InspectAPedia®   -   Search InspectApedia

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

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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

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
at CRAWL SPACE GROUND COVERS.

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

see CRAWL SPACE DRYOUT - home.

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.

    See CRAWL SPACE SEAL & SANITIZE
  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.

    See CRAWL SPACE DRYOUT
    and
    See CRAWL SPACE DEHUMIDIFICATION
  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.

    See CRAWL SPACE GROUND COVERS

    and
    See CRAWL SPACE MOISTURE BARRIERS
  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 InspectApedia.com

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.

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

Mold remediation completed in a problem crawl space (C) Daniel Friedman

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.

 




ADVERTISEMENT





Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-01-15 by (mod) - Mold in the crawl space

Marci

Mold in the crawl space ought to be removed and the crawl space converted to a dry, warmed, conditioned space; that, combined with keeping out any leaks from the crawl area will prevent mold recurrence and thus reduce the chances of sending mold up into the occupied space.

(There's probably no mildew in your crawl space - mildew only grows on living plants)

Your inspector is absolutely right. The mold genera/species won't change the cleanup and prevention measures one iota.

Whoever got people scared about "toxic black mold" has done a dis-service. Of the millions of mold genera species, there are hundreds of them commonly found growing on materials or surfaces in buildings and on building contents. Some of these are "black" or "dark" in color; those dark molds may be toxic or pathogenic, or merely allergenic, and in rare cases just cosmetic;

But those exact words, "toxic or pathogenic, or merely allergenic" apply equally to molds that are white, green, red, yellow, brown, etc.

And in fact Aspergillus sp. is one of the most-ubiquitious mold families in buidings, is often very small (far smaller than the famous black mold Stachybotrys chartarum), is therefore far more airborne, and is breathed deeply into the lung where it can give you a very serious, even fatal illness, Asprgillosis.

So forget about the mold color. (Mostly).

On occasion a doc may want to know a persons exposure to specific molds as part of medical history and diagnosis, though docs tell me they can often tell the difference between big sticky S. chartarum exposure (sticks in your nose) and Aspergillus or Penicillium exposure (gets deep into your lung) by where mold related illnesses appear.

Remove the mold and fix its cause.

On 2020-01-15 by Marci

I recently hired a "home health" inspector due to concerns about air quality and other issues in my recently purchased home. I bought the house 4 months ago and found out after purchase that the longtime residents had been heavy smokers for years.

The main odor I noted was more musty than tobacco-like. I had the duct runs cleaned, then realized how degraded they were. I had them replaced nearly 3 months ago with a noticeable reduction in the musty smell.

The inspector noted mold and mildew in the crawl space, along with some in the wood-paneled sun room. The inspector made some recommendations regarding moisture control, etc. but in one area where I had concerns about what appeared to be a blackish mold spot, he stated that the type of mold was irrelevent as the treatment was the same. This seems contrary to everything I know about black mold.

While I can't say that's what it is, I also cannot say it isn't. I've already spent a large amount of money (for me) in only a few months and feel as if I'm not much further along than I was. I have ongoing health issues and am looking for answers. Any insight would be greatly appreciated.

On 2018-10-19 by (mod) - can you run cable TV wire (presumably co-axial cable) through a foundation vent into a building

Billy,

The final legal authority to answer your question of "can you run cable TV wire (presumably co-axial cable) through a foundation cent into a building" is your local electrical code inspector

That said, Most likely, yes you shouldn't run cable through a foundation vent into a building (though that's probably quite common).

In the U.S. (if you're in the U.S.) the pertinent section of the US NEC is Article 820

Excerpt:

820.1 Scope. This article covers coaxial cable distributionof radio frequency signals typically employed in communityantenna television (CATV) systems.

There are specifications for the type of cable, its grounding, and its protection.

In particular if you are running cable through or into a fire-rated building floor, wall, ceiling, there are extra requirements and certainly running through a foundation vent opening would not be among them.

You can read versions of the NEC including the full (and lengthy) txt of NEC Article 820 and other countries' codes at the live link given in the ARTICLE INDEX for
BUILDING CODE DOWNLOADS - free downloadable PDF files of building codes & standards

On 2018-10-18 by Billy Luck

Is it a code violation to run cable tv wire through a foundation vent?

Weluck01@gmail.com

On 2018-09-29 by (mod) - yes we do find moisture and mold contamination problems in crawl spaces in the Northeast

It's absolutely the case that we find moisture and mold contamination problems in crawl spaces in the Northeast.

A conditioned crawl space is a good design but of course needs to be executed properly. And you still want to do what you can to prevent order entry or high humidity.

This article series gives a lot of detail about how to do that. It sounds as if your contractor is of the old school. It seems that building experts currently explain that conventional crawl space venting does not work to prevent moisture problems and in fact it can actually cause them.

As temperatures change and indoor outdoor pressures or winds directions change moist air enters the crawl space, moisture condenses, creating a moisture and mold problem.

