Crawl space vent shown from inside the crawl area (C) Daniel FriedmanCrawlspace Ventilation

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Crawlspace ventilation guidelines:

Current best practices compare crawl space venting versus crawl space dry-out & seal-up as a conditioned space.

Should you ventilate the crawl space, close off crawl space vents, install crawl space vent fans or dehumidifiers or what? This article describes best practices as well as common codes and standards for venting (or not venting) the space beneath buildings.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Should You Ventilate the Crawl Space?

Crawl space poly and heat (C) Daniel Friedman

Crawl space venting versus sealing:  this question was debated for decades in several countries including the U.S.. Today we understand that in humid weather venting outside air into a cool crawl space is likely increase crawl space moisture.

That's because moist warm air entering a usually cooler crawl area will drop its moisture out in that space, allowing moisture to accumulate as condensate (water) on foundation walls, floors, and in insulation where in turn it can become a key factor in problematic crawl space mold growth even if there is not other water leaking into that area.

Therefore experts no longer recommend simple passive venting nor humidistat-controlled venting except in special circumstances.

When our onsite inspection indicates a long-standing moisture problem in such an area the best current advice is to stop venting the crawl space and to convert the crawl space to a dry, conditioned space.

That means we close off crawl space vents, dry out the area, and add some heat to it.

Temporarily in a wet crawl space and certainly in a moldy crawl space that is waiting for cleaning and repairs we also recommend an exhaust fan or two blowing out of the crawl space to the outdoors. This creates a slight negative air pressure within the crawl area with respect to the rest of the building and thus it reduces the chances of moving moisture, insect allergens, bacteria (say from a sewage leak), and mold upwards into the occupied building space.

At CRAWL SPACE DEHUMIDIFICATION we explain the use of heat, dehumidifiers, or crawl space exhaust vents to try to improve the humidity level in crawlspaces.

Crawl space venting practices and building codes specifying crawl space vent areas also discussed

Watch out: in some conditions, dust containment, negative air, and more protective gear or help from professionals may be needed.

Should Crawl Space Ventilation Be Included in a Crawl Space Dryout Scheme?

Photograph: typical mold on floor joists and subflooring over a wet crawl space - © Daniel Friedman

The following is excerpted from CRAWL SPACE GROUND COVERS

In the original 1984 Solar Age article, the same experts were recommending what was conventional crawl space ventilation wisdom - specifications that were consistent with building codes.

1984 crawl space advice from Solar Age"

"A ground cover should be used in conjunction with ventilation. The HUD standard [1984] typical of others, recommended four crawl space vents with a total minimum free vent equal to 1/150 of the crawl space floor area if there is a ground cover, 1/1500 with the ground uncovered. For best results, place two vents each on opposing walls."

Really? Experts continue to study crawl space ventilation, sealing, air movement, and the effects of those choices on building indoor air quality, energy costs, and moisture - we cite some of this research at the end of this article. But by the very next year Nazaroff (1985) had shown that there was significant air movement up from a crawl space into the occupied space, enough that with crawl space vents open radon gas contamination levels doubled and 50% of the radon contamination entered the living space. And Dickson (2013) confirms and sums up a key point about venting crawl spaces in hot humid climates.

Research has shown that vented crawlspaces located in mixed or hot-humid climates tend to increase the moisture level within the space instead of keeping it drier.

Our photo (above-left) shows a severe and problematic mold contamination on the underside of the first floor of a building constructed over a wet crawl space. Ventilation with outdoor air had not helped one bit to avoid this problem.

Studies of air movement in buildings including from crawl spaces has continued to the present. In 2007 Kalamees demonstrated that air leakage upwards through a building was significantly driven by temperature (and thus pressure) differences and that normal openings around wiring and plumbing as well as windows and doors produced significant air leakage and air movement in buildings.

The typical air leakage places in the studied houses were: the junction of the ceiling/floor with the external wall, the junction of the separating walls with the external wall, penetrations of the electrical and plumbing installations through the air barrier systems, penetrations of the chimney and ventilation ducts through the air barrier systems, leakage around and through electrical sockets and switches, and leakage around and through windows and doors.

According to the questionnaire conducted, fluctuating room temperature, cold floors and draught from electric sockets were related to the houses with air leakage rate >3 m3/(h m2) at 50 Pa.

Conventional best practice crawl space moisture control has thus shifted from the 1984 view.

Crawl space poly and heat (C) Daniel Friedman

Experts observed that crawl space venting was not effective in many instances, for example depending on wind direction as well as the source and amount of crawl space water or moisture, crawl space vents were simply ineffective.

In some instances, such as blowing warm high-moisture laden air into a cool crawl space in summer months in some climates greatly increased the level of crawl space moisture and condensation, making crawl space moisture worse rather than better. See Walker (1998) for a scholarly study of the strength of the "stack effect" in moving crawl space air upwards in buildings.

Our crawl space photo (left) shows that poly was placed on the dirt floor of the crawl area and a heat source was provided, salvaging an old radiator. We'd have preferred to see the poly extending up the crawl space walls a foot or so. But we notice that this crawl space looks dry: there are not mold nor moisture stains on the floor framing overhead, and no leak stains on the crawl space foundation wall.

