liabilities of having multiple disconnected points in the foundation that could heave if not installed correctly, and exposing the area between the floor and the ground to cold winter air, which can greatly reduce thermal performance and issues with freezing water pipes, if not designed correctly. [back to top]

Insulating Concrete Forms: Insulating concrete forms, or ICF's, are various sorts of hollow blocks which are stacked where the foundation wall will be. The inside and outside surfaces are made of an insulating material, while the center is left open to be filled with concrete, either as a solid wall or as a structural grid. Most ICFs such as Ecoblock, are made entirely of styrofoam and plastic. Rastra block is made from recycled styrofoam chips held together with cement, while Durasol and Faswall blocks are made of wood chips bound with cement. All of these systems create a concrete wall of the same thickness as a standard concrete foundation; their advantage is that the resulting wall is well insulated without having to take the extra step of applying foam to the foundation. Additionally, ICF walls generally use 25% less concrete poured than a solid concrete wall. Rastra, Durasol, and Faswall can also be plastered with clay or lime plasters without any need for lath. The drawback of the unrecycled styrofoam ICFs like Ecoblock is that styrofoam has notable environmental costs. Rastra, Durasol, and Faswall contain no new styrofoam, but they contain a high proportion of cement, with its high-carbon impacts; some reports indicate that certain ICF systems have a higher cement content than solid cement walls. One must weigh the energy savings from good insulation value against the impacts of the materials. For these reasons, it is best to limit the use of ICFs to the parts of the house that are near or below ground level, building above-ground walls with friendlier materials.
[resources: Environmental Building News] [back to top]

Masonry Walls: Although concrete is currently the most common form of buidling a foundation wall, for hundreds of years in New England foundation walls were built from the stones gathered from the land, and many are still around holding up older buildings in towns across the region. There are certainly drawbacks to building with native stone: it is a slower process, it involves considerably more skill (especially for dry-laid, or mortar-free, walls), they are more difficult to insulate and seal from moisture. However, stone is an abundant, often free resource, and can be used to create a very durable and stable foundation wall when placed in the right hands.

Urbanite refers to reclaimed or salvaged masonry material, most commonly chunks of cement block or pavement. Urbanite is amaterial that can be used either in slip-formed masonry walls (when stones or chunks of urbanite are placed in a form with cement mortar to build a wall, not unlike concrete) or in some cases as rubble for a rubble trench foundation (see below).
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Gravel (Rubble) Trench Foundation: The gravel trench is an entirely different strategy for avoiding frost heave: instead of displacing water, it simply drains it away from under the house walls. This foundation is built by digging a trench exactly where the walls of the house will be. The trench need only be the width of a small backhoe bucket. The bottom of the trench is sloped slightly toward one corner, where an outlet trench drains to daylight. The trench is lined with filter fabric and a perforated drainage pipe is laid in the bottom and connected to an outlet pipe to daylight. Washed and screened gravel is placed and tamped with a pneumatic tamper, until the trench is filled. A concrete footing, called a grade beam because it is at ground level, is typically poured on top of the gravel. It is often desirable to build a short stemwall of durable materials on top of the grade beam, to further separate moisture sensitive walls from the ground.

The gravel trench foundation cannot be used for a building with a basement. It works very well for crawlspaces and for earthen floors. Unless there is a crawlspace with an insulated floor above, it is strongly recommended to insulate the grade beam and stemwall to improve the heating efficiency of the building. Very flat sites that cannot drain to daylight are not easily suited to this foundation system.
[resources: "Rubble-Trench Foundations: A simple, effective foundation system for residential structures" by Elias Velonis, Fine Homebuilding Issue #180, January 1984, pp. 66-68, Taunton Press] [back to top]

Frost Protected Shallow Foundation: This third foundation strategy prevents frost heave by insulating around the perimeter of the building so that the soil and water below cannot freeze. Following this strategy, the bottom of the foundation footing can be placed 12" to 16" below grade, saving a significant amount of concrete (if that is to be used). The foundation is wrapped in foam insulation and the building may also be surrounded by a layer of foam horizontal, or "wing," insulation, if the severity of the climate and the design particulars call for it. This strategy may require more foam than other approaches, an environmental cost which must be weighed against the other alternatives. Foamglass insulation is a non-toxic and extremely durable alternative, although its insulating value and price are not ideal. [back to top]

Rammed Earth Tires: Old car tires can be used as permanent formwork for rammed earth. They can be used to construct a retaining wall below grade for an earth-bermed house, if extensive and carefully detailed drainage is also built in. They may also be used above grade for shorter stemwalls or piers. In this application, gravel rather than dirt makes the best fill, since it is unaffected by moisture. Rammed gravel tires over a gravel trench foundation are an excellent combination, not only because of the low environmental impact of the materials, but also because the tires are outside the living space, keeping undesirable off-gassing outdoors.
[resources: www.strawbalefutures.org.uk] [back to top]

Earthbags: Instead of tires, one can use woven polypropylene bags or tubes as permanent containers for rammed earth. The term for this technique might also be known as Flexible Form Rammed Earth, paying reference to both the rammed-earth nature of the wall substance, as well as the flexible nature of the forms — the bags. The bags are filled in place on the wall and tamped to provide a very solid base for the walls above. Barbed wire is run between courses to "mortar" the bags together and help provide tensile strength. Polypro tubes can also be used for longer walls or coiled roofs. Again, earthbags may be used in bermed or below-grade locations, provided adequate drainage around the outside and stabilized earth used in the bags. Generally in this climate, a rubble-trench foundation (RTF) would be put in below grade to control ground moisture and eliminate frost heaving, with earthbag stem walls placed over the RTF; however, below-grade structural applications can work quite well if one can account for all the variables. Earthbags can be plastered upon directly, making for seamless transitions into other types of wall systems. It should be noted that the polypropylene bags are not UV-stable, and will degrade in sunlight if not protected.
[resources: www.okokok.org] [back to top]