Crawl Spaces
Crawl Spaces: The Downsides of Basements With Few of The Benefits
Bird's eye view
CRAWL SPACES ARE LIKE MINI BASEMENTS. If you don't need the subterranean living space, you can save money on excavation and foundation walls. Crawl spaces should be sealed, insulated, and passively kept warm if mechanical equipment, ducts and plumbing are left there. A crawl space is less likely to have moisture problems if the crawl space floor is higher than the exterior grade.
Crawl spaces should be insulated and sealed
Many building experts recommend against crawl spaces because they have the water problems of a basement with almost none of the storage space, at much higher cost than a slab.
Still, sometimes a crawl space makes sense. Crawl space walls should be insulated with rigid foam or closed-cell spray polyurethane foam, and should be sealed rather than vented.
Crawl spaces should be treated as if they were miniature basements, which is exactly what they are. The best current practice is to make a crawl space a conditioned area like the rest of the house. This is permitted by newer versions of most building codes, and is much better for the house and residents.
See below for:
Key Materials
Choose a durable ground cover
The minimum covering for a crawl space floor is 6-mil poly with taped seams. Other, more durable options include:
Many crawl spaces include mechanical equipment and plumbing that needs regular servicing. The material used to cover the crawl space floor must be easy to clean and durable enough to survive visits by maintenance workers.
Design Notes
CRAWLSPACES SHOULD BE INSULATED, CONDITIONED, AND DRY. This crawlspace provides lots of storage room, and room for the mechanical equipment and ducts. Open-web floor trusses make running individual duct lines easier.
A crawl space floor should be above grade
Ideally, the floor of a crawl space should be a little above the exterior grade at the foundation perimeter. Although this raises the first floor, complicating the installation of a handicap access ramp, it reduces the chance that the crawl space will be damp.
Leave plenty of headroom.The higher the crawl space, the easier it will be for workers to do a good job installing and maintaining equipment located under the house. Building codes typically call for a minimum 18 in. distance between grade and floor framing, but boosting that to 24 in. will make it easier to work down there. It's a good idea to include a generous number of permanent lighting fixtures.
Sealed crawl spaces should be conditioned. Conditioned air can be introduced by installing a supply register and a return-air grille. Another approach is to install a supply register balanced by a small exhaust fan connected to the exterior. The Building Science Corp. outlines a number of options in a thorough study of crawl space issues at http://www.buildingscience.com/documents/reports/rr-0401-conditioned-cra... space.
Builder Tips
Choose the right tape
Use an appropriate tape when sealing polyethylene seams. Choices include housewrap tape (for example Venture #938 or #1585CW), Film-Gard Tuff Tape, or fiberglass mesh embedded in mastic.
For more information on tapes, see Air-Sealing Tapes and Gaskets.
The Code
The code
The International Residential Code permits residential crawl spaces to be sealed, as long as the requirements of section R408.3 (Unvented crawl space) are followed.
Section N1102.2.8 covers requirements for crawl space wall insulation.
Although the illustration shows a crawl space with a below-grade floor, a better designed crawl space has a floor that is several inches higher than the exterior grade.
Illustration:Code Check Building 2nd Edition. click to buy .
ABOUT CRAWL SPACES
Vents are counterproductive
There are two types of crawl spaces: vented and sealed. Vented crawl spaces are associated with moisture problems and mold. Although older building codes required crawl spaces to be vented, newer versions of the code allow sealed crawl spaces. In most cases, a home with a sealed crawl space will use less energy than a home with a vented crawl space.
It may seem like a good idea to allow fresh air to circulate through the crawl space, at least during the summer. Locating vents on opposite sides of the crawl space should let air waft through, removing moisture and keeping things dry, right? Not really. What actually happens is that water vapor in humid summer air comes into contact with cool surfaces under the house and condenses into water. The dampness can foster the growth of mold, leading to decay (and the need for repairs). This is an especially likely scenario in the southeastern United States, where crawl spaces are common and summer air quite humid.
When crawl space walls are sealed and insulated and the air in the crawl space is heated and cooled, the potential for condensation problems is greatly reduced. Perimeter rigid foam insulation can be applied to either the inside or outside of the walls.
