Besides holding up the house, a foundation is also a connection between the soil and living space. How this connection is made is important for the health of the occupants and the durability of the house. The foundation should prevent problems due to moisture, radon, and termites, and also be energy-efficient. In short, a foundation should protect the house and its occupants from the negative effects of the ground.
Moisture control is important because moisture’s effects upon foundations can mean mold growth or rot. Mold is a common allergen and a health problem for millions of people. Wood rot that results from excessive moisture can lead to costly repairs.
We do not strictly control Google ad content. If you believe any Google ad is inappropriate, please email us directly here.
One of the most significant moisture sources to control is rainwater. For all foundation types, rain pouring off a roof needs to be channeled away from the building—usually via gutters, down spouts, and splash blocks. In addition, the ground around all houses should slope away to keep the water exiting from the downspouts from soaking into the soil around the structure.
There are a variety of strategies for controlling moisture in foundations. The specific techniques will vary from house to house with differing foundation types, climates, and soil conditions. Generally, moisture control involves using a piping and drainage system to divert water away from the building, as well as coatings or barriers to prevent dampness from passing into the structure.
If a building is in an area where the ground water extends above the footings of a foundation, then a system of drainage pipes or tiles should be installed to divert that water away from the building. A perforated drain pipe is usually located near the bottom of the footing, and the area above it is backfilled with gravel. Water in the vicinity will easily travel down through the gravel into the tile and away from the structure. Synthetic drainage mats now on the market can be substituted for the gravel.
Soil dampness can migrate horizontally through concrete or masonry foundation walls, or up through a concrete slab. Dampness can also rise up through a concrete footing into the foundation wall and then evaporate into a crawl space or basement. As remedial measures, cement-based foundation dampproof coatings are less odorous than asphalt or synthetic coatings, but since they are generally quite well separated from the living space, foundation coatings rarely cause problems for chemically-sensitive occupants of a house. Plastic sheeting is also often used as a barrier in foundation construction.
Radon is a radioactive gas found in small quantities in the soil virtually everywhere. Radon is invisible, you can’t smell it, and you can’t taste it, but it should be avoided because it is a known cause of lung cancer. If a great deal of radon is present in the soil, the gas can contaminate the air in your house by passing from the soil, through the foundation, and into the living space.
While it is possible to measure radon concentrations in the soil, such measurements are not always good indicators of indoor radon levels. This is because all houses are different, and they interact with the ground differently. The only way to know for sure how much radon is in a particular house is to actually measure it. This is easy to do with an existing house, but if you are planning to build a new house you must wait until the house is finished before you can determine if it has excessive indoor radon levels.
In new construction, it makes sense to take inexpensive precautionary measures when building the foundation. In an existing house, indoor radon levels can be reduced by blocking its entry points (caulking and sealing holes in the foundation), diverting it to the outdoors (through a series of pipes), or diluting it (increased ventilation). Since every house is unique, the best radon control method will vary from house to house. Often a combination of approaches will be the most cost-effective.
How Much Radon Is Too Much?
Since radon is a carcinogen, it would be preferable if we could avoid it altogether. However, since radon is found nearly everywhere, this can’t be done. We all breathe some radon all the time. The goal is to breathe as little as possible.
The concentration of radon in the air is usually measured in units of picoCuries per liter (pC/l). If 1,000 people were exposed to a level of 2 pC/l in their house over a lifetime, one of them would probably get lung cancer. At a level of 20 pC/l, about eight would get lung cancer. The EPA has suggested that if levels are above 4 pC/l, you should consider remedial measures. The risk of dying from radon-induced lung cancer at this level is about the same as your lifetime risk of drowning. For further information on what risk is posed by different levels of radon, see the brochure A Citizens Guide to Radon, published by the EPA.
Testing for Radon
Radon can be measured in a few days with a short-term test, or over several months with a long-term test. Short-term testing is more common because it is less expensive, but long-term testing is more accurate. Inexpensive ($10-25) short-term test canisters can be found in building supply or discount stores. If you can’t find a source for test kits locally, ask your local health department for a referral.
To perform a radon test, just follow the instructions supplied with the kit. The usual method is to open the canister and place it in a central indoor location for 1 to 3 days. The opening and closing times are recorded on a form, and the canister is mailed to a testing lab for evaluation. The kits usually contain a mailing envelope. The most common kits contain a few ounces of activated carbon that absorbs radon as long as the canister is open. By measuring the radon in the canister and knowing how long it was open, the lab can determine what the indoor radon concentration is. Results are usually returned by mail within a week or two.
If a short-term test reveals a radon level above 4 pC/l, the EPA recommends that you either do a second short-term test to validate that the first test was accurate or have a professional perform a more expensive long-term test. Again, your local health department should be able to help you find a nearby radon testing service. A long-term test will give you a better idea what your year-round average exposure will be. This is helpful to know because the levels in most houses vary somewhat from season to season. If a house has a radon level of 5.0 pC/l in the winter, but only 1.2 pC/l during the rest of the year, the average year-round level would probably be below the EPA’s 4 pC/l action level.
Often the news that a house is infested with termites will scare a homeowner into calling an exterminator to spray the foundation with chemical poisons. A well-built foundation, not chemical poisons, should be the first line of defense against termites. Even though the termiticides used today are slightly less toxic than those used a few years ago, these chemicals can still have a devastating effect on human health.
There are several different species of termites in the U.S., but the most common is the subterranean type. Subterranean termites live in the ground because they require the moisture of the soil to keep their bodies from drying out. When they leave the ground to eat the wood in a house, they have two ways to protect their bodies from losing too much moisture. The easiest way is to climb up inside hollow concrete blocks to get to the wood. If such a path isn’t available, they will build mud tubes. If they are either inside a block wall or a mud tube, they are safely protected from the dry air. Once such an enclosed pathway has been established, they will travel back and forth between the soil and the wood.
