There are three basic types of concrete foundations. In some areas, only a surface slab is possible. Otherwise, crawl spaces and basements are also options. Each broad category has additional requirements and limitations, largely based on the soil, water and geography in your specific area.
Surface-level slab foundations are often a poor choice, although sometimes there is no other option. Slabs are most common in areas with a shallow water table or where a deeper foundation might mean blasting through solid rock. Slab-on-grade foundations are a relatively inexpensive and quick project, but they bring complications.
Pouring a concrete slab requires careful prep work. The ground underneath must be free of roots, stumps and other potential problems. It needs to be compacted and covered with crushed rock. In some areas, a vapor barrier also is required. In places with freezing temperatures, the concrete slab needs extra structural support to help keep the slab from cracking. Perhaps the biggest disadvantage to slab-on-grade foundations is the fact that utility lines are buried under a thick slab of concrete. This makes repairs expensive, and leaks can increase the risk of mold exposure in the home.
Crawl Space Foundations
If possible, dig underground rather than simply laying a slab. A crawl space prevents many problems that can be brought on by slab foundations. It may save the effort and cost of building a full basement, but it doesn't offer the advantages of a basement.
For a crawl space, you will need to pour a concrete footing, and then build short foundation walls. Insulated concrete forms are a popular and sustainable option. These are light but strong foam blocks that hold poured concrete, then remain in the walls and foundation, reducing the amount of concrete required and, again, improving insulation. You also need to carefully add a vapor barrier to keep moisture and mold from creeping into the house.
Crawl spaces allow relatively easy access to plumbing and other utilities, especially if you build deeper than the typical 16 inches, making it a crouching space rather than a literal crawl space.
Finally, retrofitting a crawl space to add a basement is expensive, so don't build with a crawl space if you think you might add a basement later.
If your soil, water and budget allow it, we recommend building a full basement. In fact, you should take it a step further and add some extra depth to the foundation walls, allowing for a full eight feet of clearance below the joists, support beams and pipes.
Basements require extra waterproofing materials, such as liquid asphalt with rubber additives, particularly if you plan to finish the basement space eventually. It can be easy to recover the additional cost of a full basement. Resale value will increase significantly, and finishing the basement is a relatively inexpensive way to add some extra living space.
Basements are the best, but most expensive, concrete foundation option, followed by crawl spaces. Slab-on-grade foundations are not recommended unless absolutely necessary.