Buildings

Energy Efficient Foundations

Q: One builder we talked with said that building on a slab foundation is definitely the most energy-efficient way to build. Another builder was just as sure that a crawl space is the only way to go. My father-in-law, who lives in the mountains, insists that a basement is the only kind of foundation that makes sense. And my wife really likes houses built on posts like they have along the coast. What kind of foundation really makes the most sense from an energy efficiency point of view?

A: The short answer is that any type of foundation can be part of an energy-efficient house if the details are done right. Many factors, such as soil type, slope of the site, local experience, and material availability enter into deciding what to rest your house on for the next 50-100 years. Besides supporting the weight of your house, your family, all your relatives at Thanksgiving, your piano, waterbed, and hot tub, the foundation needs to control the flow of heat, moisture, and air. Each of the foundation types you mention can accomplish all of that if designed and built properly.

As you've already noticed, people have very strong feelings about the "right" way to build a foundation. Start asking around, and you can hear horror stories about major failures involving each type of foundation:

"Slabs are damp and cause mildew in the house."

"Crawl spaces are wet and musty and if you put your ductwork under there, it will suck mildew and radon and termite spray into the house."

"A basement is where you dig a well and spend the rest of your life trying to keep water out of it."

"If you build your house on piers or posts, the floors will always be cold." As with many aspects of houses, myths abound, and people believe them strongly. Of course, one reason these myths stick around is that these problems really do happen.sometimes.

My favorite foundation myth is the one about the need to ventilate crawl spaces. This particular myth is so popular that it's been put into the building code! The idea is that moisture gets into the crawl space, and we need to let it out, particularly in the summer. So, we cut all these holes in the foundation wall & open them up when the relative humidity is about 95%, so we can let that humid air into the crawl space and that will dry it out. Huh? In the winter, when the outside air tends to be drier much of the time, we're told to close the foundation vents.

Some people advocate unvented crawl spaces, but most builders and inspectors still reject this idea out of hand. Of course, if we make the crawl space a little taller and call it a basement, then, for some reason, it no longer needs to be ventilated.

The real solution, of course, is to prevent moisture from entering the crawl space to begin with. There are several techniques for keeping the crawl space dry. They are covered in our publication Closed Crawl Spaces: An Introduction to Design Construction and Performance available at www.crawlspaces.org.