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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.

make crawl spaces generous

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.

Building codes may require only 18 inches of clearance in a crawl space, but increasing that to at least 24 inches makes it easier to work in this space. Either the floor or the crawl space walls should be insulated.

Shallow Frost-protected Foundations

The National Association of Home Builders' Research Center has developed a unique way to reduce the amount of concrete in foundations. Rather than pour footings below the frost line in cold climates, researchers placed footings just 2 feet below grade and then insulated the slabs in two ways. Walls were insulated conventionally with 2 inches of rigid closed-cell extruded polystyrene foam. Then, horizontally, a section of rigid foam 2 feet wide was placed around the perimeter of the slab 24 inches below grade.

These added layers of insulation keep frost from being driven deep into the ground where it could crack the slab. Model building codes have recognized frost-protected shallow foundation design principles for more than a decade. In most regions of the continental United States, these foundations can reliably be placed as shallow as 16 inches below grade. Performance has been proven in Europe, and frost-protected foundations have been used in very cold climates in the United States—in North Dakota, for instance—with great results.

It takes a lot less concrete to form one of these slabs when compared to the more conventional approach of placing concrete stem walls below the frost line before the slab is poured. That means savings for the buyer, as well as less energy and material use—all without sacrificing performance.

Rigid foam insulation

Rigid foam insulation

Rigid foam insulation mi

Frost line

BEST pRACTICES

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.

• Foundations, including concrete slabs, should be insulated to reduce heating and cooling loads. Insulating the outside of the foundation is better than adding insulation on the inside. If you do insulate inside, use rigid foam instead of fiberglass batts.

• When planning a slab-on-grade house, use a frost-protected shallow foundation design. It uses less concrete and requires less excavation.

• To combine structure and insulation, consider insulated concrete forms.

• Using foundation forms made from dimensional lumber will waste material. Use plywood or metal forms instead.

• Make sure the concrete you order contains fly ash. That makes good use of an abundant industrial by-product, and makes better, more durable concrete.

• Use moisture-proof coatings and perimeter drains to keep moisture out of foundation walls and lower the risk of mold, while making basements more comfortable.

• Raise crawl space height to 24 inches to make access easier and include the crawl space in the conditioned space of the house.

• Engineer the foundation for the loads it will actually carry—and not simply to industry standards—to conserve resources and make the house more affordable.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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