The purpose of a foundation is to support a house and transmit its loads to the ground or soil. Well-built foundations made from the right materials can make a significant difference in the integrity of the house and the health of those living in it.

There are many ways to build a foundation. The specific materials and techniques will depend on the style of the house, the lay of the land, the type of soil and the normal depth of frost penetration in the winter. In general, any foundation can be built in a healthy manner.

To design a healthy foundation, you should be concerned with:

• Soil conditions; to minimize uneven settlement
• Material selection; to minimize outgassing into the living space
• Moisture control, to minimize dampness and mold growth
• Radon mitigation; to reduce lung cancer risk
• Deterring termites; to avoid the need for pesticides
• Energy efficiency; to save on energy costs and minimize condensation.

Soil Conditions

When built on weak or unusual soil, foundations may be prone to uneven settlement. This can cause cracking or damage to the foundation. Foundation cracks are not only unsightly, they can be unhealthy as well—pollutants can easily pass through very small cracks in a foundation, particularly when the lower portion of a house is depressurized.

Cracks can also allow moisture to pass through a foundation. And subterranean termites can invade a house by passing from the soil through cracks in the foundation

When unusual soil conditions exist, it’s a good idea to have a qualified architect or engineer perform some soil-analysis testing as a part of the design process.

Material Selection

In some cases, part of a foundation may be exposed directly to the living space. So while a basement wall supports the structure of the house, it can also be the finished wall of a basement bedroom or recreation room. When a foundation is directly exposed to the living space, material selection can influence indoor air quality.

Concrete and masonry are popular choices for constructing foundations. Masonry materials are usually well-tolerated by sensitive people, but some masonry products contain chemical additives which can be problematic for some people. The same is true for concrete and masonry sealers.

Pressure-treated wood containing arsenic salts is being used more and more for foundations. Often called All-Weather Wood Foundations (AWWF), these foundations have a number of advantages: they’re less costly, easy to insulate and easy to finish.

However, pressure-treated wood can contaminate basements with arsenic dust. If the arsenic gets into the occupied part of a basement, children playing on the floor can easily get it on their clothes and hands and inadvertently into their mouths. With proper design and tight construction techniques, contaminants from building materials should be well-separated from the living space.

Moisture Control

Moisture control is very important with all foundations for a number of reasons.

If the structural design is flawed, wet soil freezing in the winter can apply tremendous pressure to a foundation and actually push the building with enough force to cause structural damage. Excessive moisture can also result in decay, deterioration of mortar joints, or structural damage.

More commonly, moisture passes through a foundation into a basement or crawl space where it results in mold growth and musty odors. While mold in basements should be avoided at all costs, mold growth in crawl spaces can also contaminate the living space via leaky ductwork or in a loosely constructed house.

In general, foundations should be designed to prevent moisture from migrating into any enclosed spaces (i.e., basements, crawl spaces, or above-ground living spaces).

Moisture passes through a foundation by four mechanisms:

• liquid flow
• capillary action
• air movement
• diffusion
  

Liquid Flow

Liquid flow occurs when water actually flows through cracks in a foundation. The water can originate above ground (e.g., rain) or below ground (e.g., a high water table), and it can easily move through cracks in a foundation into a basement or crawl space.

Capillary Action

With capillary action, water is sucked into concrete in the same way water wicks into a paper napkin dipped into a glass of water. In a foundation, soil moisture can wick up by capillary action through the bottom of a footing, into a foundation wall and be released from the upper, exposed portion of the wall—either inward or outward. This is also called rising damp.

Air Movement

Moisture can also be transported through cracks in a foundation by differences in air-pressure. This is the same mechanism that accounts for moisture and pollutant transport in above grade construction.

Diffusion

Finally, a certain amount of moisture can be transported through a foundation by diffusion—moisture molecules traveling through the solid surface of the foundation itself.

Radon

Radon is a radioactive gas that is often found in the soil. It can get into a house by diffusing through the solid materials making up the foundation. It can also enter a house if there is an air-pressure difference to pull it indoors through random cracks or holes in the foundation. As with above-ground moisture and pollutant transport, air-pressure differences move significantly more radon indoors than diffusion—often one-hundred times more.

Radon reduction usually involves one or more of the following mitigation strategies:

• Sealing cracks and pathways that air-pressure differences push or pull radon through
• Manipulating air-pressures in, or under, a house so radon won’t be pulled through the cracks and pathways that can’t be sealed
• Increasing the ventilation rate in the house to dilute the concentration of radon that does enter the living space.
  

