Of the three main foundation types—slab, crawl space, and basement—crawl spaces generally lead to the most performance problems. Think about it. They've got exposed, damp soil right below all that unprotected wood that composes the floor system. Vents bring in cold air in winter and warm, moist air in summer. Vents and crawl space access openings also allow many different types of critters in, everything from cats and dogs to possums and snakes. In fact, I came face to face with a possum, which turned out to be dead, in the crawl space shown here. I've also seen a baby copperhead in an urban crawl space. Let's look at some of the major problems with crawl spaces.

Problem #1. Exposed soil and foundation walls put a lot of moisture into crawl space air. Even when it may not look damp, that soil is evaporating moisture. In the crawl space shown here, plastic covered part of the soil, and the exposed soil looked dry. But when you look under the plastic, you can see that there really is a lot of moisture there.  The darker area of soil is where I pulled the plastic back to expose the wet soil.

Problem #2. Uninsulated or poorly insulated floors make cold floors in winter. They also can allow condensation to form on the cool underside of subflooring in summer, when the crawl space humidity is high. This happens most often near uninsulated sheet metal boots to which air conditioning ducts attach.

Problem #3. Crawl space vents let in moisture and critters. In a hot or mixed humid climate, crawl space vents let more moisture in than they let out. That's because the warmer outside air can hold more moisture. When it enters the crawl space and replaces the air that was down there, the humidity goes up! It's true. Advanced Energy in North Carolina has done the research and proved it. Crawl space vents make no sense. And when the vents aren't screened well, or there are holes in the foundation walls, you get critters. Sometimes they just leave their waste products and make a little noise. Sometimes they die and stink up the joint (photo at right). At the bottom of this page are some other photos of critter problems.

Problem #4. All that moisture in the crawl space causes things to grow--microbial infestations, i.e., mold. Animal waste, mold, dirt...all that stuff is in the air, and guess what. It gets right into your house. Your floor is far from airtight. If there's a bathroom sitting over that crawl space, I can guarantee you that you have at least one enormous hole, the one that the plumbers cut for the bathtub drain. Some people call it the six-pack hole because it's often a little bigger than a six-pack of beer, so the guy upstairs can pass the six-pack to his buddy working down below. Here's a picture of one (left). You can see up into the walls behind the bathtub, so crawl space air gets up in there and then comes out under baseboards, through outlets and switches, and wherever else there might be a pathway. At right is a floor joist covered with mold.

Problem #5. That bad crawl space air can be sucked into the house even faster when there's an air handler and duct system in the crawl space. Any leakage on the return side of the duct system (the side that's pulling air back to be heated or cooled) pulls in crawl space air. One of the worst culprits for this is what's called the panned joist return or just the panned return. Instead of running an actual duct all the way to the vent, the installer takes a shortcut and uses a floor joist cavity by covering it with sheet metal (see photo on right). These constructions are notoriously leaky.

The Solution. So, how does one reduce or eliminate these problems? The answer is crawl space encapsulation.

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