Crawl space dehumidification methods.

Here we describe how to make best use of dehumidifiers or heat to remove crawl space moisture.

We also explain that crawl space ventilation by exchanging outdoor air with crawl space air does not work well to dry out most crawl spaces even if the air exchange rate is quite large, and it is unlikely to work predictably if the crawl space is tightly sealed.

Other measures to dry out a crawl space such as removing sources of water entry and use of moisture barriers are detailed in companion articles.

How to Use Heat and/or Dehumidifiers in Crawl Spaces to Keep Moisture Levels Down

Watch out: as we detail at CRAWL SPACE SAFETY ADVICE, entering a wet crawl area is dangerous, involving shock hazards and possible chemical or sewage contaminants.

And see our complete list of crawl space dryout steps at

CRAWL SPACE DRYOUT

How to install a continuous dehumidification system in a crawl space: (with a permanent drain hookup and a condensate pump if necessary so that condensate will be disposed of automatically and so that the crawl space dehumidification system can run unattended. Use an A/C condensate pump to a building drain if a gravity drain connection is not feasible.

First remove the mold, remove any moldy insulation, then correct the water entry problems, then clean the surfaces, then you can put your dehumidifier to work to keep the space dry.

This installation could have done a better job on the plastic but the dehumidifier is well placed near the center of the crawl space.

[Click to enlarge any image]

If your crawl area is large, add one or more small fans blowing towards the dehumidifier from remote areas of the crawl space. You'll find this makes an enormous improvement in the rate of dehumidification.

Don't even bother to try to dehumidify the crawl space if the area is taking on standing water or puddles. First you'll have to solve the water entry problem.

Cleaning up puddles or active foundation leaks with a dehumidifier won't work any more than you can suck the dust off a the living room carpet by standing across the house in the kitchen and waving your vacuum cleaner wand in the air. (This is also why an indoor "air cleaner" cannot remove a problem mold or allergen source in a building.)

Watch out: Don't put a new, expensive dehumidifier into a moldy crawl space if you're going NOT to clean up the mold first. Doing so will increase the airborne mold level in some cases - yes as things dry out the mold will begin to release more spores than ever.

Also you'll contaminate your dehumidifier with mold spores.

We like to set our crawl space dehumidifiers to 45% RH or lower. We've made lots of humidity measurements. When the humidity right close to the dehumidifier is 45%, you'll find that more distant crawl spaces will have a higher humidity level, especially close to the foundation walls. So if you're trying to dry out the whole area, don't set your dehumidifier above 45% RH.

A dehumidifier in a crawl space will also provide some heat in that area; if the crawl space is too cold (despite perimeter insulation) it may be necessary to add a small level of heat there. Some building also permit introduction of dry heat into these areas.

Make sure your crawl space electrical wiring is safe and meets current electrical codes. Receptacles (such as the electrical outlets you may want to use to power your crawl space fans or dehumidifier) should be GFCI protected and all of the circuits there such as wiring for lighting should be AFCi protected.

See AFCIs ARC FAULT CIRCUIT INTERRUPTERS for details on the 2008 National Electrical Code requirements for AFCIs and GFCIs.

Add a heat source to the crawl area to help keep it dry. If plumbing supply or drain lines are in the crawl space that will be another reason to add heat if your building is located in a freezing climate.

You don't need much. An air supply register cut into an existing supply duct in the crawl area may be enough, or a small section of heating baseboard if your building uses hot water heating. In crawl spaces where these heat sources are not convenient, add a small electric baseboard or oil-filled electric heater with a thermostat that turns it on at low temperatures.

Can Exhaust-Only Venting for Crawl Spaces or Bi-Directional Crawl Space Air Flow / Vent Systems Help Dry a Crawl Area?

OPINION: well yes, ... maybe, and sometimes no, it may make things worse. Here we discuss all four possible cases involving make-up air and crawl space exhaust vent systems.

Some building experts like to add an exhaust vent to the sealed up crawl area, opining that a humidistat-controlled exhaust vent will help reduce crawl space moisture levels.

But keep in mind that for a fan to move any air out of a building area make-up air has to enter that area. In the case of a building crawl area, if we run an exhaust-only vent system, make-up air will come from two areas - from outdoors or from inside the building, or perhaps from a combination of the two.

Case 1: Crawlspace Makeup Air Drawn from Outdoors: a humidity SNAFU

I outdoor air is drawn into a crawl area to satisfy the negative pressure created in the crawl area by the exhaust fan we risk sometimes drawing air into the space at higher humidity levels than the space already had.

