Building an Unvented Crawl Space
Sealed crawl spaces have less mold and stay dryer than vented crawl spaces — and they often save energy
Residential foundations vary widely from one corner of the U.S. to another. Builders in some regions love basements, while builders in other regions swear by slabs on grade. Although most builders have a theory to explain these regional preferences, the main reason for these variations is habit, not logic. In areas of the country where basements are rare, there usually aren’t any technical barriers to building basements; and up north, where basements rule, it’s perfectly possible to build on a slab.
Slabs have several virtues: they are inexpensive and they keep all of a home’s living area above grade, away from dampness and mold. Basements also have their virtues: they keep plumbing pipes from freezing, provide a good place to install a furnace and run ductwork, and provide a useful area for storage.
Crawl spaces cost almost as much as a basement, with none of a basement’s advantages
Crawl spaces are more of a puzzle, and it’s hard to come up with a reason to like them. I’m sure that as soon as this blog is published, a builder from North Carolina will write in with an eloquent defense of the crawl space. I don’t have a dog in this fight, however, so if you really want a crawl space, go ahead and build one. Just be sure you get the details right.
If you’re perverse, and you want to build a damp, moldy, nasty crawl space, just do two things: insulate the crawl space ceiling with fiberglass batts, and vent the crawl space to the exterior. If you live in the Southeast, within a few short years the fiberglass batts will begin to hang down at odd angles like drunken stalactites. Every summer, the open vents will introduce huge amounts of moisture into the crawl space. You’ll end up with a classic moldy crawl space — one that represents a significant source of moisture for the house above.
In a hot, humid climate, venting a crawl space is counterproductive. During the summer, the outdoor air in North Carolina holds more moisture than the cooler crawlspace air. When humid outdoor air enters the crawl space vents, it soon hits cool surfaces — concrete blocks, water pipes, and air-conditioning ducts. Condensation forms and begins to drip. The more you ventilate, the wetter the crawl space gets. If you set up a fan to double the ventilation rate, you’ll just make the pipes drip faster.
Crawl space vents can also cause problems during the winter, when they introduce outdoor air that can cause pipes to freeze.
Unvented crawl spaces are permitted by building codes
In most areas of the U.S., sealed crawl spaces work much better than vented crawl spaces.
Most building codes permit the construction of unvented crawl spaces. In the 2006 International Residential Code, requirements for unvented crawl spaces can be found in Section R408.3. If an unvented crawl spaces has a dirt floor, the code requires exposed earth to be covered with a continuous vapor retarder with taped seams: “The edges of the vapor retarder shall extend at least 6 inches up the stem wall and shall be attached and sealed to the stem wall.”
The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. upstairs. Option 1 requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” In other words, install an exhaust fan blowing through a hole in the rim joist or an exterior wall; make sure that the fan isn't too powerful. (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.)
Option 2 requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.)
Advantages of unvented crawl spaces
Unvented (sealed) crawl spaces:
- Stay dryer than vented crawl spaces;
- Protect pipes from freezing;
- Require less insulation than vented crawl spaces (since the area of the perimeter walls is less than the area of the crawl space ceiling); and
- Bring ducts within the conditioned envelope of the home — an improvement that usually results in energy savings compared to vented crawl spaces.
According to researchers who conducted a careful study of vented and unvented crawl spaces in North Carolina, homes with sealed crawl spaces with insulated foundation walls use 18% less energy for heating and cooling than identical homes with vented crawl spaces with insulation between the floor joists.
A crawl space can work well in a dry climate
However, similar energy savings cannot be expected in dry climates. Researchers comparing the energy performance of homes with different crawl space designs in Flagstaff, Arizona found that homes with insulated floors used less energy than homes with sealed crawl spaces with insulated foundation walls. According to a report on the research, “This seemed counterintuitive; ducts are a notorious source of heat loss. With all the Flagstaff homes’ ductwork in the crawl space, one would expect better performance from the warmer, wall-insulated crawl spaces. But according to Cyrus Dastur, the Advanced Energy building scientist who directed the research, those homes’ lack of floor insulation let heat radiate from the first floor to the crawl space, robbing more heat from the house than was saved by keeping the ductwork warm.”
While vented crawl spaces often perform poorly in the humid states of the Southeast, they perform well in most Western states. According to an article in Home Energy magazine, “In the drier regions of the West, and even — surprisingly — in the marine climates of the Northwest, vented crawl spaces work acceptably most of the time. The hot-dry conditions in summer and the cold-moist conditions in winter do not cause the same problems that hot-humid conditions cause in the rest of the country. ... The Washington State University Extension Energy Program (WSU-EEP), as part of its work for Building America, monitored four test houses in Vancouver and Moses Lake, Washington, for over a year and found that the vented crawls rarely, if ever, reached dew point and that they remained above 80% RH only for brief periods of time.”
