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Green Basics

Slab Foundations

UPDATED 11/15/2012 Slab-on-Grade Foundations Work Everywhere

Concrete slabs are usually less expensive than full-basement foundations and crawl spaces. Slabs require less site work, can be installed in less time, and require less concrete. As long as they are detailed properly, they can be used successfully in even the coldest parts of the United States.

Slab foundations can be the finished floor

Because of the high mass of concrete, slabs can absorb and hold radiant energy in a passive solar design, and they’re ideal for radiant-floor heating systems. Concrete can be acid-etched or dyed to become a very attractive finish floor; in some cases, such floors may cost less than other flooring options.

There are, however, a few drawbacks. Concrete slabs are not the best choice for sloping sites. Plumbing and other in-slab utilities must be carefully planned. Wiring and ductwork are more difficult to install, and alterations after the fact are challenging and usually expensive.

Insulated slabs save energy

Install a continuous layer of rigid foam insulation under the slab, except in the hottest climates or in termite-infested areas where local codes may prohibit the practice. The insulation buffers the slab from outdoor temperature swings, keeps the slab warm and dry, and lowers energy bills.

The U.S. Department of Energy estimates that even in a mild climate, R-10 slab insulation saves 20% more than it costs (in 2008 dollars) — a pretty good return on your money. A continuous layer of foam is particularly critical if the slab includes in-floor radiant tubing. Rigid foam insulation is also a good capillary break, helping to keep a slab dry as well as warm.

One potential disadvantage of foam-insulated slabs is that the insulation can shelter termites from view as they tunnel upward and into wood framing. Because of this risk, some jurisdictions prohibit the use of foam…

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2 Comments

  1. Jay Danielowski | | #1

    insulated monlithic slab
    Why do you need crushed rock under the perimiter of the shallow footing? I am building on sandy soil

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Jay Danielowski
    Jay,
    The vulnerability of any shallow foundation is susceptibility to frost heaving. If you are building in Hawaii, of course, you may have no frost worries.

    A layer of crushed stone under a shallow slab ensures good drainage. If the material under a slab is well drained and dry, it won't be susceptible to frost heaving.

    If you think that your climate and your soil conditions are such that you can omit the layer of crushed stone, you should probably talk to an engineer to confirm your hunch.

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