Common Termite Control Strategies
Termites have usually been controlled with toxic chemicals that poison the soil around a house, but metal shields are often recommended as an alternative. The location of metal shields is the same for all foundation types—between the concrete or masonry foundation and any wooden framing members. What many people don’t realize is that termites can easily get around the shields and still attack the wood. They simply travel as far as they can in a hidden and protected way, then build mud tubes around the shields to reach the wood. What the shields do for the homeowner is not prevent a termite infestation, but rather force the termites into building their tubes out in the open where they can be more easily spotted during a regular inspection. While shields are an acceptable way of dealing with termites, it should be remembered that termites can travel through very small cracks. If the seams between the pieces of the metal shields are not tightly fitted and sealed, the termites might be able to squeeze by the shields in a hidden location without building any tubes. If this happens, the termites could attack the house without being detected by a quick inspection.
Some of the newer methods of insulating foundations with foam board materials can provide pathways for subterranean termites that are difficult to block with metal shields. Many designers don’t worry about this and rely on chemical treatments instead. It would be far healthier to take the time to design an adequate shielding system. However, I don’t consider termite shields a permanent solution because, sooner or later, you could get an infestation anyway, and then you would still need to select a low-tox treatment method.
Drywood and dampwood species of termites can be found in the southern and western United States. They do not need to live in the soil and they can attack any portion of a house. Therefore, the metal shields are ineffective against them. Shields are also ineffective against carpenter ants.
If a house is built of a material that termites can’t eat, then no particular control is necessary. For example, a house constructed entirely of concrete, masonry, or steel will be immune to the ravages of termites, as will a building made of redwood or cedar. Chemically treated lumber won’t be attacked by termites either, but the negative health effects associated with most treated wood make it something to avoid in healthy house construction. Some highly toxic treatments, like creosote and pentachlorophenol, are no longer used in residential construction because of the health hazards associated with them. The widely used waterborne salts, many of which contain arsenic and chromium, are also coming under fire for the same reason. For chemically sensitive people, cedar is probably not a viable option either because it is so odorous.
One less-toxic chemical alternative that is effective against termites is the wood treatment called Tim-Bor, made by U.S. Borax. It can be used in both new and existing houses, but Tim-Bor is water soluble, so it cannot be used on wood exposed to weather or to soil dampness. Besides insects, it is toxic to plants and fish, so must be applied with care so that it doesn’t kill your lawn, get into streams, or pollute sewage treatment plants.
Alternative Termite Control Strategies
A variety of non-chemical control methods are available for combating a termite infestation in an existing wood framed house. However, they aren’t always as effective as chemical controls. For dampwood or drywood termites, electrical probes can be used to electrocute them, or liquid nitrogen can be used to freeze them to death. For subterranean termites, a species of nematode is being marketed as a biological control method. Nematodes are microscopic worms that can be mixed with water and injected into a termite nest. They get inside the termites’ bodies and eat them.
Sand of a certain grain size can be used as backfill around a building to deter subterranean termites because when they try to dig through it, their tunnels cave in. However, this method may not be effective if the sand gets damp, because tunnels through damp sand are less likely to collapse. This method of control can be costly to apply to an existing house because of the expense involved in excavating the soil around a house and replacing it with sand.
Unlike termites, carpenter ants don’t eat wood; they simply tunnel through it for nesting. They can enter a house virtually anywhere. Carpenter ants are usually associated with damp or rotted wood because it is easier for them to excavate, so the primary control strategy involves reducing the moisture content of wood—keeping it dry.
Many people think of energy efficiency as applying only to walls and ceilings, but floors and foundations need to be well-insulated, too. Heat leaks out of a house in all directions—up, down, and sideways. If you insulate your walls and ceiling but forget about the floor or foundation, you haven’t done a complete job. Energy efficiency also has an effect on controlling moisture problems. If a foundation is uninsulated, it will be cool, and any moisture in the air can condense on those cool surfaces, resulting in mold growth. While insulation alone can’t always solve a moisture problem, it is often part of the answer.
Different foundation types require different approaches to energy efficiency. The Superinsulated Home Book is a good reference for more in-depth information.
Concrete is one of the primary construction materials used in foundations, and I’m often asked about any negative health effects associated with it. While there are some ingredients in concrete that could potentially bother some chemically sensitive people, I haven’t seen much evidence of an actual problem. In my experience, concrete is generally quite inert.
Concrete is typically a mixture of Type I Portland cement (other types are only used in specialized commercial construction), aggregate (sand and gravel), and water. Sometimes concrete will contain various chemical additives, called admixtures, that give it specific properties. There are many different admixtures in use today. The most common are water-reducing agents for shrink resistance, air-entraining agents for freeze protection, and plasticizers for improved flowability. Admixtures are usually added in such small quantities (a few percent by weight) that they are not a health problem. I have had a couple of chemically sensitive people report being bothered by concrete when it is warmed by the sun. This may be the result of outgassing from the admixtures, or it might be due to something that was spilled on the concrete, perhaps a solvent or a cleaning product. This kind of outgassing could be a consideration with a concrete patio where a homeowner might spend a lot of time, but it is not likely to be a problem with a concrete foundation.
Solid concrete walls are usually constructed by pouring concrete between wood or metal forms. Once the concrete has hardened, the forms are removed. Forms are usually coated with a release oil so the concrete won’t stick to them. I have seen fuel oil used for this purpose. Since concrete is absorbent, the oil can soak right into it, leaving the concrete with a persistent odor. Release oil isn’t always necessary if forms are waxed or painted, or if they are cleaned promptly after they are removed. Contamination from release oil is much more problematic than contamination from the use of admixtures. In commercial construction, concrete may be sprayed with a chemical curing agent to help it achieve its maximum strength. This could bother sensitive people, but curing agents are rarely used in residential construction.
Since radon occurs naturally in some rocks, many people are concerned that the sand and gravel used in concrete could release radon. There are certainly cases of this occurring. In fact, there are houses built with radioactive materials from uranium or phosphate mining operations. However, concrete is not a significant source of radon in new construction. Most radon enters homes from the soil, not from concrete.
Concrete has been used successfully for hundreds of years without release oil, admixtures, and curing agents. Many of these products are proprietary mixtures for which manufacturers won’t release complete ingredient lists, so it is difficult to determine which products actually are the least toxic. I find that it is easier to use regular concrete with no admixtures or curing agents, and forms with no release oils, than to specify nontoxic substitutes. In most cases, it is easy to do so.