Creating an unvented crawl space
If you live in a humid climate, and you still want to build a crawl space — or if you are trying to correct problems in an existing moldy crawl space — here’s how to go about it.
- To help keep the crawl space dry, correct any grading problems on the exterior so that the grade slopes away from the foundation.
- Remove all rocks and debris from the crawl space floor, and rake the dirt smooth. Ideally, the crawl space floor will be higher than the exterior grade, although keeping the grade high on the interior of a crawl space is not always possible.
- If the home is located in an area where radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. is common, install a passive radon collection system in the crawl space floor.
- If the crawl space is subject to water entry, be sure to slope the floor to a sump equipped with a drain or a sump pump.
- Install a durable vapor barrier — for example, a 20-mil pool liner or Tu-Tuf poly — over the floor and extending up the crawl space walls, to within 3 inches of the top of the wall. Leave a 3-inch-wide termite inspection strip at the top of the wall.
- Attach the top of the vapor barrier to the wall with horizontal battens, secured to the wall with masonry fasteners.
- Seal the seams of the vapor barrier material with a compatible tape or mastic; many builders use duct mastic embedded in fiberglass mesh tape.
- Consider installing a 2 in. or 3 in. thick concrete slab (a “rat slab”) to protect the vapor barrier.
- If this is a new-construction crawl space, and you can’t afford a rat slab, you may want to install a temporary (sacrificial) second vapor barrier — usually a layer of 6-mil poly — on top of the permanent vapor barrier; once construction is complete, this temporary poly is rolled up and discarded.
- Insulate the interior of the walls and rim-joists with rigid foam — many builders use Thermax, a polyisocyanurate foam that does not require a thermal barrier or ignition barrier — or spray polyurethane foam. Another option: insulate the exterior of the foundation walls. If your crawl space has stone-and-mortar walls, you can’t insulate the walls with rigid foam; the only type of insulation that makes sense for stone-and-mortar walls is closed-cell spray polyurethane foam.
- Install a floor register in the floor above to allow air to flow between the living area and the sealed crawl space below.
- Install an exhaust fan or a forced-air register to meet code requirements for conditioning the crawl space. Be sure that the fan does not exceed air flow requirements for the size of the crawl space, since exhaust fans carry an energy penalty.
- Install good lighting — most crawl spaces will benefit from at least six fixtures, spaced evenly across the crawl space ceiling — controlled by a switch located near the entry door.
New-construction crawl spaces often require temporary dehumidification to remove construction moisture. Once the home is dried in, it’s a good idea to install a stand-alone dehumidifier in the crawl space and run it for three or four months until the interior relative humidity stabilizes.
Any combustion appliance (for example, a water heater or furnace) in a sealed crawlspace should be a sealed-combustion unit.
For further details on building a sealed crawl space, consult the resources listed in the “More Information” sidebar.
Even well-detailed crawl spaces may not make sense
A well-detailed crawl space is a thing of beauty, as shown in the photos (below) of renovated crawl spaces. (Perhaps such crawl spaces aren’t beautiful to all eyes, but they are to mine. I used to work as a home inspector, and I have spent far too many hours crawling under houses, in damp caves littered with debris and animal droppings.)
That said, it’s important to emphasize that even a well-detailed crawl space represents a problematic foundation design. Bill Rose, a renowned building scientist and a research architect at the Building Research Council at the University of Illinois, remains skeptical of crawl spaces. “I’m very cautious about crawl space construction — maybe it ought to be abandoned,” said Rose. “It would cost as much as a basement to get a crawl space right.”
One problem with sealed crawl spaces: the air quality in such a crawl space may be poor unless the crawl space is equipped with an exhaust fan that runs continuously. Of course, such a fan can be part of a whole-house exhaust ventilation system; but when (not if) the fan eventually conks out, the homeowner is unlikely to notice. That raises concerns over air quality in the crawl space — and also in the home above.
According to the previously cited Home Energy article, “Additional radon testing showed that radon levels in the closed crawls — with a relatively low dilution rate — were roughly 10 times the levels measured in the vented crawls.”
If a builder chooses to condition a sealed crawl space using “Option 2” from the two code-approved options — that is, by installing a forced-air register rather than an exhaust fan in the crawl space — it’s easy for any radon or moisture in the crawl space to circulate throughout the house.
The more you think about crawl space problems and crawl space remedies, the better a slab on grade begins to look.
Last week’s blog: “Alternatives to Clothes Dryers.”
- Aire Solutions
- Charles Buell - Buell Inspections
- Crawlspace Doctor
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