MORE ABOUT CRAWL SPACES
SEAL CRAWL SPACES TIGHTLY
All crawl spaces require a layer of 6-mil (or heavier) polyethylene plastic spread over the floor of the crawl space to help keep moisture and soil gases from getting in. The plastic should be continuous, taped at any seams, and mechanically attached and sealed at the perimeter. For a superior crawlspace, consider covering the polyethylene with a thin slab.
For more information on best-practice details for sealed crawl spaces, see Building an Unvented Crawl Space.
Where radon is a hazard, the crawl space can be safely vented by installing perforated plastic pipe in gravel beneath the polyethylene ground cover and running the stack up through the roof.
Crawl space walls should be damp-proofed just like a full foundation wall to prevent water from migrating inside. If the floor of the crawl space is lower than the exterior grade, the foundation must have perimeter footing drains.
Newly built sealed crawl spaces may contain high moisture levels, especially if the crawl space was open during a spell of rainy weather before the house was closed in. In such circumstances, it may be prudent to install a portable dehumidifier in the crawl space, at least temporarily, to remove moisture that accumulated during construction. After six months, the dehumidifier can probably be safely removed.
FURTHER RESOURCES
BuildingScience.com:
Conditioned Crawl Space Construction, Performance and Codes
Advanced Energy's Crawlspace Information Site
Building an Unvented Crawl Space
Green Basics Index
Related Topics in the Green Building Encyclopedia
Image Credits:
- Chuck Lockhart
- Daniel Morrison
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3.
Wed, 03/24/2010 - 11:44
Wed, 03/24/2010 - 11:44
conditioned crawlspaces
by Anonymous
There is no cheap way out, remove the floor insulation,close the vents,remove the mold,insulate the box and beam ends and fondation walls,seal the dirt with NEUTOCRETE,heat and condation the space to make it become a part of the home , like any other room.
2.
Wed, 11/18/2009 - 04:50
Wed, 11/18/2009 - 04:50
A lot of questions
by Martin Holladay, GBA Advisor
Danny,
You will probably get more answers to your many questions if you post this in our "Q&A" section.
I'll address one of your questions. You wrote, "While doing energy audits we come across 50-year-old houses with no insulation in the floor system. We obviously recommend adding floor insulation; however we are concerned that we may be creating a problem by adding insulation in a vented crawl — basically cooling the bottom of the floor joists and creating a surface for condensation."
You needn't worry. If the floor of an air-conditioned house has no insulation, the floor sheathing facing the crawl space is cold, encouraging condensation. Insulating the floor (especially if spray foam is used) will separate the cold floor sheathing from the humid air of the crawl space. The face of the foam insulation facing the crawl is now much warmer, so there will be less condensation, not more.
1.
Tue, 11/17/2009 - 23:51
Tue, 11/17/2009 - 23:51
Crawl space retrofits
by Danny Kelly
Here in NC most houses are built on crawl spaces and have high moisture levels most of which have at the minimum a small amount of mold and a musty smell. We do not feel comfortable doing a closed crawl unless the mold is removed and the musty air is removed. This is a very expensive option - remediating the mold, running air scrubbers and then doing a full closed crawl system. Down here when doing a closed crawl, most will recommend adding a supply duct into the crawl thus the crawl and living area are somewhat connected which is why we do not like this option for a retrofit. We do closed crawl on all new construction. Had anyone come up with a cheap solution or a poor man's closed crawl that we can do to improve the situation but without breaking the bank. We have thought of adding a vapor barrier, leaving the floor insualation and closing up the crawl vents. Maybe adding a dehumidfier. Anyone see any problem with this scenario. Another concern we have - while doing energy audits we come across 50 year old houses with no insulation in the floor system. We obviously recommend adding floor insulation however we are concerned that we may be creating a problem by adding insulation in a vented crawl - basically cooling the bottom of the floor joists and creating a surface for condensation. We have also considered installing a bath fan in the crawl vented to the exterior to put a slight negative presuure on the crawl maybe drawing some conditioned air out of the house whcih may solve our condensation problem but kind of defeating the purpose of trying to save energy in the first place. There has to be a better solution for those without $5000 - $10,000 to spend for a full remeiation. Thanks for reading.
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