A building’s foundation is extremely important to its longevity and performance. As such, it is often the one element where homeowners and builders will tend to choose the “tried and true” techniques and avoid “experimentation.”
This is unfortunate, because the “tried and true” methods and materials typically involve the highest environmental impacts and often the lowest energy efficiency. Most North American homes use vast amounts of concrete in their foundations, and concrete is a perfect example of the kind of energy-intensive building material that has led us to our current environmental state. The production of the portland cement that is the “glue” in concrete requires using large quantities of fuel to heat limestone to very high temperatures to change its chemical composition. In the process the carbon dioxide trapped in the stone is released into the atmosphere (along with additional CO2 released by the fuel used to heat the rock). Cement manufacture is one of the world’s leading sources of greenhouse gas emissions.
Widespread and prodigious use of concrete is only possible when vast amounts of cheap energy can be used to quarry, heat, process and transport the material. Every rise in energy costs will be reflected in a rise in concrete costs. Where once this material was the cheap, obvious answer when building foundations, it is becoming less so all the time.
In the attempt to make concrete foundations more energy efficient, concrete is often combined with foam insulations. These insulations also have dramatic environmental impacts. If we can eliminate concrete use in foundations, we also tend to eliminate foams (though not always). In the following discussions about more sustainable foundation materials, careful thought must also be applied to the insulating of these foundations, and insulation options will be addressed for each system examined.
In considering more sustainable foundation systems, a builder is forced to consider a number of challenges to typical expectations. In much of North America, foundations have been twinned with conditioned, subgrade living space: the basement. In many markets, having a basement is so normal that it can be hard to convince a homeowner to imagine about a house without one. It is difficult to create a sustainable basement and — unless the home is in the driest, best draining of soils — impossible to create a basement that doesn’t rely on several layers of petrochemical products to stay dry.
As you will see in this section, there are many ways to create stable, long-lasting foundations that have reasonable environmental impacts. Most of them, however, do not make basement foundations and those that do come with significant labor requirements. The fact of the matter is that building large, conditioned basements has been a privilege of having cheap energy at our disposal. We are nearing the end of commanding that privilege.
There is one great benefit to moving away from conditioned basement foundations: cost. The cost savings that can be realized by using a sustainable, grade-based foundation are substantial, and can be used to lower the price of the entire project or traded off against sustainable materials or systems that would otherwise drive up the overall cost. It is possible to build with higher-cost renewable energy systems at a competitive cost due to savings on the foundation.
There is no doubt that the most skepticism and wariness about sustainable technologies will happen here, at the foundation. As with any change, the underlying assumption — the “foundation” — is the hardest to change. Yet this is the place that most needs changing.
Building Science Basics for Foundations
A foundation transfers loads from the building to the ground and anchors the building to the ground. To adequately perform this role, a foundation must have enough compressive and shear strength to handle all gravity loads (the weight of roof, walls, floors) and imposed loads (occupants, furniture, snow, rain, wind, earthquakes) placed on the building and prevent the building from moving on the ground.
In areas with cold climates, the foundation must provide stability even when frost has penetrated the soil surrounding the building. When soils containing water freeze, they can expand up to 10 percent in volume and exert pressures upward of 100,000 pounds per square inch, enough to lift or shift a building. When frozen soils thaw, they can become supersaturated with water, resulting in dramatically reduced bearing capacity, enough to cause a building to sink. There are two basic strategies for achieving frost protection for a foundation:
Footings below frost depth. This strategy involves digging into undisturbed soil to a depth lower than the expected frost depth. Building codes will prescribe frost depths regionally. The foundation then becomes a wall that rests on this sub-frost footing and extends to a suitable height above grade to start the floor/walls of the building. Frost walls, basements and piers fall into this category.
Shallow, frost-protected foundations. This strategy involves installing an insulation blanket horizontally around the perimeter of the building to prevent frost from entering the soil beneath the footings. The footing can be at grade or just below grade, minimizing the amount of excavation and material required to build the foundation. Grade beams and slabs fall into this category.
Many of the materials examined in this chapter can be used for either kind of foundation, but some can only be used for one or the other.
The foundation also separates the building from the ground, and this separation must include keeping ground moisture from rising into the building and surface moisture from getting into or under the building.
The foundation must also keep out insects, rodents and other unwanted guests trying to enter the walls or the living space. These pests will vary by region, as will the strategies for keeping them out.
A foundation can play an important role in the energy efficiency of the building. A properly insulated foundation thermally protects all edges of the building. Where floors and/or walls attach to the foundation, preventing thermal bridging and unwanted air movement is particularly important. Strategies for achieving a well-sealed, well-insulated foundation will change depending on materials used and climatic conditions. Don’t fall prey to the common mistake of assuming that “heat rises” and therefore it’s not important to insulate around and under foundations. Heated air rises, it’s true, but heat energy moves effectively in any direction by radiation and conduction. A warm building in contact with colder soils will continuously transfer heat to the ground, which has an almost infinite capacity to absorb that heat. If you don’t want to attempt to heat the entire mass of the Earth’s crust, insulate your foundation adequately!
Durability is of exceptional importance when it comes to foundations. All the other components of a building can be repaired, restored or replaced as they age. Foundations can also be fixed, but it’s rarely easy and usually expensive to do so. If a foundation has a short lifespan, the building above it is usually condemned to the same short lifespan. All of the various building science aspects of the foundation will have an impact on its lifespan, as will the nature of the materials used.
No foundation can be considered sustainable unless it combines adequate strategies for meeting all of these building science objectives and does so with materials that can last a long time in a demanding environment.