Selecting a Foundation Type
Any foundation type can be built in a reasonably healthy manner. It is often more a matter of suiting the foundation to the site. For example, a steep hillside may lend itself to a pier foundation. In warm climates, concrete slabs with little insulation are popular and cost-effective. In colder climates, basements often make more sense. In the middle latitudes of the U.S., foundation types are divided fairly equally between slab-on-grade, crawl space, and basement, depending on personal preference. There are both advantages and drawbacks to all foundation types, so no particular style is correct for all situations or climates. Good foundation design simply requires common sense: You should build a solid, energy-efficient, dry foundation that will allow for a way to keep radon and termites out of the house.
The first step in the process of selecting a foundation is actually related to one of the last steps in the construction process. You need to determine what type of finished flooring will be used. This is because some floor coverings are easier to install on specific foundation types. For instance, ceramic tile will adhere easily to a concrete slab-on-grade, but it is simpler to nail tongue-and-groove wood flooring to a wooden subfloor over a basement or crawl space.
The specific details of a particular foundation design will vary from one part of the country to the next because of differing climatic and geological conditions. The Builders Foundation Handbook and the Building Foundation Design Handbook both contain very good information about all aspects of foundation construction in a variety of climates. Following is a discussion of the basics.
Pier foundations are the least common type; however, they can be one of the best choices from a health point of view. Pier foundations are the easiest to build in a healthy manner because the house and the ground are so well-separated from each other. They have traditionally been a good choice in hot, humid climates, but they can work well in any locale.
The primary drawback to a pier foundation is that it can appear out of place in most subdivisions. However, on the right building site a pier foundation can look very dramatic. I built an office for myself several years ago using a pier foundation. Located on a hillside in the woods, it fits into the landscape quite well.
A pier foundation needs a drainage system if there is a high water table, but since there is minimal contact with the soil, no other moisture control measures are necessary. No radon control is required either. With the space between the ground and the house open to the air, any ground moisture or radon can dissipate harmlessly into the atmosphere.
In new construction, metal termite shields can be placed on the top of each pier. Since the only points of contact with the ground are the piers, they can easily be inspected for signs of termite tubes. If necessary, spot treating the ground where there is an infestation can be done with nematodes. With a pier foundation, the floor system should be well-insulated and contain a diffusion retarder just like the walls of a house. If there are any exposed plumbing lines and heating ducts they should also be insulated, but no additional precautions are necessary.
Concrete slab-on-grade foundations are popular in warm climates because they can be built at less cost than the other types. In cold climates, this type of foundation is sometimes associated with cold floors, dampness, and mold growth. This is because many concrete slab foundations have been built without proper moisture prevention strategies or enough insulation. If constructed correctly, slab-on-grade foundations can be trouble-free. The Model Healthy House has this type of foundation, and it is neither moldy nor damp.
If ground water is present in the soil around a slab foundation, a series of drainage tiles should be installed around the perimeter of the building to divert the water away. To prevent dampness in the soil from migrating up through a concrete slab into the living space, two approaches can be used. First, a layer of gravel or crushed stone can be placed on the soil beneath the slab. If a slab is in direct contact with the ground, the concrete will absorb moisture from the damp soil by capillary action much like a napkin dipped in water. Gravel placed under a slab acts as a capillary break. The second technique is to use a plastic sheet between the slab and the ground to act as a moisture retarder.
A plastic sheet beneath a concrete slab will also function as a radon barrier. Since radon can pass through any cracks in a concrete slab and enter the living space, a floor slab should be adequately reinforced with steel to minimize the chance of cracking. In addition, all openings, such as those around plumbing or electrical lines, need to be sealed with caulking. The goal is to have a perfect barrier between the living space and the soil. Since we live in an imperfect world, a radon removal tube should be installed in the gravel layer under the slab.
Several termite control strategies were discussed earlier, but there are some points that pertain specifically to concrete slab construction. Since it is relatively warm under the center of a concrete slab in the winter, that is where subterranean termites often build their nests. Because of this, all of the wood-framed walls in contact with the slab are vulnerable to termites, and they should all have metal termite shields under them. This is something that is not always easy to do effectively. The use of steel reinforcing in a concrete slab will minimize any cracking that could become pathways for subterranean termites to travel through. However, termites can still chew their way through openings in a slab that were caulked for radon control. A house built with concrete, masonry, or metal walls, rather than wood, will be immune from subterranean termite attack and will not require any control strategies.
To insulate a concrete slab from the cold ground, sheets of foam insulation can be placed under the slab, or in a variety of locations around the perimeter. This is not only an energy-saving feature, but it also will minimize any moisture problems due to condensation. Moisture generated indoors (from bathing, laundry, etc.) can condense on a cool concrete slab.
It can be difficult to incorporate some of these techniques into an existing house with a slab-on-grade foundation. For example, a barrier of plastic sheeting may never have been installed under the floor when the house was built. If you plan to install a wood floor on top of the slab, you can place plastic sheeting between the wood and the concrete. Another option would be to rely on air conditioning or dehumidifiers to lower the indoor humidity levels.
Insulation and drainage tiles can usually be added to the perimeter of an existing house, but the necessary excavation might be costly. Cracks in a floor slab can be pathways for both radon and termites. The best option for the termites would be one of the alternative approaches I discussed earlier, perhaps termite-eating nematodes. One way radon can be controlled is to bore holes in the slab and run pipes from the holes to the outdoors. Suction fans connected to the pipes will pull radon from under the slab and exhaust it into the atmosphere before it can have a chance to enter the house. If the ground under the house has been treated with toxic termite chemicals, a radon suction fan will vent any outgassing from them into the atmosphere along with the radon, thus helping to prevent the noxious chemicals from entering the living space.
A crawl space can be a convenient place to run furnace ductwork, plumbing lines, and electrical wiring. Since the air in a crawl space can contain radon, mold spores, and moisture, crawl spaces need to be thought of as being outdoors and well-separated from the living space in order to be healthy. Crawl spaces are probably not a good idea in hot, humid climates because the warm, moist air can enter the cooler crawl space and condense on the walls.
Moisture control is complicated by the fact that a crawl space is sort of a no-man’s-land. It isn’t heated or air conditioned, but isn’t exactly at outdoor temperatures either. In the winter it is warmer inside the crawl space than it is outdoors, especially if the heating ducts are located there. In the summer it is cooler. What this means is that moisture control strategies can vary, depending on the season. Because of this, crawl spaces are more difficult to design properly than other foundations.