The result of humid air entering the crawl space is most often an increase in crawl area humidity level. We can avoid this dilemma by using a more costly humidistat that compares both indoor and outdoor humidity levels and only operates if outdoor humidity is lower than crawl space humidity.

Case 2: Crawl Space Makeup Air from Other Building Interior Areas: an IAQ Advantage but a Radon concern

If a crawl space has been tightly sealed against outdoor air entry, when the exhaust fan operates it will seek makeup air from leaks between the crawl space and the building interior - from the floor above or from an adjoining basement. If air inflow into the crawl space is from the conditioned living space above, it is possible, indeed likely that often (though not always) the incoming make-up air will be at a relatively low humidity level and the crawl area humidity level may drop in turn.

There is an advantage of leaking or deliberately drawing air from the conditioned space of a building through the crawl area to ultimately be blown outdoors: the improvement of building air quality.

If a crawl area or its insulation risk harboring mold, allergens, or other airborne hazards, keeping the crawl area at negative pressure with respect to the building occupied space prevents convection currents from carrying potentially problematic particles up into the occupied space.

We have recommended this approach using a simple and small continuously-operating crawl space vent fan, venting out through the crawl space foundation wall, for buildings suffering from IAQ complaints that appear to originate below the structure.

Watch out: placing the crawl space under negative pressure with respect to the building upper levels may improve IAQ above, but if the structure is built over radon-gas bearing soils you risk creating high radon levels in the crawl space and worse, subverting an installed radon mitigation system.

Watch out: also that in some buildings where a whole house fan or other building air exhaust vent systems are installed, when the building air from the occupied space is being exhausted the system may overcome and subvert the crawl space vent system, drawing harmful particles or gases into the occupied space.

Case 3: Crawl Space Makeup Air Drawn from Both Outdoors and Inside the Building: unpredictable results

Depending on varying conditions we list below, the make-up air to satisfy a crawl space exhaust vent fan may sometimes come from just outdoors, sometimes just indoors, or from a variable mix of the two sources. The result in any case is that with mixed source air leaks into a crawl space, the performance of a crawl space exhaust vent fan in dehumidifying the area will be unpredictable

Case 4: Tightly Sealed Crawl Space Has Inadequate Make-up Air: exhaust effort is futile

For a crawl area that has been tightly sealed against all areas, its floor, foundation walls, the perimeter rim joist, and the floor above, an exhaust fan is unlikely to be very effective.

For a comparatively large crawl area, running an exhaust-only fan will produce some sensible movement of air out of the crawl area, but it should be quite apparent that if the crawl space is tightly sealed so that little or new new2 air from an external source can enter the area, the exhaust fan is making a near-futile effort to move sufficient air to change crawl space conditions.

CRAWL SPACE REINSPECTION: Inspect the crawl space periodically to make sure your crawlspace dryout measures have been effective. How often do you need to inspect the area? It depends ... on site conditions and building history.

At least once a year you should look for any new leaks such as a leaky plumbing drain or an outside water entry problem. If you have been having trouble keeping water out of the crawl area, you should check more often until your confidence is restored.

Watch out: for steps 1-7 above, in some conditions, dust containment, negative air, and more protective gear or help from professionals may be needed.

Also see our other crawl space dryout and safety discussions beginning

at CRAWL SPACE GROUND COVERS

where we describe crawl space venting, crawl space poly over dirt, and crawl space heat, to illustrate current best-practices in keeping a crawl space dry.

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.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-08-20 - by (mod) -

Anon

I agree that what you describe is not a good job of putting down poly in a crawl space. You can fix the problem with a whole additional layer of poly, overlapping a couple of feet at seams, or taping seams; the cost of those materials is trivial.

On 2020-08-16 by Anonymous

I live just North of Myrtle Beach SC one mile from the ocean. The crawl space has been encapsulated, but the 6ml poly on the sandy ground was not sealed with caulk or tape at the seams. And the poly is held down with roofing nails with washers. The joints overlap only a few inches. The poly comes up the walls and support columns two feet. All the wall vents have been sealed. There is a dehumidifier with a pump that keeps the space consistently at 55% RH. Is there a reason to be concerned about the floor covering not being sealed properly other than that the dehumidifier maybe runs longer than if properly sealed?

On 2020-02-07 0 - by (mod) -

Blowing unconditioned air risks blowing in moisture, accumulating moisture, and causing a mold problem in a crawl area.