None of the foundation options discussed in this chapter need to be used in isolation. It is entirely feasible to meet the needs of a particular project by using combinations of different foundation strategies. Combinations are used to match building loads and needs with site conditions, costs and design of space.
Common combinations include the use of partial basement foundations to create space below the building for services, mechanical systems and/or cold storage. A section of basement foundation can be combined with frost walls, piers or slabs. Pier foundations are often mixed with other systems for additions or to create sections of a building on sharply sloped ground.
When combining foundation styles, it is important to ensure that differential settling will not be problematic and that the places where unlike materials meet are detailed for adequate structural and thermal performance.
Whats not included in this Section
There are foundation materials that are feasible to use, but do not meet the sustainability standards for inclusion in this book. Drawing such lines is always controversial, and there will be green building advocates and practitioners who see this as folly. However, we will present our opinion on these materials and readers who disagree will find ample information in other sources to work with.
Foam ICFs are similar to the cement-bonded wood fiber ICFs detailed earlier in this chapter, except the forms are made from one of three different kinds of foam insulation: expanded polystyrene, extruded polystyrene or polyurethane foam.
Measuring the impacts of foam insulation will reveal high embodied energy figures and carbon outputs, but these alone are not really reason enough to ignore this category of products. As the foam industry is quick to point out, there is a degree of energy savings from using these products (though the same savings are available with similar amounts of other insulators) that can render the embodied energy moot over time. The deeper issue is not measured by straight energy analysis, or even life cycle analysis as it is typically carried out. Neither of these approaches take into account the full impact of the petrochemical industry that produce these products. The full “chain of custody” for foam products needs to address the environmentally disastrous processes of oil exploration, extraction, shipping and pipelining, refining and processing that finally result in supposedly benign foam insulation. Numerous dangerous and environmentally persistent chemicals are key ingredients in creating these materials, and even if they are not proven to be dangerous at the final point of use, their manufacture, storage and use have widespread environmental implications. Human history has taught us nothing if it hasn’t shown us that we are hopelessly unable to “contain” the dangerous chemicals we create and use.
Foam used in buildings is also treated with flame retardants (otherwise the foam is too dangerous to use in a building) that are proving themselves to be dangerous to soil, water and the human nervous system.
Foam ICFs combine all the suspect characteristics of petrochemical insulation with high-impact concrete and rebar usage. This combination of materials will also prove very difficult to disassemble and separate at the end of its life, meaning it’s most likely headed for landfill and not recycling. The whole system is just too dubious in its environmental impacts to recommend in any way, especially when the existing alternatives are every bit as feasible and practical.
Pressure-treated wood foundations are built just like above-grade frame walls, using wooden studs, sill plates and top plates and sheathing the wall in structural plywood on the exterior, except that all the lumber and plywood is pressure-treated to help prevent rot and insect infestation below grade. This can be an affordable, quick and low embodied energy way to build a basement foundation. It has two major drawbacks: the toxicity and environmental side effects of the pressure-treating process and the inherent dangers of putting a material below grade that nature intends to decay in that position.
While the copper-arsenate pressure-treating formulations of the past have now been outlawed because of their toxicity, their replacements use even higher quantities of copper. Copper is a limited resource with highly problematic mining techniques as a major side effect, and its presence in soils and groundwater can create toxicity issues around the building.
Pressure-treatment processes can help wood to remain sound below grade, but it requires great diligence to ensure that the treatment process is intact on all cuts made to the wood, and penetrations made by fasteners can be problematic. Used above grade, wood building materials have an opportunity to undergo natural drying processes that help make them viable. Wood that gets wet below grade is likely to stay wet for a long time, increasing chances of rot and mold, even with pressure-treated surfaces.
Why do we recommend wooden pier foundations (with warnings about possible lifespan issues) but not pressure-treated wood foundations? It is relatively easy to replace piers because the entire beam and floor system are exposed above grade. Bracing the building and replacing piers can be done quite simply, and so if the piers are on a 30–60-year replacement cycle it is feasible to maintain such a building over the long term. But a rotting foundation wall that is buried under the building is much harder and more expensive to deal with, and so problems are more likely to cause the demise of the building or require costly and disruptive repairs.
There are new wood treatment systems making their way onto the market that have significantly fewer environmental issues and offer the promise of long-term protection from rot and infestation. At this time, none of the new treatments are applied to plywood and have not been tested for use as a foundation material
The number of “green” homes built on giant concrete slabs may indicate that they should be among the options given in this chapter. However, the sheer amount of concrete and rebar that go into a typical slab make them very resource intensive. Most slabs are made to be structural, and this means they have a thickened edge around the perimeter of the building and an extensive rebar and/or mesh grid throughout. Often this thickened edge will use as much or more concrete than building a narrow concrete frost wall, and definitely more reinforcing bar.
Another issue that makes a slab foundation difficult to endorse is the amount of foam insulation that typically accompanies a slab, and the weakness of that insulation around the perimeter of the slab where heat loss is at its most dramatic. In most slab construction the outside edge is insulated with foam, but the quantity tends to be minimal as the addition of a wide foam (or other insulation) band around the foundation can create awkward details at the base of the wall. The insulation is rarely seamless between foundation and wall at this junction and creates a large thermal bridge.
As with the other options “excluded” here, it is certainly feasible to build a home on a slab, and to make efforts to use more responsible concrete and insulation. However, given all the excellent solutions available that do not have the inherent environmental problems of slabs, our advice to those trying to meet high sustainability goals is to avoid pouring a slab.