As with other foundation types, drainage tiles should be used to divert any ground water away from a crawl space. In addition, the floor of a crawl space should be covered with some kind of barrier to prevent soil dampness from evaporating. Plastic sheeting is the usual choice, but sometimes concrete is used. The walls of a crawl space, being in contact with the soil, should be coated with a damp-proofing material. Some experts are now recommending that either a plastic barrier be placed beneath footings, or a dampproof coating be applied to the top of footings. This minimizes rising dampness.
Most building codes require that crawl spaces be ventilated. This is to allow any moisture that does happen to enter the crawl space to escape to the outdoors. Ventilation is often provided by 8" x 16" aluminum grilles spaced every so many feet around the foundation. The vents should contain insect screening to keep bugs out of the crawl space. Many homeowners close these vents in the winter in the interest of saving energy and to keep the plumbing lines from freezing. While this may protect the plumbing and save on the heating bills, it can trap moisture within the crawl space and lead to mold growth.
In the summer, the air in a crawl space will be cooler than the outdoor air. If the vents are left open, soil moisture can escape; however, humid air from the outdoors, or from the living space, can enter and condense on the cool crawl space walls. This can be a serious problem in very warm and damp climates. Some experts have suggested that crawl spaces in hot, humid climates not be ventilated in order to better control moisture. However, this is in violation of some current building codes.
The first method of dealing with radon in a crawl space is to isolate the crawl space from the soil. Covering a bare dirt floor is important to prevent radon from entering the crawl space directly. The same large sheets of polyethylene plastic used for moisture control will also act as a radon barrier. They should be carefully sealed to each other and to the perimeter walls with tape. Some builders prefer covering the floor with concrete because it is more durable.
Another way of dealing with radon in a crawl space is to seal the floor system, thus forming a barrier between the living space and the crawl space. In new construction this can be done by placing a well-sealed layer of builder’s foil or plastic sheeting between the subfloor and the finish floor and caulking all electrical, plumbing, and heating system penetrations in the floor.
Ventilating a crawl space throughout the year will help radon escape to the atmosphere rather than enter the house. It is also possible to install a suction fan and pull radon from the soil beneath the plastic sheeting and vent it into the atmosphere.
The primary termite control strategy in crawl spaces is to use the metal shields discussed earlier. It is also important to have the floor of the crawl space at least 18" from any wood framing members. This is because termites can build their mud tubes out in the open, away from the foundation walls, but the tubes aren’t sturdy enough to rise as high as 18". Termite killing chemicals used around a crawl space will not easily be able to migrate into the living space if the house is constructed in an airtight manner, and any ductwork in the crawl space has had all of the joints well sealed.
Many builders insulate the walls of crawl spaces with foam insulating boards. This improves energy efficiency only if a crawl space is unvented. In a vented crawl space, outdoor air entering through the vents negates the purpose of the insulation. Since health concerns related to moisture and radon dictate a vented crawl space, a more effective place for insulation is between the floor joists. With the penetrations in the floor caulked and sealed, the insulation will be separated from the occupants in the living space. One problem with insulating the floor is that the crawl space will tend to be colder in the winter. Therefore, any plumbing lines located in a cold crawl space will need to be insulated to prevent them from freezing. Heating ducts in the crawl space should also be insulated to prevent heat loss and waste of energy.
Many of the same techniques used in new construction can be incorporated into an existing crawl space. Plastic sheeting (from building supply stores) can be rolled out on the floor of a crawl space. Be sure to overlap all seams by about a foot or tape them securely. You should also tape the plastic around the perimeter to the foundation walls.
If ground water is present, a drainage tile can either be installed around the outside, or the inside, of the foundation. Exterior excavation can be done by machine, but it may result in destroyed shrubbery, sidewalks, and driveways. Interior excavation is less disruptive, but must usually be done by hand. Dampproof coatings can be more effective on the outside of a wall, but they often result in some moisture control when applied to the interior surface.
Floor insulation and foundation vents can usually be added to an existing house with a crawl space without a great deal of difficulty. It is also relatively easy to seal the joints in the ductwork with tape or mastic, insulate the ductwork, and caulk electrical and plumbing penetrations in the floor system at the same time.
Basements should be considered living spaces that need to be heated, air conditioned, and ventilated along with the rest of the house. Since they have so much surface area in contact with the ground, healthy basements require a considerable amount of care in both design and construction.
Many people build basements because they consider it an inexpensive way to gain extra space. However, the costs involved in building a healthy, finished, dry, properly insulated basement can sometimes exceed that of constructing main floor space. Given the choice, most homeowners would rather have above-ground living space than basement space, if the costs are similar.
For a basement to be healthy, it must be kept dry. If not, mold can attack anything stored there, including books, clothing, furniture, etc. Moisture control strategies include those discussed with the other foundation types: drainage tiles to divert liquid water away from the structure, dampproof coatings on the exterior, plastic sheeting under the floor slab, and either plastic sheeting under the footings, or a dampproof coating on top of them. Moisture control must be carried out more conscientiously in basement construction than with other foundation types because the basement is living space.
If a basement is constructed in an airtight manner, there will be no pathways for radon to enter. Poured concrete walls are more airtight than masonry walls, but it is still important to caulk all electrical and plumbing penetrations. In most cases it is a good idea to install a radon removal tube under the floor slab just in case any excess radon does find its way into the living space. Sometimes, the radon removal system and a drainage system can be combined. For example, a suction fan can be attached to a tight fitting cover on a sump pump. Since drainage lines are rarely completely filled with water, the suction fan will pull radon through them and vent it outdoors.
The termite controls for basements are similar to other foundation types. Keep them out through crack resistant construction, use metal shields to force them to build their mud tubes in visible locations, and use building materials like concrete and steel that they can’t eat. If a house must be chemically treated, a radon suction fan will help prevent the chemicals from migrating into the living space.
Basements should be insulated for energy efficiency. In new construction, this can be easily done on the outside of the foundation wall with a foam insulating board. A basement can also be insulated on the inside of the foundation. A typical method involves building a conventional wood- (or metal-) framed wall just inside the concrete or masonry basement wall, filling the cavity with insulation, and covering it with drywall. It is absolutely essential to control moisture in the basement before doing this, or else you might trap moisture within the wall and end up with mold and rot. Such a wall should have a moisture retarder to prevent moisture generated in the living space from penetrating the wall and condensing on the cool foundation. It should also be constructed in an airtight manner.