Below we summarize the best way to avoid wet or damp crawl space problems under buildings. If your crawl area is already wet or damp, also

Five Best Practices to Control Crawl Space Moisture

  1. Seal the crawl space from outdoor air - close off those crawl space vents - and convert the crawl space to conditioned space, providing a small amount of heat where climate dictates, to help keep the area dry and above freezing.
  2. Seal the crawl space against leaks into the building interior. By closing off openings around pipes, wiring or framing between the crawl space and adjacent or overhead building areas we stop un-wanted air movement from the (usually) cooler crawl area into occupied building spaces.

    This step prevents movement of moisture, mold, allergens or anything else into the building from the crawl area. Further by stopping possible convection currents between the crawl space and upper building areas we not only will stop natural air movement up into the building from the crawlspace, we'll also stop the simultaneous drawing of seasonally warm humid outdoor air into the crawl space.

    The best way to prevent un-wanted air movement from a crawlspace into a building is to seal off all air leaks between the crawlspace and the house.
    See AIR SEALING STRATEGIES for details about sealing building air leaks.
  3. Identify and cure sources of crawl space moisture, such as roof drainage spilling around the foundation. In roughly 90 % of inspections performed by experts, we find that wet or moldy crawl spaces or basements that had been blamed on "high water table", "rising damp", or "built over a spring" were actually being caused by gutter and downspout defects, perhaps combined with in-slope grade that concentrated roof drainage right against the building foundation.

  4. Use a moisture barrier such as 6-mil poly continuously over the crawl space floor and lower crawl space walls, up to grade level, sealed as we described above. In new construction the poly may be installed under a crawl space slab or gravel. In crawl spaces that are rarely entered, placing sand or gravel over the poly is probably not necessary, and its use can hide depressions in the poly that may actually be holding ponding water on top of the poly in some cases.

  5. Inspect the crawl space periodically, at least once a year, to be sure that the poly moisture barrier is working as intended. As we just suggested, an outside water leak, such as roof spillage entering through a foundation wall, or an inside water source such as from a leaky plumbing supply or drain pipe, can place water on top of your crawl space poly moisture barrier, leading to a costly building moisture and mold problem.

    At CRAWL SPACE DRYOUT - home we show photos of just this problem. -- DF

Remember these are minimum values for average conditions. Your building may need special measures. If, after identifying and fixing outside sources of a wet or damp crawl space, you still find high water levels right under the crawl space floor, you may want to install a sump pump as well.

Crawl Space Ventilation Standards, Codes, & Research

Water trapped in poly vapor barrier © D Friedman at Mold remediation completed in a problem crawl space (C) Daniel Friedman

Our photo at above left shows accumulated water above an improperly-located plastic vapor barrier that was stapled to the bottom of the floor joists over a crawl space. Moist air leaking into the floor space carried water that condensed enough to accumulate as puddles on the upper side of the poly. The wet insulation became a mold reservoir as did the wood framing and plywood subfloor above.

At above right is a properly cleaned and sealed crawl space that had been mold contaminated. The white device atop my black clipboard is an air sampler.

U.S. HUD Ventilation Codes for Occupied or "Interior" Areas of Manufactured Homes

The following is excerpted from Kurnitski (2000) but readers should note that the air change ventilation specifications those authors cite is for the occupied space, not the crawl area.

All HUD-code homes are required to have a ventilation system installed. Title 24 CFR Part 3280.103b ‘Light and ventilation’ states that…

“Each manufactured home shall be capable of providing a minimum of 0.35 air changes per hour continuously or at an equivalent hourly average rate. The following criteria shall be adhered to.

Natural infiltration and exfiltration shall be considered as providing 0.25 air changes per hour.

The remaining ventilation capacity of 0.10-air change per hour or its hourly average equivalent shall be calculated using 0.035 cubic feet per minute per square foot of interior floor space. This ventilation capacity shall be in addition to any openable window area.

The remaining ventilation capacity may be provided by: a mechanical system, or a passive system, or a combination passive and mechanical system….”

Currently, there are two main types of ventilation systems that are employed by the manufacturing housing industry to meet the 0.10 air change per hour requirement. Both types are used in the hot and humid climate; an exhaust only system that is located in a hallway or utility room and an outside air supply system that is ducted from the roof to the return airside of the air handler fan. The exhaust ventilation system is manually controlled with a simple on-off switch. The outside air supply system is linked to the operation of the air handler and controlled with an automatic damper (if applicable).

Crawl Space Ventilation Standards & Research

This article series describes the steps needed to get into, inspect, clean, and then dry out a building crawl space. We give a step by step crawl space entry, inspection, cleanout, dryout and keep dry guide explains how to get into or inspect a crawl space even if there is no ready access, how to assess crawl space conditions, how to stop water that is entering the crawl area, how to dry out the space, how to clean up and if necessary disinfect or sanitize the crawl space, and how to keep out crawl space water and moisture in the future.


Continue reading at CRAWL SPACE WATERPROOFING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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