On 2020-02-07 by BCG

What can anyone tell me about blowing unconditioned attic air into the crawl space as a means of reducing crawl space humidity? Advisable? Not advisable? Why, or why not? Thanks.

On 2020-01-21 - by (mod) -

Larry

I understand your motive and appreciate wanting to save on both energy usage and money.

But IMO it's not so good for your home to let the humidity in the crawl area reach above the 55% to which it was set.

I figure that if the RH is already below that target the dehumidifer ought to stop running on its own, and if it's running the RH must be above the target.

Am I wrong?

Have you got a meter to check RH in the crawl space?

On 2020-01-21 by larry agee

A few months ago I had a company install a vapor barrier and dehumidifier(healthy climate HCWHD3-070, 70 pint capacity), with a range of 40% to 80% in the crawl space under our house(approx. 1400 sq.ft). here in Charleston, sc. They initially set it at 55%. being as it's now winter, and low humidity, I crawled under and re-set it to 60% in order to save on the electricity bil

. i'm thinking that during the low humidity months I can keep it at this setting, then change it back to 55% from may thru September. what do you think? thank you. tinalarryagee@comcast.net

On 2019-02-09 by (mod) - problems with putting the crawl area under negative pressure: Radon?

A reader commented by email:

In Case 2 there is a warning that placing a crawl space under neg pressure could cause radon levels to increase. I don't understand this. the radon mitigation systems I've seen are fans that pull air out of the crawl space. Isn't that the same as adding an exhaust vent fan to the foundation vent opening?

Mod comments:

Typical radon mitigation uses a method called sub-slab suction: a fan is connected to one or more holes in (and sealed-to) a basement or crawl space concrete slab, creating negative pressure under the slab and venting above the roof line. By making the under-slab pressure negative with respect to the building the propensity for radon gas to seep up into the building's occupied space is significantly reduced.

Making an entire crawl area negative and venting to the outdoors has a potentially similar effect, as you suggest.

OPINION, However I'm leery of drastically increasing the radon level in an open space under the building - I'm not confident that it's as reliable an approach for such an un-sealed surface.

If the sub-slab suction fan is turned off there won't be anywhere the same surge of radon up into the building as there would be possible if we've created a crawl space buffer of high-level radon right below a building's occupied space. So the approach you suggest needs to be very reliable. It's not an approach I've encountered.

At least not without an effort to seal off the crawl space surface.

On 2018-08-15 by (mod) - will my dehumidifier remove water from between layers of flooring or a ceiling below?

Nope. not effectively. It can take weeks to months for water spilled into a building cavity to completely dry. If the materials are mold-friendly (like drywall) there's also a risk of mold growth in the ceiling cavity.

My procedure is to drill a hole to drain trapped water, and if there's no access from above, I might cut a drying opening below. Yes there will be ceiling repair needed, though that is often needed anyway following water damage, and it's better to let an electrician or other suitable experienced repair person confirm that the electrical wiring and devices are safe. And dry.

On 2018-08-11 by Phyllis

My kitchen sink overflowed down into the electrical light of the tenant below me. After clean up, I put a dehumidifier and a fan above that spot in my apartment above neighbor's.

I started that the next day after accident. I have been drying out with the dehumidifier for about five days. Do you think this will dry out any water that got down between the floors?

On 2017-12-01 by (mod) -

De

Good for you,

In fact those are sawed off trees that were used as posts in this antique house

On 2017-11-30 by Delited2meetU

The image with the dehumidifier in the crawl space, are those trees on either side of it ?

On 2017-09-03 by Jeff

I have a full crawl under house that I sealed and dehumidify. I have 12 courses of block so really a basement with dirt floor. I want to put in floating decking so I can work there. ID put it over vapor barrier. Any thoughts. I'd put 1/3 exterior subfloor material instead of decking.

On 2017-06-18 by (mod) -

CW

Thank you for the careful reading and for taking the time to comment. You're absolutely correct, and I don't know what happened to the word "NOT" but I will edit and correct the articlee to put NOT back where it belongs.

On 2017-06-18 by CW

Is this statement correct, or is there a word missing in the first sentence, copied from your website: "Don't put a new, expensive dehumidifier into a moldy crawl space if you're going to clean up the mold. Doing so will increase the airborne mold level in some cases - yes as things dry out the mold will begin to release more spores than ever.

Also you'll contaminate your dehumidifier with mold spores". Maybe I'm missing something. I thought the first line would say words to the effect of don't install an expensive dehumidifier if you're NOT going to clean up the mold....no?