You should heat the basement in the winter to minimize problems associated with moisture condensing on cool surfaces. If the basement walls are well-insulated, moisture condensation on the walls will be less likely. It is also important to exchange the air in a basement continuously with the furnace system or a ventilation system, to keep the air from becoming stagnant. Air conditioning or dehumidifying in the summer will also help to remove moisture from the basement air.
If you have an existing basement that is excessively damp, you should first determine where the moisture is coming from and, if possible, prevent it from entering the basement. A minor dampness problem can often be solved by using a dehumidifier, but if the soil around the basement is saturated with water it may be necessary to install a drainage tile around the foundation. Such a tile should direct the water to a storm sewer, drainage ditch, or sump pump. If the soil surrounding the basement is damp, but not saturated, the walls can be coated with a dampproofing material. While dampproofing can be applied to the inside of a basement wall, it will usually be more effective if it is on the outside of the wall.
With an existing house, installing a drainage tile or an exterior dampproofing could involve the considerable expense of digging a trench around the entire foundation. Solving a moisture problem in an existing basement can sometimes be very costly, and each house can require slightly different techniques, so it is best to get more than one opinion about a solution before starting any repairs.
There are a number of ways to handle radon in an existing house. For example, radon can be kept out of a basement by sealing cracks and electrical or plumbing penetrations, or it can be diverted to the atmosphere by installing a suction fan connected to a pipe installed through the floor slab.
The Model Healthy House Foundation
After analyzing all of the factors involved in choosing a foundation, including cost, I decided to construct a concrete slab-on-grade foundation system for the Model Healthy House. One of the main influences was the fact that I wanted to use ceramic tile for a floor covering. Ceramic tile will yield one of the most inert floors possible. Ceramic tile can be adhered easily to concrete, so a slab-on-grade is a good economic choice. Although a hardwood floor can be fastened to a slab, it is easier and less expensive to attach it to a wooden subfloor of some type. Therefore, a wood floor is usually more cost-effective over a basement or crawl space than over a concrete slab.
Placing the House on the Lot
I built the Model Healthy House on a 3.62-acre parcel in southern Indiana. The land is wooded and the area is relatively unpolluted. A small country cemetery borders the property to the south and a paved road runs along the east and north sides. The nearest two houses are 700 feet to the south (beyond the cemetery) and 500 feet to the west. There was an existing septic system on the property, so I located the house on ground high enough so that a sewer could flow by gravity from the house into the septic tank.
Using some white strips of cloth tied to trees or to temporary wooden stakes, I roughly spotted the corners of the 28' x 56' house where I felt it fit nicely among the trees. I positioned the house so it wouldn’t be too near large trees so their the roots wouldn’t interfere with the foundation. I then walked around the site, looking at it from different angles, and located the garage and driveway. Then I changed my mind and relocated everything. By the third try, I was satisfied that all was in the right place. One of my requirements was that the house be on an east-west axis so that the majority of the windows faced south. This insured that the house received as much free solar heat as possible in the winter.
The house ended up roughly in the center of the lot, with the garage and drive to the north. The driveway meets the road at the northwest corner of the property near the neighbor’s drive. Thus, the house is relatively far from the road, so automobile exhaust at the house will be minimal.
The existing house to the west is heated electrically, so there is no polluting chimney. The existing house to the south is heated with wood and it could be a concern in the winter, especially at night when the smoke tends to hang near the ground. The air filtration system I selected for the Model Healthy House will be able to handle any such smoke. If necessary, the ventilation system can be shut off in the event that the outdoor air is temporarily polluted.
With the house location decided, I cut down the trees that were in the way. Then I hired a bulldozer to remove the stumps and scrape away the upper layer of topsoil. A concrete slab should not be placed over loose topsoil because it generally won’t provide a firm enough base. If the ground were to settle unevenly, the concrete could crack.
Once the area was cleared, I accurately located the corners of the house with stakes and batter boards. This allowed me to create a layout of strings to precisely mark the top surface of the foundation. I later measured all aspects of the foundation from these strings. The tops of the batter boards must all be level and in the same plane. I accurately located the strings so that all distances were correct and all corners were square. With the house all strung and squared up, I used the same procedure on the garage. I placed the 24' x 24' detached garage 17' away from the house to minimize the chance of any exhaust gases contaminating the house.
Once the strings were up, I used them to mark the width of the footing on the ground so that a trench could be dug. The size of the footing is 16" wide and 8" deep. The bottom of the footing was located 32" below the level of the string. The result is 8" of foundation above the ground and 24" below ground. The 24" depth is set by the local building code as the frost depth. In colder climates it would need to be deeper.
Constructing Footings and Foundation Walls
Once the trenches for the footings were dug, I placed steel guide stakes at four- to six-foot intervals in the bottom of the trench so that their tops were precisely 24" below the string. This marked the top of the 8" thick footing. Sometimes wooden stakes are used for this purpose, but they can be an invitation to termites. Stakes cut from a 1⁄2" (#4) steel reinforcing bar (rebar) are a better choice. In order to minimize cracking of the footing, I strung two long lengths of #4 rebar in all of the trenches. It was wired to the steel stakes so that it would be midway between the top and bottom of the footing. Rebar gives the concrete considerably more strength.
Next, I filled the trenches with concrete up to the level of the guide stakes. I ordered the concrete without any admixtures. The concrete was delivered in large trucks, dumped into the trenches, pulled into place with a rake until relatively level, then smoothed with a hand trowel. The steel guide stakes make leveling easy. The next day the concrete was hard enough for me to begin laying concrete block.
Concrete block construction is fairly straightforward. It involves masons building a wall using the sizes of block shown on the Plans. The blocks are held in place with a mortar mixture typically containing Type I Portland cement, powdered lime, clean washed sand, and water. Sometimes the cement and lime are packaged together in bags called “masonry cement.” In cold weather, when freezing can affect the water in the mix, an antifreeze admixture may be added. It is doubtful if admixtures in mortar will have an effect on the occupants’ health since they are added in such small amounts. However, sensitive people may wish to have masonry work done in warm weather so that antifreeze isn’t needed. In the Model Healthy House, I used standard masonry cement (Type N) for the mortar in the concrete block foundation.
