by Alex Glenn and Tommy Blair
Roughly 15% to 20% of homes built in the U.S. each year have crawl space foundations. They're cheaper to build than full basements and more functional than a slab, offering a convenient place for plumbing, wiring, ductwork and heating or cooling equipment, as well as some bulk water resiliency.
Twenty years ago, nearly all crawl spaces were ventilated with outside air in an effort to control moisture. Most building codes required such venting.
The problem is that atmospheric venting is ineffective, to put it mildly. It can actually cause moisture problems, especially in humid climates when warm, moist air enters the crawl space and condenses on the framing.
Many builders and remodelers tried to address these problems by bringing in even more outside air, either passively by building more openings into the foundation, or actively by installing fans in the crawl space. This usually made the problems even worse.
Things began to improve in the early and mid-2000s after field research by Raleigh, N.C. Advanced Energy in mixed-humid climates found that properly detailed closed crawl spaces (with no atmospheric vents to the outside) not only avoided those moisture problems but also made homes generally healthier and more comfortable. Such homes had warmer floors, reduced drafts, less dust, fewer pests, and more stable indoor relative humidity.
Thanks in part to that research, most codes now allow properly detailed closed crawl spaces.
The key phrase is properly detailed, and in fact, builders are more willing to close their crawl spaces once they've been shown those details. Many have an initial fear that they will actually introduce problems, but this fear goes away once they know how to do the job right.
Proper detailing is more than eliminating vents: it's a systematic approach to air-sealing, insulation, water management, and space conditioning. Remember that you're bringing the crawl space into the home's conditioned building envelope, so you need to treat it with appropriate care.
We've listed the details Advanced Energy recommends below. We've also listed common errors we see, along with their corresponding solutions.
Crawl space walls should be insulated with R-values appropriate to the local climate. The optimal choice over poured concrete or block is rigid foam designed specifically for humid applications. Batts are acceptable for wall sections framed with treated lumber (which we sometimes see on the upper portion of sloped foundations).
Encapsulate the batts. Batt insulation will only provide the advertised R-value if it’s encapsulated on all six sides. That means putting a rigid air barrier, such as OSB, on the interior face of any framed wall sections.
Don't insulate the floor. While installing batts between the floor joists can work if done right, we generally don't recommend this. Wall insulation combined with good air tempering (see below) will keep the floor just as warm, and because crawl spaces almost never have drywall on the ceiling, the batts inevitably pull away from the floor.
Remember the door. Any door to the outside needs the same insulation value as the walls as well as good weatherstripping and a secure latch.
Slope and Drain
Slope the ground to one wall with a perimeter drain or to a sump pump. There's no need to measure the slope; instead, just make sure that it's obvious to any observer that it is sloped.
A 6-mil or thicker polyethylene vapor barrier over the ground serves two purposes: It keeps soil-borne moisture out of the crawl space and creates a draining surface for water that does get into the crawl, for instance from a plumbing leak. Extend the poly at least 6 inches up the foundation walls and behind the foam insulation. Securely fasten the top edge to the wall and seal it with crawl space liner tape or equivalent material caulk.
Lap it right. Hard as it is to believe, we've seen some builders reverse-lap the poly. It needs to be lapped shingle-style, with the upper sheet lapped over the lower so that water flows to the drain.
Most air infiltration will be through the band area so this needs to be carefully sealed.
Don't use caulk. It's nearly impossible to do a good job crawling around the space with a caulking gun, so we recommend open or closed cell spray foam.
Temper The Air
The mechanical system should provide moisture removal and should keep framing and other elements warm enough that moisture won't condense on them.
Make sure it's dry. Some builders install a dedicated heating and/or air conditioning supply register in the crawl. In humid climates, however, if the homeowners routinely turn off the air conditioner during spring and fall, the space can become humid enough for condensation and mold to appear. And, of course, if the AC system is oversized it won't have a chance to dehumidify the air.
A dedicated dehumidifier will solve the problem. It should be sized for the crawl space (you might need a couple of them) and should dump water into that perimeter drain or sump pump, rather than to a pan that has to be emptied.
Making It Affordable
Advanced Energy has worked with builders as part of its SystemVision program for nearly two decades now. Many of these have transitioned to closed crawls.
Most builders, once they understand the system and complete a few homes, find that closed crawl spaces don’t cost more to build than vented ones. For instance, the total cost of rigid foam on the walls installed before the floor is framed shouldn't be more than batts installed between the floor joists later on.
In fact, we have helped Habitat for Humanity transition to closed crawls in their crawl space homes, and they have generally found the comparative costs to be a wash.
Builders may need to complete a few homes to master the process, but the benefits more than outweigh that effort. As mentioned, closed crawl spaces are associated with better moisture control and air quality. They also lower the potential for surface mold growth, rotting wood, and termite and carpenter ant infestations. That means fewer callbacks and lower long-term costs for the builder.
For those wanting to know more, Advanced Energy offers additional resources, including articles, videos, and research reports; click here.
Alex Glenn is a building scientist, energy consultant and training specialist with Advanced Energy in Raleigh, NC
Tommy Blair is a residential subject matter expert with Advanced Energy in Raleigh, NC, providing education, quality assurance/control and technical guidance to a diverse set of clients and utilities
Properly closing a crawl space requires a systematic approach to air-sealing, insulation, water management, and space conditioning.