On 2018-09-29 by Karina24

Dan, do you find issues with mold in crawl spaces here in the Northeast? It's generally presented as a problem in the Southeast, but it sounds like you are seeing mold in crawl spaces in the New York area.

The reason I'm asking is I have specified a conditioned crawl space in a multi-family project that I'm working on in upstate New York, but the general contractor wants it to be vented, I think at the recommendation of his mechanical engineer. So I'm trying to get a feel for whether or not I should insist on having it conditioned.

On 2017-11-03 by (mod) - does the ground vapor barrier change the venting area requirements?

No, Joddie.

On 2017-11-03 by Joddie

Is the total venting area required for a permanent foundation under HUD guidelines reduced with the installation of the ground vapor barrier.

On 2017-01-21 - by (mod) - response to reader prefers to vent crawl spaces

JJ

Thank you for questioning this matter and article. Notwithstanding an occasional surge in dislike for science, facts, objective data, when people face the high costs of mold contamination, rot, insect damage, and structural damage, the work by building scientists becomes more interesting,

I don't think it's a matter of simply "in some cases ventilation didn't help" as much as a matter of improvement in building science and the benefit, now of half a century of observation of crawl spaces, vented, un-vented, well-vented, inadequately-vented, and probably better equipment to detect and measure problems.

In 1910 buildings oil was cheap, buildings were barely insulated or not insulated at all, and windows, doors, walls, and crawl areas were as drafty as all get-out.

I've inspected old buildings from the 1700s and 1800s that had water regularly running in and through their basement or crawl space. When the building was drafty and barely heated and not insulated, the moisture mostly got out without making much trouble.

Then things changed.

By the oil crises of the 1970's energy costs had risen so dramatically that we made buildings tighter, better-insulated.

Moisture or water in a crawl area is a problem for most buildings though it's not always noticed promptly.

Small crawl space vents that admit warm moist air in summer, allowing that water into a cool crawl space, cause trouble for the building.

Sure a crawl area in a very dry area where no moisture enters in air the matter is not such a worry.

Contemporary expert opinion is supported by observation and research is that

- older, small conventional foundation vents admitted moist air enough to cause trouble but not enough air to keep a crawl space dry by natural circulation (as might occur under a building built on piers without a skirt)

- blowing or circulating air even by force in to the crawl space - something we tried for a time - through the crawl area made matters worse - that forms a one-way moisture pump IN.

- blowing air out - negative air in the crawl space, has some temporary applications (mold remediation)

- closing off crawl vents, getting water away from the building and out of the crawl pace, and putting down a good 6 mil poly barrier over dirt in a crawl space stops what otherwise functions as a natural moisture pump - sending moisture from the soil up into the structure.

Where a crawl area has been so closed off it might be useful to run fans and a dehumidifier during the dryout process.

We'd prefer not to rely on fans for a building to survive the elements, and we'd prefer to lower our energy cost for the structure. So once we've fixed gutters and leaders, outside soil grading, other leaks into the crawl area, such active drying will very often not be further needed.

I've cut open structures and cavities that were "vented" by a one-inch hole, even two-inch holes to see how effectively those areas had been vented and dried. They were not effectively dried at all, even when the remediator using that approach added a blower fan.

Both insulated wall cavities and uninsulated interior partitions and other areas so-vented were wet and moldy in problem buildings.

On 2017-01-21 by JJ

I can't really understand the logic behind all this. Just because in some cases, ventilation did not help, that does not mean the absence of ventilation WOULD help. Turn an empty tote upside down over a pile of mud and leave it there for months.

Check it from time to time and you will see that even in the hot summer, it will stay moist. But if you put even a 1 inch hole on either side, the mud would have dried completely in a short time. The time it takes would be dependent upon the flow of air combined with the temperatures involved.

And I say that it is important WHERE you place crawlspace vents. The wind comes in differently at a given location. So one needs to design a system to effectively function.

I have seen my fair share of crawlspaces that stayed dry thanks to vents. And I will be using them on a new construction of my own. The absence of air movement hinders evaporation which of course promotes all kinds of nasty problems.

The idea of using dehumidifiers and/or heat to dry out a crawlspace is a crutch due to the lack of understanding or ignorance of how air flow is used functionally. Think about the gray, cloudy winter time when everything is blanketed by the sky. The promotion of mold is amazing. Everything stays wet and gross. Take away the blanket and that all changes. The warm air of the summer works if you know how to use it.


...

Continue reading at CRAWL SPACE WATERPROOFING or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

Suggested citation for this web page

CRAWL SPACE VENTILATION CODES at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to CRAWL SPACES

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Questions & answers or comments about how to dry out a wet crawl space & prevent future crawl space water entry.

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification
when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca

Comment Form is loading comments...

Citations & References

In addition to any citations in the article above, a full list is available on request.



ADVERTISEMENT