Conventional foundations, while amply strong, have fundamental energy efficiency and sustainability problems—that can be cured. First, conventional foundations are often designed and built without regard to their potential for energy loss (or, looked at the other way, without regard to the contribution they can make to a home’s overall energy performance). Second, the concrete they are typically made from has high embodied energy and therefore, a high carbon footprint. Finally, foundations are typically overbuilt, resulting in an unnecessary use of materials.
Basements. In regions where full basements are the norm, they are often just uninsulated concrete boxes. What’s forgotten in all of this is the amount of heat lost through foundation walls, crawl spaces, and concrete slabs. Although foundations are thermally connected to the rest of the house, too often they’re not designed as part of the thermal envelope. How much this common oversight actually adds to the cost of heating a house depends on a variety of factors, yet insulation can considerably increase energy savings.
One solution is to insulate the exterior of the foundation wall before backfilling. In most heating climates, 2 inches of closed-cell extruded polystyrene, which is more moisture- resistant than expanded polystyrene, should be the minimum. This should be applied over the moisture proofing on the outside of the foundation wall. The insulation helps to keep the basement at a steady temperature—warmer basement walls minimize condensation and the mold and mildew that go along with it if the basement is made into a finished space.
Turning an unfinished basement into living space usually involves adding new interior walls. Builders sometimes use 2 by 4s to build a conventional wall and insulate it with fiberglass batts. A vapor barrier applied to the warm side of the wall, or directly against the foundation, is supposed to keep moisture away from the concrete. But moisture that accumulates inside the wall is trapped, which can lead to mold and decay. A better approach is to attach semipermeable expanded or extruded polystyrene foam panels directly to the foundation wall and tape the joints. This keeps most air and moisture from reaching the concrete and allows the wall to dry to the inside. Gypsum drywall can be applied over furring strips for a finished wall surface, but no impermeable wall finishes should be used.
Slabs are rarely insulated, which means they constantly wick heat or cold to the inside of the house. In areas where the number of cooling degree-days (a quantitative index that reflects the amount of energy needed to cool a building) is very high, heat gain at the edges of a slab foundation can account for as much as 15% of the cooling load. In heating-dominated climates, a lack of insulation often means cold floors in rooms at the perimeter of the house. Edge insulation is an easy answer. It should be installed inside the forms before the concrete is poured. For houses in which radiant floor heating is incorporated in the slab, not only should the edges of the slab be insulated with rigid foam, but also the entire underside of the slab.
The thickness of slab insulation should be determined by climate, but use a minimum of 1 inch of closed-cell rigid foam. When the forms are removed, the insulation sticks to the concrete. When siding is installed over the framing, z-channel flashing can be installed over the foam to protect it. Any foam that’s exposed after the foundation has been backfilled can be covered with cement board or a coat of stucco.
Wood form material is one of the largest sources of wood waste from building a house. Dedicated plywood or metal forms for full-basement concrete walls and stem walls can be used many times. But slab-on-grade foundations are often formed with 2 by 12s, which may come from old-growth trees. They may hold up through three or four foundations, depending on how carefully they are removed from the previous slab, but eventually the boards become unusable. By then, they are covered with concrete and permeated with the chemical release agents that keep the concrete from sticking, so they can’t be recycled.
Although the initial cost of alternatives such as plywood and metal forms is higher, they save the trouble and expense of replacing forms made from dimensional lumber. They also save good lumber from needless destruction. The release agents themselves are often a petroleum-based product or simply diesel—a waste of oil and a potential environmental hazard. Using a biodegradable vegetable-oil-based form-release agent is better.
If you don’t like the idea of the plastics that go into many types of insulated concrete forms, consider blocks made from cement-bonded recycled wood-chips—Durisol wall forms are one such product. They can be installed above or below grade, come in several widths, and are available with inserts of mineral wool to boost insulating values.
One advantage of the wood-block forms is their ability to absorb and release high levels of moisture in the air without damage and without supporting the growth of mold. Some or all of the wood fiber used to make them comes from post-industrial waste. From an insulating standpoint, they also perform well, ranging from R-8 for an 8-inch block to R-20 for a 12-inch block with a 3-inch mineral wool insulating insert. Blocks are dry-stacked and then filled with concrete.
According to Environmental Building News, concrete production produces 8% of global-warming carbon dioxide. One way of reducing this number is to combine concrete with fly ash, a waste product from coal-fired power plants. Using fly ash has a double benefit: It not only provides a way of recycling fly ash but also reduces the amount of portland cement required in the concrete.
Fly ash is a difficult by-product to dispose of. What makes this marriage interesting is that fly ash bonds chemically with cement to make the concrete stronger, more water resistant, and more durable than a batch that uses portland cement alone. Typically, 15% fly ash is added to the mix to yield concrete with a compressive strength of 3,500 pounds per square inch—500 psi greater than a conventional concrete mix. In some parts of the country, fly ash is added to concrete at the ready-mix plant—the builder doesn’t even have to ask for it. Some builders in California are experimenting with mixes that contain as much as 50% fly ash and 50% cement, and find they are working very well.
Concrete containing fly ash does have some drawbacks. It sets up slowly, meaning that construction might be delayed a day or two after the pour, depending on weather. That is what makes it more water-resistant, however. In cold weather, it may require other admixtures to accelerate the setup time.
Crawl spaces are common in many parts of the country, and keep the house off the ground enough to allow for the installation of wiring and plumbing. Building codes typically call for a minimum distance between grade and the floor framing of 18 inches. But raising the height of the crawl space to at least 24 inches will keep all the trades happier.
At a minimum, the floor should have as much insulation as the walls. More is always better. Installing insulation between floor joists keeps the floor warm, but it makes the crawl space an unconditioned space. If ductwork runs through an unheated space, heat loss and the risk of condensation and leaks increase. A better method is to insulate the exterior of concrete walls with 2 inches of rigid foam insulation, just as a foundation is insulated. This creates a heated or conditioned crawl space.
Building codes that require vents in a crawl space can be a problem. Here, code hasn’t kept up with building science. It’s better to keep air and moisture out and make the crawl space part of the insulated envelope, and it’s worth having a talk with your local building official if vents are still required. In any case, the ground should be covered with 8- to 10-mil polyethylene and sealed at the perimeter wall. A layer of sand under the poly can keep stones from puncturing this layer. Seams should be sealed with polyethylene tape. If there are piers in the crawl space, plastic should be sealed around them as well. This keeps the moisture out of the air under the house. The only exception to this approach is when a high water table periodically pushes water into the crawl space. In that case, venting will be necessary. Power vents or fans along with a sump pump might also be required.
So, you’ve got your plans, the site is prepped, and you’re ready to start building: Remember to keep these tips handy to get your project off to a good start.
David Johnston’s work in green building has been embraced by homeowners, building professionals, and sustainability advocates in the United States and internationally. In 2007, he was named the International Sustainability Pioneer by the European business community. Johnston’s previous book, Green Remodeling, has been hailed as the definitive guide to green remodeling.
Scott Gibson is a contributing editor to Fine Homebuilding magazine, and a freelance writer and editor.
This article was adapted from Green from the Ground Up (The Taunton Press, 2008).