Crawl Spaces

Crawl Spaces: The Downsides of Basements With Few of The Benefits

Bird's eye view

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:

  • Tu-Tuf cross-laminated polyethylene (Sto-Cote Products; 800-435-2621).
  • Swimming pool liner material.
  • A thin concrete slab over 6-mil polyethylene.
  • 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

    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 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

    untreated wood clearances
    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 .


    More insulation is better
    Once you've built a well detailed sealed crawl space, the best way to make it greener is to increase the thickness of the rigid foam insulation on the crawl space walls. There's nothing wrong with installing the same R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. on crawl space walls as you have on above-grade walls.


    Concrete Crawlspace Details
    Concete Block Crawlspace Details


    LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -for Homes 1/2 point available under MR2.2 (Materials & Resources) for fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info or slag as replacement for 30% or more of Portland cement in foundation

    NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Ch. 6 — Resource Efficiency: up to 4 pts. for recycled-content (fly ash or slag substitution for Portland cement in concrete) (604.1).


    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.



    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.

    For more information, see All About Radon.

    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.

    Conditioned Crawl Space Construction, Performance and Codes

    Advanced Energy's Crawlspace Information Site

    Building an Unvented Crawl Space

    All About Radon

    Image Credits:

    1. Chuck Lockhart
    2. Daniel Morrison
    Tags: , , , , , , ,
    May 14, 2017 9:39 AM ET

    Response to Steve Knapp
    by Martin Holladay

    Thanks for letting us know about the broken link. I've fixed the problem.

    May 14, 2017 9:18 AM ET

    Broken link
    by Steve Knapp CZ 3A Georgia

    The BSC link no longer works.

    Jan 17, 2013 7:42 PM ET

    Response to James Reeves
    by Martin Holladay

    You have at least two problems. One is financial; the other concerns your moldy house.

    We can't help you with your financial problems, unfortunately. If you can't afford to perform the needed repairs to your house, it hardly matters what our advice is.

    If you have a dirt-floored crawl space, you should perform the work described in this article: Building an Unvented Crawl Space.

    Concerning the extensive repairs required for the rest of your house, it seems clear that you need the help of an experienced contractor. If you can't pay for the necessary repairs, however, you are obviously between a rock and a hard place.

    Jan 17, 2013 6:55 PM ET

    Dirt crawl space, musty smell, ceiling mold please help me
    by James Reeves

    Sorry for length of this am disabled need urgent advise, expert help. I really hope someone can help me, l live in Carrying Place, Ontario, Canada were located about 1 1/2 hour drive east of Toronto, Ontario.
    My wife her mother 5 years ago was put in nursing home with Alzheimers disease then 2 years ago her dad got Alzheimers Disease and placed in same nursing home.

    They lived in their home from 1986 to 2010, for the last 10-12 years could smell musty smell in the home. Many tried to tell them, but they said they didn't smell it and would get upset, maybe living there they there their smelling senses never sensed it.

    Jan/2011, my father inlaw gave the property to my wife, his name still on deed. The house was debt free, since l have many health problems house being in town we planed to move there, selling our present home thinking would cost much to be liveable and l be closer to the hospital stores driving hard for me.

    So l removed all carpets furniture and anything else with material on it thinking smell would go away. But it didn't leave, Bought 4 - 50 pint dehumidifiers, run them 24/7 for 5-6 months abit of difference but still strong smell.
    So next l removed all interrior doors, all window, door baseboard trim still no change.

    We then had to get a a $ 25,000 line of credit, to remove and replace drywall, insulation.
    The complete house has cathederal ceilings, outside walls 7 ft hogh when beams meet in the middle walls are 9 1/2 ft high. So we removed the false ceilings nearly every ceiling has allot of water stains and dinningroom is 11 ft wide x 13 ft long tiles were very bad mold roof 6 years old damage from a prior leak many years ago l assume.

    Inside house looking up the stained tiles, mold ceiling was original roof, on the attic side of the mold stained tiles is tar and gravel about 5-6 inches thick the tiles are on top of beams barely can be cut with chainsaw. Prior owner before my father inlaw bought it in 1986, put a complete stick type plywood roof about 8-10 inches about the mold stained tile.

    My father inlaw about 6 years ago hired a roofer tp put new shingles on top of old shingles the old shingles has a green type moss above kitchen area weren't removed when new shingles were put on.
    There was never any insulation between the original tar/gravel attic side of the roof and the 2nd outer roof.

    How do l insulate in the 8-10 inch gap between the two roofs? We have to remove both layers of shingles and likely allot of plywood below shingles hoping to do that in spring of 2013 no money to insulate before then should l insulate when l remove both layers of shingles and should l remove all mold on inside section of dinningroom and then when we do shingles cut those tiles out of dinningroom while shingles are off?

    When we removed drywall and wall panelling on outside walls 45 ft or more of outside walls had to be cut out replaced from sill to ceilings, and another 20 ft of wall same in garage removed killing our budget.
    Had to sister floor joices in livingroom, kitchen, dinningroom also.
    Crawlspace dirt floor related?
    Is all this damage, to floors, walls from the dirt floor crawling space, high hummidity, black mold type stain even on nearly all cement blocks on inside crawling space.
    Main crawling space from dirt to floor joices is about 5 ft high but under the 13 x 18 ft spare bedroom dirt is about 2 ft from floor joices no vents,, nothing to remove hummidity and has 100% block wall between the 2 crawlspace the main crawl space has 2 small window vents across from each other.

    New drywal on all walls and new insulations in all outside walls, 45 plus feet complete new walls, all new windows doors running 2 dehumidifiers on main floor one each end and 1 in crawlspace should windows screens be closed stuffed with insulation and garage/shop atached to house access to crawspace open should it be boarded up?

    Spring/2013 if can get funds cheap help need to replace roof, shingles and some how fix smelly musty smell and cover both crawl space dirt floors need help bad near bankruprt email me at if you can help or advise me thanks for reading
    can send photo's if would be of help thanks
    Jim Reeves, Ontario, Canada

    Mar 24, 2010 12:44 PM ET

    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.

    Nov 18, 2009 5:50 AM ET

    A lot of questions
    by Martin Holladay

    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.

    Nov 18, 2009 12:51 AM ET

    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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