Crawl spaces are very common in our mixed humid climate here in North Carolina. Although much concern has been expressed about them in building science literature in recent years, this message has not filtered down to most homeowners. Indeed, even some builders do not yet understand some of the issues.
What is a crawlspace? Crawl spaces are empty areas that separate a structure from the ground over which it is built. Typically two to five feet in height, they have subflooring above them, and foundation walls for sides; many have exposed soil for a floor. Unlike a basement, they are not considered living space. Some houses have multiple crawl spaces, or a basement and one or more crawl spaces. Crawlspaces are usually open to the outdoors by foundation vents and access doors. Many homes have "mechanicals" in the crawl space: water heaters, furnaces, the "indoor" component of the air conditioner or heat pump, and/or the air handler (the main fan for the HVAC system). When there are mechanicals in this space, there is usually ductwork that brings indoor air to the "return side" of the air handler/furnace/air conditioner and "supply side" ductwork to take it back to the indoors.
What problems do crawlspaces pose? Moisture is the most pervasive problem. Even though the soil in the crawlspace may look dry, it conveys moisture from the ground. Rain falling in the yard, or gushing from downspouts, makes the outside ground wet. This water wicks toward any drier dirt it can find, and will move around and under a foundation wall to do it. And keep in mind that cinder block and untreated concrete foundations are porous.
A second source of moisture is our humid North Carolina air. In the summertime, it flows through our foundation vents and condenses on cooler surfaces. This especially includes ductwork and HVAC cabinets that are carrying conditioned air, but can also include floor joists and even the ground. As this happens, humidity in the crawl space rises, and mildew and mold set up shop. Eventually, this can cause floor joists to warp and even fail. Termites and other insects are drawn to moist wood. Not good.
Besides moisture, there can be soil gases down there. Radon seeps up from the ground in many areas of North Carolina, and methane is not unheard of.
Once this soup forms in our crawlspace, where does it go? A little escapes through the vents, thankfully. However, much of it creeps through holes in subflooring (such as penetrations for plumbing and wiring) and up and around rim joists... into the house. Actually, most of the air that leaks into houses comes from the crawlspace. Yuck.
What to do? The solution has two critical components. There are more advanced strategies, and then there are variations on the themes.
The two critical components are really two strategies with the same goal: separate the indoors from the outdoors. The first component is air sealing with caulk, spray foam, and/or mastic. This includes penetrations in the floor above and in the foundation walls. Yes, this includes the foundation vents; though it may still be counter-intuitive, understand that they are part of the problem, because they let in humid outdoor air. Close off any openings that lead to outdoor spaces (such as those under porches), either permanently or with a structure containing a door.
The second component is a complete, durable crawl space liner. Heavy plastic (12-mil) sheeting is one option; a better though more costly option is reinforced polyethylene that is more durable and also a fire retardant. Some contractors use sheeting that functions as a radiant barrier. All seams should be sealed either with tape designed for this purpose or with mastic; the former is much easier. The sheeting should be fastened to the foundation walls and any floor supports, about six inches above the level that water would rise to if flooding resulted from a burst pipe. (Be sure to leave a 3" termite viewing strip at the top.) These "sides" are then attached securely to the plastic "floor." Note that this design is helpful for soil gases as well as moisture.
It is a good idea to also consider ventilating the crawlspace. One option is to install a supply vent (about one per 1000-1500 square feet). It should include a damper to keep air from moving up into the rest of the HVAC system when the fan is not blowing. This conditioned air further dehumidifies the space, and keeps pipes warmer in the winter. There is a mild energy penalty, but in balance, it is worth it.
Another advanced component of this ventilation strategy is a balancing dose of "return air." With a supply vent placing some conditioned air into the crawl space, a mild negative pressure can occur indoors. The typical solution is to place a duct from the outdoors (with a screen and a filter) to the return side of the HVAC system. (A backdraft damper and perhaps a flow restrictor are desirable additions.) This brings fresh clean air into the house and helps even out pressures. The incoming air passes through the evaporator coil in the air handler, dehumidifying the air. Again, there is a mild energy penalty, but the effect on indoor air quality is worth it. Depending on the other features of your HVAC system, an air cycler can be another helpful upgrade; it works with the system's air handler to bring in a programmed amount of air.
An option that is worthy of consideration, especially if your home has sub-par floor insulation to begin with, is to then insulate the foundation walls. This can be done with closed-cell spray foam or rigid insulation boards made from foil-faced polyisocyanurate (recommended), XPS ("beadboard"), or EPS ("pink- or "blue-board"). Decide about this option before the crew starts work on the sealing of the crawlspace; it is best to coordinate those tasks. Also, this is probably the best time to have the same crew seal penetrations in your attic as well, if that hasn't yet been done.
In crawl spaces with severe moisture problems, such as standing water or continual moisture incursion, it may be necessary to also use other methods. Under almost any circumstance, it is a good idea to have drain pipes that carry water from downspouts to a spot 10+ feet from the house that allows flow away from the house. A French drain system on the outside of the foundation and thoughtful downspout/drain pipe systems can be parts of the solution also. A French drain along the crawlspace side of the foundation perimeter that feeds into a sump pump in the lowest corner is sometimes needed. This keeps excess water from the area under the liner.
Some companies will suggest installation of a dehumidifier in the crawlspace. If you do the strategies listed above, you likely will not need one. And a dehumidifier is NOT a good stand-alone solution to crawlspace moisture. Dehumidifiers are costly to install and to run; don't do it unless you have done the above and still need it.