Masonry construction always begins at the corners. With a plumb bob, the intersections of the layout strings (house corners) are projected down and marked on the footing. These marks are then connected with a chalk line. Enough masonry corner blocks are laid in place to reach the level of the layout strings. Special care should be taken to get the corners plumb and square with evenly spaced, level rows because the rest of the foundation is based on the corners. The uppermost corner blocks are aligned precisely with the layout strings.
Once my corners were complete, I filled in the rows of block between them. I embedded L-shaped anchor bolts in the concrete block foundation wall to a depth of 16", as required by the local building code. The cores of the blocks around the anchor bolts were filled with mortar to hold the bolts in place. I later used the anchor bolts to attach the house walls to the foundation. An anchor bolt is needed at each house corner, next to doors, and they should be spaced no more than 6' apart. Anchor bolts usually extend about 21⁄2" above the foundation so that a 11⁄2" piece of lumber can be bolted in place. In this case, I built the walls with steel framing, which is very thin, so the bolts didn’t need to extend nearly as high. I left about 1" of the threads exposed on each bolt.
Installing the Foundation Drain
Next, I laid a 4" perforated plastic drain around the outer perimeter of the block wall and covered it with crushed stone to a level that was just below the surface of the ground. The stone was then covered with red rosin paper before the top layer of soil was placed. Available in rolls at most lumber yards, the rosin paper keeps the fine soil particles out of the stone so it doesn’t get clogged. According to Sheets 2 and 4 of the Plans, at the northwest corner of the house, I ran a 4" non-perforated pipe by gravity into a nearby drainage swale. The purpose of this drainage system is to divert any standing water in the ground away from the foundation. If there is a high water table in an area, the drainage system takes on greater importance, and it would be a good idea to locate it near the bottom edge of the footing rather than on the top of the footing as I have done. There are various drainage boards on the market that can be substituted for the stone backfill. Their purpose is to provide a pathway for any water to travel down into the footing drain.
Installing the Plumbing Lines
Once the foundation walls were in place, I installed the below-grade plumbing lines, including the pipes for both the water supply and sewage drains. This is a relatively standard procedure. In most cases, the plumber will work with a floor plan and calculate all of the dimensions for the various pipes. I took the trouble to show all of the required information on Sheet 4 of the Plans. This includes the layout, pipe sizes, material, and their locations.
The plumbing drain lines extend to the outside of the foundation where they then run to the septic tank. There are a variety of regulations that must be followed in installing a plumbing system. The regulations are designed to prevent any water contamination and to insure that the system will function correctly. These requirements are familiar to all licensed plumbers.
The only water supply line under the slab is the main line coming from the water utility company. It must be deep enough to protect it from freezing in the winter. I used extra-heavy Schedule 80 PVC plastic pipe to ensure that it would never develop a leak. Some plumbers will locate most of the water supply lines under the slab because the installation is easier, but if a leak should ever develop under the slab, it would be very costly to repair. To be on the cautious side, I decided to place the hot and cold supply lines above grade within the walls of the house where they would be easier to get to, should a leak ever develop.
The slight outgassing of plastic piping materials can occasionally be bothersome to some hypersensitive people, but when placed below a concrete slab, they are well-separated from the living space and they cannot outgas into the air of the house. The utility’s water main running along the road is plastic and I didn’t see the addition of the 1" PVC supply line into the house as a major additional contribution to drinking-water contamination. In any case, the water filtering system I installed will remove any pollutants added to the water by the plastic piping.
The glues and cleaners used to connect plastic pipe fittings are fairly noxious, but they air out very quickly. Health considerations are more of a concern for plumbers who are directly in contact with them than for homeowners. I only use such materials with plenty of ventilation in order to dilute the dangerous gases.
The drain connections for the two bathtubs in the Model Healthy House needed to be below the surface of the concrete slab. Here I used plastic forms that created a void in the concrete. Such forms usually result in an open hole in the slab that would easily allow radon to enter the house. To prevent this, I secured a piece of foam insulation board to the bottom of the form with 100% silicone caulking and caulked the drain pipe exiting the form. I have found 100% silicone caulking to be very long-lasting. Once cured, it has no odor, so I use it a lot in healthy house construction.
When I later hooked up the bathtub drains, I cut out the top of the form and caulked the perimeter of it to the concrete slab. The end result is a sealed void containing the bathtub drain. I created a similar void in the concrete around the toilet drains with a performed foam cap. During the finishing stages of the house, after the ceramic tile was in place, I dug the foam away with a utility knife and attached a plastic water closet flange for mounting the toilet. After the foam was carved away and the flange was glued in place, I resealed the pipe to the concrete with a can of single-component polyurethane aerosol-foam insulation so there were no gaps for radon to pass through.
I elected to install a 4" radon removal tube under the concrete floor slab. It consists of a plastic pipe that has small perforations along its length, connected to a non-perforated pipe that runs up through the slab and vents outside the house. The perforated pipe is the same material that is used for drainage around the outer perimeter of the foundation. While the Model Healthy House is located in a part of southern Indiana that isn’t known for high levels of radon, there is no accurate way to predict indoor radon levels. An inexpensive radon test can only be performed after the house is built. If such a test shows indoor radon levels to be high, I will hook a suction fan up to end of the tube. The fan will draw the radon-laden air out from under the concrete slab and exhaust it to the outdoors where it will dissipate harmlessly into the atmosphere. This is a highly effective method of handling radon, known as sub-slab depressurization.
At this stage of construction, I had no idea whether or not radon would eventually be a problem. But since the piping system would only add a couple of hundred dollars to the cost of the house, I considered it cheap insurance. If the piping wasn’t installed now and then it turned out to be necessary later, it would be much more costly to do after the fact. Most builders are willing to spend a little extra up front on the chance that a radon removal system might be needed once the house is complete.
Once the below slab piping for both radon and plumbing was in place, I leveled out the crushed stone fill (gravel is an acceptable alternative) around the perimeter, and the insulation was installed. The stone has several purposes. First, it is easy to move around and create a level surface. Second, it will allow any ground water to drain away, insuring that the slab will remain dry. Third, it will allow a radon removal system to function by making it easy for a suction fan to move air between the pieces of stone.
