Building Science 101: Foundations
Traditional homes in Maine were built on rubble stone foundations, with or without a bit of mortar to hold the stones together. Those foundations were often deep enough to double as a root cellar.
By Michael Maines
HERE IN MAINE, we build houses with basements and concrete foundation walls. At least that’s the conventional wisdom; there are many reasons to rethink that approach.
Traditionally, homes in Maine were built on rubble stone foundations, with or without a bit of mortar to hold the stones together. Those foundations were often deep enough to double as a root cellar, sometimes held a rainwater cistern and were later adapted to accept central heating plants and water heating devices, but only the nicest homes had cellars high enough for a person to stand up straight. These foundations leak air and water, contribute to poor indoor air quality and can be difficult to improve or change. Later, concrete blocks were often used. As the technology for forming and placing poured concrete foundations has evolved, they now dominate the market.
Downsides to concrete foundations
Concrete foundations reinforced with steel bars are strong, durable and somewhat resistant to groundwater in filtration. Their primary purpose is holding up buildings, and they now include hardware to keep buildings from moving in a storm or earthquake. They also provide some relatively inexpensive square footage, convenient for locating and accessing mechanical systems and, in many cases, they are finished to provide living space. In some situations, they are still the best option. But they have several downsides worth considering:
A foundation is a hole in the ground that water wants to enter. Especially in areas with a consistently high water table — the level of groundwater below grade — but also in places where the groundwater is only occasionally high, it’s difficult to keep water from getting into the foundation, due to unrelenting hydrostatic pressure. (Hydrostatic pressure is the force of water or another fluid pushing outward due to gravity and increases exponentially with the depth of the fluid.) Having a good perimeter drain system helps, especially when the house is built on a slope so the drains can lead to open air. But many foundation drains lead to a sump pump, which requires electricity to keep groundwater at bay. Concrete can be specified with extra Portland cement, or with additives such as Pozzolan admixtures that densify the concrete, plugging its pores to reduce water in filtration. Maine’s building code requires “damp-proofing,” typically a thin asphalt emulsion applied to the walls below grade to slow water penetration, but it does not resist much hydrostatic pressure. More effective systems use an elastomeric membrane, a fluid that is sprayed or rolled onto the concrete to create a rubber-like waterproof surface, or a waterproof membrane applied in sheet form to create an impervious barrier. Another approach is to use a dimple mat, like a plastic egg crate that directs any groundwater to the perimeter drain before it can cause problems. Sometimes a mix of systems is used.
Even with those systems, and especially without those systems, concrete foundations tend to be damp. The surface of the concrete may appear dry because the moisture evaporates into the indoor air. But moisture is present and provides sustenance for mold and fungus, leading to the basement smell we are all familiar with and contributing to poor indoor air quality throughout the house.
Concrete is a poor insulator. At R-0.08/in, the only common building materials that conduct heat more readily than concrete are steel and aluminum. Concrete is 15 times more conductive than wood framing and 50 to 80 times more conductive than foam insulation. As long as it’s not cracked, concrete is considered airtight, which can lead occupants to believe that uninsulated concrete walls aren’t wasting energy. Insulating basement walls is required by the building code for new homes, but some builders use an exception that allows the first floor to be insulated instead, which is almost never an effective approach because of all the bypasses to the basement space. Other builders install foam insulation on the exterior but cut if off just below grade so they don’t have to cover it. Not only is that a code violation, it leaves the part of the foundation wall that sees the coldest air, the portion above grade, unprotected.
Maine has a lot of ledge (called bedrock in most other places) close to the surface so some foundations require blasting, which adds cost and complexity, and may annoy neighbors or harm wildlife.
Production of Portland cement, the “glue” that holds aggregate together to create concrete, is a major emitter of greenhouse gasses — comprising about 8% of all emissions worldwide. There are ways to reduce the amount of Portland cement needed in a foundation, but the simplest is to reduce the amount of concrete used.
With all the potential problems, why do we still build with foundations? Alternatives are becoming more common, especially in the high performance building world. For those who still build basements, they can be designed and built to be safe and comfortable; doing so costs significantly more than the traditional, bare-bones approach, though in most cases it’s still less expensive than building new space above grade. But building science knowledge about how to design and build a good foundation is still hard to find, in my experience. Even when I provide details for how to build a foundation that will be warm and dry, with good indoor air quality, I almost always see the builder has made changes when I visit the site.
Greener Options
For these reasons, like many green designers, architects and builders, I prefer to avoid poured concrete foundations with basements when possible. Here are some of the options I have used:
A raft slab foundation on a project built by Maine-based ecocor. Photo courtesy of Michael Maines.
Crawl space. This is basically a short basement and is built similarly. There are two versions: ventilated and unventilated. Ventilated crawlspaces are supposed to allow enough air ow, through exterior vents, to keep the air dry and the surfaces free of condensation (and the resulting mold and fungus growth). Unfortunately, what actually happens in every ventilated crawl space I have been in is that the vents allow warm, moist air into the crawlspace during humid months, where it condenses on cool surfaces. The first floor is supposed to be insulated, but the most common products used for this are fiberglass batts, which do not hold up well against fluctuating humidity and do little to block air ow through the insulation, short-circuiting its efficacy. The vents bring cold air into the crawl space in winter, so people block them with foam plugs and never open them again. In short, ventilated crawl spaces don’t work as intended.
