Problem-Free Closed Crawls

The advantages of closed, conditioned crawl spaces have been well documented, but many builders need help with the details

Problem-Free Closed Crawls

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.

Documented Benefits

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.

The System

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.

Ground Cover

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.

Air Seal

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.


re: Problem-Free Closed Crawls

Monday, July 29, 2019 10:26:44 AM

Sorry, but the methods in this article are at least 10 years out of date.  We have been building very successful closed crawl-spaces for at least 20 years by insulating the floor just as you wold for a vented crawl-space, and then insulating the walls (and sometimes parts of the floors) of the crawl-space so that there is less heat loss out of the crawl-space than there is from the house into the crawl-space through that insulated floor.  R-10 rigid insulation on the vertical walls of the foundation is usually all that is required.  With this method we not only assure a completely dry crawl-space, we cut by half the heat loss through the floor (because the under-floor area is always at least half of the difference between the desired inside temperature and the current outside temperature).  Most of the studies of the methods in this article actually show higher heating costs, whereas my system results in lower heating costs by an average of 11% over a conventionally vented crawl-space.

Why does the air barrier need to be rigid?

Monday, July 29, 2019 12:42:18 PM

In the Encapsulate the Batts section it's implied that the air barrier must be rigid.  Does it actually need to be rigid and if so, why?  My understanding is that with fiberglass batts the important thing is that they aren't compressed and air isn't allowed to flow freely through them.  You'd prevent air flow with a non-rigid barrier and I don't think you would get enough air pressure to significantly compress them...

Doors don't NEED to have the same R-value

Monday, July 29, 2019 12:51:48 PM

The article states: 'Any door to the outside needs the same insulation value as the walls as well as good weatherstripping and a secure latch.'  That's simply not true.  The point not to forget about the door is a valid one, but there's no need for it to have the same R-value as the walls.  If a lot of wall insulation is used, it may be impossible to find a matching door and it probably won't be the right economic choice.

re: Problem-Free Closed Crawls

Wednesday, August 14, 2019 9:56:06 AM

Thank you for sharing your experience and comments. Advanced Energy takes pride in providing information and advice on improving energy efficiency, comfort and indoor air quality in homes.

Although our crawl space research was published in the mid-2000s, the methods and science behind our recommendations are still relevant today. Our Princeville, North Carolina, project showed that closed crawl spaces better control moisture levels than do well-built wall-vented crawl spaces. While relative humidity in the wall-vented crawl spaces exceeded 80% for most of the spring and summer, the closed crawl space designs controlled relative humidity below 65%. The research also found that the homes built on closed crawl spaces saved up to 15% on annual energy usage for heating and cooling compared to those built on wall-vented crawl spaces.

To this end, the guidance and recommendations presented in the present article are examples that have performed successfully in our field testing or in projects by professional installers in North Carolina. We recognize, though, that there are many closed crawl space designs that can perform acceptably and meet the requirements of specific states or different climate zones. For example, our research was conducted on test homes in the southeastern United States (a mixed-humid climate), which requires unique consideration of moisture levels, remediation and insulation strategies.

We also acknowledge that our advice and techniques are not definitive specifications. Builders, property owners or other contractors planning to install closed crawl spaces are encouraged to adjust these designs and processes to local site conditions, code requirements, home designs, construction processes and occupant needs. Closed crawl spaces share the same vulnerability as other foundation types or residential water control components: If they are installed poorly they can do more harm than good.

For some additional insight, we provide below a sampling of modeling for the different crawl space configurations in new construction and existing homes. All estimates are in dollars per year.

Using REM/Rate:

  • New Construction (Efficient Home)
    • No insulation, vented crawl: $1,067
    • R-19 floor, vented crawl: $896
    • R-10 wall, unvented crawl: $916
    • R-10 wall + R-19 floor, unvented crawl: $880
  • Existing Home
    • No insulation, vented crawl: $1,317
    • R-19 floor, vented crawl: $1,129
    • R-10 wall, unvented crawl: $1,145
    • R-10 wall + R-19 floor, unvented crawl: $1,106
    • R-10 wall, unvented crawl with air sealing: $1,096
    • R-10 wall, unvented crawl with attic insulation: $1,115
    • R-10 wall, unvented crawl with attic insulation and air sealing: $1,065

Using BEOpt (slightly different house to provide a range):

  • Existing Home
    • No insulation, vented crawl: $1,641
    • R-19 floor, vented crawl: $1,614
    • R-10 wall, unvented crawl: $1,577
    • Unable to model R-10 wall + R-19 floor (not recommended)

You might be wondering, why does the closed crawl space come out as less efficient in some cases? Semi-conditioning a space does have a cost. The benefit of semi-conditioning is a less humid, more comfortable and more durable home with inherently better air quality because we are limiting the potential for biological growth and moisture intrusion. There is some energy savings potential when adding insulation to the subfloor in a closed crawl space that has insulation on the foundation walls; however, the application is cost prohibitive compared to the annual savings potential.

We’ve also modeled the same existing home in REM/Rate in which the funds are diverted from insulating the subfloor to other retrofit options. In addition to the cost benefits of air sealing and insulating the attic plane, we would expect a greater level of comfort and indoor air quality by removing potential infiltration sources in the attic, which is a source of hot, humid and often not “fresh” air we would want to introduce to a conditioned space. Coupled with the benefits of a closed crawl, we see this as a win/win over the belt and suspenders approach of insulating the foundation wall and subfloor.

We provide additional crawl space information and resources here and here and encourage you to explore these sites and reach out to us through for further discussion.

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