I used approximately three times as much foundation insulation as is common in this section of the country as a part of my superinsulation package. The extra insulation cost a little more at this stage, but it will help to ensure very low utility bills for the life of the house. Extruded polystyrene (XPS) foam insulation is readily available everywhere under several brand names. This is one of the few types of insulation that is suitable for use underground or in damp locations. Expanded polystyrene (sometimes called beadboard or EPS), although it has a similar sounding name, is not believed to be as moisture-resistant. The only other product on the market that can be used successfully below-grade is a product called Foamglas made by Pittsburgh Corning. Foamglas smells strongly like rotten eggs because of the hydrogen sulfide used in its manufacture, it is three times as expensive as XPS and, being a commercial product, it is generally not available to residential contractors.
XPS is one of the materials that has been implicated in destroying the ozone layer because of the CFC (chlorofluorocarbon) chemical used in its manufacture. Fortunately, manufacturers are changing their formulations and HCFCs (halogenated chlorofluorocarbons) are now being used. While they still have a very small effect on the ozone layer, they are many times more benign than CFCs.
It is possible that the HCFCs outgassing from the foam insulation could bother some sensitive people, but in most installations the insulation is well-separated from the living space. I haven’t found outgassing of the insulation to be a problem, in fact, many sensitive individuals can tolerate XPS quite well. Since my choice of insulation was quite limited, I chose XPS over Foamglas as the lesser of two evils.
XPS is sold in a variety of thicknesses. I used a double layer of 2" material. The Plans called for 4" around the perimeter, just inside the upper row of 4" concrete block. This is the most important location to insulate well because it is where the slab is closest to the outdoors. This is where the most heat loss will occur. In addition, I used 4" of XPS under the slab for a distance inward of 6'. The center of the slab has no XPS under it because it is already insulated by the earth beneath the house. Even though soil isn’t a very good insulator compared to XPS, the heat loss pathways from the center of the slab to the outdoors are quite long, so the center is actually insulated better than the edges. Where the XPS needed to be cut around plumbing lines, I found a serrated steak knife quite helpful.
Once I had all the insulation in place, the remainder of the stone was leveled out. I left a trench in the stone fill running down the middle of the house. This was to provide a space for extra concrete that would support the load-bearing wall that was later built in that location. At the ends of the trench, I installed the XPS so that there were no areas around the perimeter left uninsulated.
I covered the crushed stone with a layer of polyethylene sheeting, overlapping the seams by about one foot. Most builders use a thickness of 4 mil, but 6 mil is sturdier. The poly acts as a diffusion retarder, to prevent both radon and ground moisture from entering the house. It was spread in place and run up the sides at the perimeter. Once the concrete slab was completed, I trimmed away any extra material at the edges with a utility knife.
It is important to prevent cracking in the concrete floor slab in order to eliminate pathways for pollutants like radon and moisture to enter the living space. This is an important health consideration. Crack prevention is also important from an aesthetics standpoint. In a house with carpeting, a cracked slab wouldn’t be visible, but with our healthy ceramic tile floor, it could be very unsightly.
One of the most important things to do to prevent a floor slab from cracking is to provide a good base for the concrete. This involves removing any loose topsoil and using a compacted crushed stone or gravel base. Pea gravel or #11 crushed stone (my choice) are both self-compacting, so they don’t require the use of mechanical compaction equipment. Any uncompacted soil or fill material will mean that the slab will eventually settle and such settlement could mean a crack.
The next thing to do is to reinforce the slab. Most builders use welded-wire reinforcing fabric for this purpose. It works reasonably well, but it really isn’t the best product for the job. I chose to use a heavier rebar reinforcing. The Plans called for 1⁄2" diameter (#4) rebar to be placed at 18" on center in about the middle of the 4" thick slab. To prevent cracks, it is also important to use a good quality concrete. I chose concrete with a 4,000-pound-per-square-inch compressive strength, and used the minimum amount of water. Concrete mixed with extra water can be subject to shrinkage cracking as it cures. In order to further minimize the chance of shrinkage cracks, I elected to use a reinforcing fiber admixture in the concrete. This consists of very small plastic fibers that are mixed in directly with the concrete when it is delivered. They aren’t meant to act as structural reinforcing like steel, but they do help to stop shrinkage cracks from forming.
The precautions I took to prevent the concrete house slab from cracking may seem excessive to some builders and may not all be necessary in every part of the country. However, it should be remembered that even when all these measures are taken, there could still be the remote possibility of a small crack forming in the slab. That is why I took all steps possible.
Placing the Concrete Slab
When the premixed concrete was delivered to the site, it was first leveled roughly with a length of 2x4. Next it was smoothed with a large trowel known as a bull float. This evens out the surface and causes some of the fine aggregate to rise to the top, resulting in a stronger surface. As the concrete started to harden, it was troweled to give it a smooth and hard surface. This can be done by hand, but for such a large area, a motorized trowel is easier and faster. Since the weather was very hot, I misted water on the surface for 24 hours to keep the concrete from drying too rapidly.
The Garage Foundation
The garage is not attached to the house so that any odors from a hot automobile (gasoline, oil, rubber) or from materials stored in the garage will be well-separated from the living space. Many garages contain lawn and garden chemicals, cans of paint, gas cans, lawn mowers, and a variety of other products. Odors from all these can migrate into a house attached to a garage. A detached garage is much healthier. If you have an existing house with an attached garage, it may be possible to isolate the garage from the house. This can be done by making the wall between the garage and the living space airtight. For example, caulk any gaps in the wallboard, along the floor, or between the wall and ceiling, and use energy-saving gaskets under the electrical covers. The goal is to seal any holes that could allow pollutants to pass through the wall into the house.
In the Model Healthy House, the garage foundation does not have a separate concrete block wall or a separate concrete footing. Instead, I poured what is called a one-piece monolithic foundation. This is a less costly approach. It was done because the garage was not to be heated, so insulation wasn’t required. A system of forms is used for a foundation of this type. Some contractors have metal forms, but I built mine out of plywood and 2x4s. While the plywood does contain a formaldehyde-based glue, the forms were removed once the concrete was hard and they did not contaminate the concrete. Forms need to be braced very securely to prevent the weight of the wet concrete from causing them to bow outward. It is often common practice to spray oil or some type of other type release agent on the forms to prevent the concrete from sticking to them, but this can leave the concrete with a long-lasting odor. If the forms are removed the next day and cleaned quickly, release agents should not be necessary.