A raised slab, with a frost-protected shallow foundation grade beam around the perimeter. Photo courtesy of Michael Maines.
Sealed crawlspaces can be a good option: they cost less than full foundations and contain less climate-damaging concrete and foam. They can still provide access for mechanical systems, and the higher floor level should result in less groundwater in filtration than with full foundations. Having spent plenty of time in crawl spaces, they are not much fun to work in, and of course they can’t be later converted to living space. But, if done properly, they can be effective.
Slab on grade. Often associated with lower-cost homes, they do cost less than foundations with basements, because they require less excavation, less material and less labor. But they can also be used on high-quality homes. There are a few ways to build slabs on grade; one commonly used is called a mono-pour with turned-down footings. They can work well if detailed correctly, but most are not insulated as well as they should be, and most do not extend above grade as much as they should — our building code requires at least 6” from grade to any wood framing or siding (having crushed or tumbled stone at grade does not obviate this requirement, despite arguments I’ve heard). In fact, most conscientious builders prefer to allow 8-12” above grade, to minimize water splash-back and resulting damage. A variation is what I call a raised slab, or a slab with a frost wall or perimeter beam. This starts with what looks like a short foundation wall, either extending below the frost line, to ledge or protected from frost with foam insulation. The interior is back filled and insulated, and in most cases a concrete slab is poured on top. A few designers and builders, including myself, are now experimenting with foregoing the concrete slab and placing a sub floor directly over the insulation. There are many details to consider, but so far reports from the several projects around the country with this type of foundation are all positive.
Another variation of the slab on grade is called a raft slab. Based on European systems, rigid foam is used to create an insulated bathtub-like structure in which to place a structural concrete slab. Several manufacturers are selling foam products for this type of system, or you can make your own.
A full-height poured concrete “daylight” or “walkout” foundation. Photo courtesy of Michael Maines.
Alternatives to concrete
What if you want to avoid concrete altogether? There are a few options:
Helical piers, tough metal posts drilled into the ground that can support a house or deck, are an alternative to concrete basements. While not yet widely used, they have a long, successful track record in the US and Canada. Photo courtesy of Techno Metal Post.
Permanent Wood Foundation (PWF). Our building code allows foundations to be built using pressure-treated lumber, with a list of requirements. PWFs first gained popularity in the 1980s and many foundations are still in good condition from that era. Others have failed, often as a result of groundwater pressure and in filtration. Many of us who are trying to reduce our carbon footprints have scoffed at wood foundations in the past, but are giving them a second look now. To date I have not used a wood foundation, but may soon on an addition to my house, which I intend to make as low-carbon as possible.
Pier foundations. This category covers a lot of ground. The first new house I helped build, back in the early 1990s, was on 10” diameter concrete piers. (They stuck much farther out of the ground than I now know was safe, but the house is still standing.) When round concrete piers are not aesthetically appropriate, you can buy or make tapered square versions. For a house, the piers need to extend below the frost line, and should be reinforced so they don’t break when frost pushes on them. They also need to be engineered for lateral resistance — while supporting “gravity loads,” engineers’ term for vertical loading, are fairly easy to calculate, providing resistance to wind and earthquake loads, which generally act laterally on a house, can be harder to figure. On one job I was involved with, in a high-wind coastal zone, the engineer specified a section of solid concrete wall to provide bracing against lateral loads because the piers we used elsewhere were not enough. A similar approach I plan to use on an upcoming project is a mini-foundation: a small, full-depth foundation to provide a space for utilities and to brace the structure, but the rest of the house will be on piers.
Another type of pier gaining in popularity is helical metal piles. They look like a round galvanized steel fence post with an auger at the bottom. An operator drives them into the ground like a big screw, with different post sizes and depths providing different bearing capacities. Techno Metal Post of Maine, owned by Mike Brochu, just installed ten helical piles for an addition I designed. At an average of $300 per pile, it was much less expensive than a concrete foundation would have been, and much easier, faster and less expensive than digging and placing concrete piers. Plus the carbon footprint is smaller.
A third type of pier system is post frame construction, also called pole barn construction. This approach has structural posts that support the walls, floors and roof directly buried, so they also provide some lateral resistance. While there are some well-built homes that use this approach, it is generally considered to be more appropriate for unheated outbuildings due to the difficulty with details like air-sealing.
With all of the pier systems, a framed floor is needed, and many people worry about cold floors. With attention paid to air sealing and to insulation, including reducing thermal bridging, a raised floor should feel about the same as any other floor, and heat loss should not be much different. An advantage is that unlike concrete slabs, which require insulation with relatively high carbon emissions (foam or mineral wool), floors built on piers can be made from carbon-sequestering wood and insulated with carbon-sequestering cellulose. It can be challenging to deal with mechanical systems in a house built on piers; including a mini-basement as noted is one option.
A framed floor is needed with any pier system. With proper attention to air sealing and to insulation, a raised floor should feel about the same as any other floor. An advantage is that unlike concrete slabs, floors built on piers can be made from carbon-sequestering wood and insulated with carbon-sequestering cellulose. Photo courtesy of Techno Metal Post.
Which type of foundation is right for you? The answer to most questions in building science is, “It depends.” There are advantages and disadvantages to each of the options. Just consider that perhaps a conventional, full foundation may not be the best option for you, and make sure you are working with someone who understands how to properly implement the system you choose. G&HM
This article appeared in the Fall 2020 edition of Green & Healthy Maine HOMES. Subscribe today!
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