Since radon can’t be measured until after a house is built, it makes sense to plan for simple radon mitigation during construction. Roughing in a sub-slab radon depressurizationsystem might add no more than $100-200 to the cost of a house. When installed after a house is built, a complete system typically costs over $1,000.

The mechanisms that allow radon to enter a house can also cause other VOCs to enter as well, such as metabolic gases from soil microbes, termiticides, lawn chemicals, chemicals found in hazardous landfills, contaminated ground water and hydrocarbon spills. This means that whenever the basement is depressurized, any gases in the soil around the house will be pulled into the basement unless mitigation measures are in place.

Termites

Subterranean termites can pass through tiny cracks in a foundation to get at the wooden structure of a house.

While subterranean termites feed on the wood in a house, they live in the soil. Their bodies dry out very easily so, if they are not regularly exposed to the moisture in the soil, they cannot survive. As a result, they constantly travel back and forth between the soil and the wooden parts of a house.

General strategies

In new construction, it’s important to keep lumber scraps, wood debris, stumps, etc., out of the backfill around a foundation. When this type of material is buried, it can be easily found by termites and is an invitation to infest the house itself.

The wood portions of a house should be at least 18" from the soil. This will make it more difficult for termites to get from the soil to the wood. Most importantly, there should never be any direct contact between wood and soil.

One of the best methods of avoiding termites is to use building materials that termites cannot eat, such as concrete, masonry, metal or termite-resistant wood.

Termite barriers

Termite barriers are often installed in conjunction with foundations. They work by denying termites access to a house.

Chemical Barriers

Chemical barriers are traditionally the most common termite barrier. Toxic chemicals are injected into the ground around the house which forms a barrier subterranean termites won’t cross. To be effective, the chemicals must extend underneath as well as around the structure.

The poisons used as chemical barriers to control termites constitute a significant health threat to human beings. In the same way radon and ground moisture gets into houses, termite chemicals also move from the soil into the living space—primarily with air that moves through holes and cracks in the foundation and secondarily by diffusion.

Termite Shields

Termite shields are metal barriers that are placed on top of a concrete or masonry foundation wall to block termites from getting up into the wooden part of the house.. The shields force termites to build mud tubes around the shields so you can see where they are. Shields are not often used because they can interfere with the siding and the interior finish in basements.

Sand Barrier

A relatively new low-tox approach to subterranean termite control is the sand barrier. With this strategy, a layer of sand is placed under and around a house. If the sand has a certain grain size, the termites can’t tunnel through it—the individual grains of sand are too large and heavy for them to move out of the way and the spaces separating the grains are too small for them to fit between. Sand barriers show a great deal of promise—but they must be installed conscientiously.

Steel Mesh Barriers

Stainless-steel mesh barriers are used under and around foundations to prevent termites from entering a building and getting to the wood.

The big drawback to termite barriers is that their installation must be done perfectly for termites not to find a way around or through them.

Termite Bait   

A relatively new and less toxic termite-control method is the bait-block technique. With this approach, blocks of wood are strategically placed around a structure. These act as bait and they are sometimes treated with an attractant, or feeding stimulant, along with a termiticide. They draw termites away from a building and also kill them.

Nematodes

A biological method of attacking subterranean termites involves the use of a particular species of nematodes. These microscopic worms are mixed in a water solution and injected into the wood or soil near termite colonies. Nematodes are natural predators that seek out the termites and destroy them. They will live for up to two years—depending on moisture conditions.

Actual tests of nematodes have yielded mixed results with some applicators claiming 95% effectiveness, and others reporting fifty percent. Some states require nematodes to be registered as pesticides and applied by certified pest-control professionals.

Energy

Controlling energy losses with insulation is important when part of a foundation is directly exposed to the living space. This will help keep utility bills low and minimize condensation and mold growth by keeping the interior surfaces of a foundation close to room temperature.

The energy efficiency of a foundation also has an effect on health. This is because the temperature of the ground is generally different than the temperature in the occupied space. This can lead to cool surfaces near parts of the foundation that are exposed to the soil, and a high relative humidity near those surfaces that can result in mold growth.

Foam insulation boards are often used to insulate foundations. Outgassing is minimal from these products, especially once they have been covered with soil or a finishing material. However, there can be a problem with foam insulation with regards to insect control—termites can travel virtually unnoticed between the foam and the foundation wall—or they can tunnel through the foam itself.