Since a garage isn’t considered living space, radon control is less important, so a minor crack can be tolerated. Therefore, I didn’t use the heavy 1⁄2" (#4) rebar reinforcing. Instead, it was reinforced with 6" x 6" welded-wire fabric. However, the thicker portion around the perimeter does contain two #4 rebars. As in the house foundation, crushed stone provided a good base and a poly barrier was placed under the slab to keep moisture from rising up from the ground. This type of uninsulated slab construction is very common in houses and would be an acceptable healthy foundation in warm climates.
Before the concrete started to harden, I embedded several 1⁄2" diameter, 8" long anchor bolts around the perimeter. The anchor bolts were later used for holding the garage walls in place.
The last job for the foundation of the Model Healthy House was to build the two stone walls by the house and garage entry porches. These walls both have a footing that is 20" wide because they support a wider 12" concrete block below grade. At ground level, I laid a 4" concrete block wall, 3 courses high, on top of the 12" block. This left me with a 4" wide ledge on each side for the facing stone. I selected sandstone from a local quarry that was broken into pieces about 4" thick. Brick could easily have been substituted for the sandstone.
When I laid the 4" concrete block, I inserted small metal ties in the mortar joints. These were then worked into the joints between the stones to help anchor them to the block. With a mason’s hammer, I was able to chip away at the stones to get them into rectangular shapes so that they could be laid together tightly. In order to get a dry-laid look, I bedded only the backs of the stones in mortar. The result looks like a wall that was laid up without any mortar. The larger capstones had to be bedded more securely because of their exposure to weather and abuse. The finished walls are attractive and functional. They are handy for setting packages on while unlocking the door, or for sitting on.
Grading the Land
Once the house and garage foundations were complete, I graded the land so that rainwater would drain away from the house. This generally involves a bulldozer of some sort. Because there were so many trees around the house I didn’t want to damage, I had to hire an operator with a fairly small piece of equipment. It took a little longer, but no trees were destroyed. Some raking and final shaping by hand were then necessary, and the bare ground was seeded with grass seed suitable for shady areas.
I had the crushed stone delivered for the driveway the same day the grading was done so it too could be leveled and smoothed by the bulldozer. After constructing the sidewalk between the house and garage, and laying the concrete stepping stones between the house porch and the redwood deck, I was ready to begin the actual framing.
On the subject of chemically treated lumber, most of the material that is available is CCA treated. CCA stands for Copper-chromated-arsenate, meaning it contains copper, chromium, and arsenic. Though not as widely available, you can also purchase lumber in some parts of the country, that is treated with a somewhat less-toxic preservative called ACQ. It contains ammonia, copper, and quaternary ammonia, and it does not have the same greenish tint that CCA lumber has.
I’ve heard of a few good alternatives to conventional form oil since the Model Healthy House was completed. First of all, there are now some commercially available water-based form oils that are less noxious than the usual oil-based varieties. Second, I’ve talked to builders who have used an off-the-shelf cooking oil successfully with chemically sensitive clients. While this can be expensive, it is certainly a creative solution. For other sensitive people, I’ve also heard of contractors spraying a diluted dish soap or household cleaner on the forms as a release oil. These solutions seem to work fine, but there is one drawback—if the forms are already contaminated with conventional form oil that was used on previous jobs, the concrete may still become contaminated. So they work best with new, uncontaminated forms. The alternative solution I like the best is to simply line the forms with plastic sheeting prior to filling them with concrete. Once the concrete has hardened and the forms are removed, the plastic it simply peeled off. This protects the concrete from contaminated forms, it releases easily, it is fairly easy to install, and contractors generally don’t mind doing it.
One question I’ve often been routinely asked has to do with a concrete slab in the winter: “Isn’t it cold to walk on?” Well, the answer is yes and no. Being well-insulated, the floor slab in the Model Healthy House is certainly warmer than it would be if it were uninsulated. While it isn’t quite as warm as the 68°-70° F air in the room, even if it were, it would still be cooler than body temperature (98.6° F), so it would feel cool to walk on. One way around this would be to embed heating coils in the slab. These radiantly heated floors are becoming popular as a substitute for forced-air heating or baseboard heaters. One of their big advantages is warm floors in the winter. Personally, I don’t mind slightly cool floors. For about 9 years, my wife, Lynn, and I lived in a house with an insulated slab covered with ceramic tile that was almost identical to that of the Model Healthy House. During the winter, I often walked around barefoot, and didn’t find it uncomfortable at all. Lynn, on the other hand, didn’t like the coolness on her feet, so she generally wore slippers. Of course, we had a number of area rugs (for example, in front of the kitchen sink, bathroom lavatory, and bathtub) so our feet were insulated from the slab in those locations by the rugs. Which leads me to an important point. We learned that very thick rugs insulated the floor a bit too well—there was a cooler area under the rug that had a higher relative humidity which resulted in some minor mold growth. This was only a problem with our thick Oriental rug, but not with our thinner woven rugs. This would not be a concern with a radiantly heated floor.
(From Healthy House Building for the New Millennium: A Design & Construction Guide, published by The Healthy House Institute.)
(This article is from the archives of the original Healthy House Institute, and the information was believed accurate at the time of writing.)
(Note: The views expressed in this article are those of the author, and do not necessarily represent those of The Healthy House Institute, LLC.)
The Healthy House Institute (HHI), a for-profit educational LLC, provides the information on HealthyHouseInstitute.com as a free service to the public. The intent is to disseminate accurate, verified and science-based information on creating healthy home environments.
While an effort is made to ensure the quality of the content and credibility of sources listed on this site, HHI provides no warranty - expressed or implied - and assumes no legal liability for the accuracy, completeness, or usefulness of any information, product or process disclosed on or in conjunction with the site. The views and opinions of the authors or originators expressed herein do not necessarily state or reflect those of HHI: its principals, executives, Board members, advisors or affiliates.