On 2016-09-20 by (mod) -

Chris:

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On 2016-01-21 by (mod) - crawl space is at 80% humidity so I need to dry it out.

Dehumidify first, speeding it with fans;

Then identify the water source and fix that if you can - such as improper roof drainage.

On 2016-01-20 by Diane

My crawl space is at 80% humidity so I need to dry it out. A contractor today suggested a dehumidifier and encapsulation. Do we need to do both? What are the benefits of doing both?

Question: are my crawl space "dryout" fans blowing moldy wet dust and debris and maybe chemicals from a wet crawl space into the living area?

I recently moved to an 800 square foot single story historic house in Pacific Grove. The home has a very shallow crawl space (as little as 6" near the perimeter- the foundation is only 12" deep) and the crawl space was covered with a moisture barrier when I bought the home. It has a solid concrete wall bisecting the underneath of the house (the long way).

I had termites and was advised by the inspector that the house is very damp underneath. They recommended removal of the moisture barrier and installation of fans to dry out the space. I have been having symptoms like I get when I am exposed to dust (dry, red nose) and feel very uncomfortable ever since the fans went in. This is driving me absolutely nuts.

The crawl space is wetter now than before (it has rained). I can't put in a french drain because there is only about 6" clearance between my house's crawl boxes and the fence, also it is on top of the sewer lateral.

I live on a hill in an area known to have underground springs but nobody knows exactly where they run.

I think they should remove the fans and put the moisture barrier back.

Do you agree? Is there anything else I can do? Perimeter moisture barrier?

Thank you. There is lots of mold down here and I am concerned that just putting the moisture barrier back will cause mold or rot. - S.A. 2/7/2013

Reply: how not to "dry out" a wet moldy crawl space

Sounds as if you got some advice that was good in intent but not competent.

I do on occasion recommend adding a fan to increase air movement in a crawl area or basement where a dehumidifier is at work, as that will increase the rate at which the dehumidifier can dry out the area. But just blowing air around in a wet moldy crawl space seems like a bad idea.

Removing the moisture barrier from the crawl floor and blowing fans turns the crawl space into a moisture pump, moving moisture from soil into the crawl space air. Perhaps if the fans blew OUT of the crawl space that might have been better, but the proper approach is to find and fix sources of water entry, seal (poly is ok) the floor, and dehumidify the area.

Watch out: there is an added risk of blowing pesticide contaminated dust and debris into the home if the applicator used a surface spray - something that's not usually done for termites. Usually for termites the pest control officer places a termiticide in the soil around the home; but in a home with a dirt crawl space they may be unable to take that approach because of the risk of chemicals surfacing in the crawl area and entering the living area - making occupants sick. So I'm not sure what has been done about your termite issue but that too needs expert review.

I suspect that dust, possibly allergens (insect fragments, mold, even soil particles, potentially other particles) have been stirred up; and if the fans were not blowing out of the crawl area, it may have been pressurized by the air movement; if that's the case, the arrangement may have increased the movement of particles up into the living area from the crawl space.

More likely you need to stop the fans, find and fix outdoor water sources like roof or surface runoff spilling by the foundation, put the poly back down to stop pumping water into the crawl area, and after the mold problem has been evaluated and most likely removed (cleaning the wood surfaces, tossing out insulation), then you might get a fan and dehumidifier back at work to keep the area dry.

Question: How do I dry out a crawl space quickly?

While this article did address the dehumidifier to a large extent, it really did not give instruction on how to rapidly dry out the crawl space after the water issues had been resolved so that the other remediation steps could be taken. More instruction on that would be extremely useful. - C. Brown

Reply: Rapid Crawl Space Dryout Procedure

Thanks for the comment C. Brown. In response to your well- CRAWL SPACE DRY-OUT PROCEDURE, from which I excerpt below:

To dry out a crawl space rapidly after standing water has been removed, and thus to try to reduce further moisture and water damage to the rest of the building (of course effects may already have taken a toll so a corollary rule will be how to inspect the building for hidden moisture related trouble that originated in the crawlspace) we take the following steps:

By this interpretation we use multiple fans to increase air circulation in the crawl area, thus picking up moisture rapidly, combined with one or more fans exhausting - blowing the moist air outdoors.

Beware that in hot humid weather, bringing outdoor humid air into a crawl area can make for new condensation issues. Some have experimented with a humidistat that changed direction of airflow depending on which air (inside or outside) was the less humid.

If we cannot exhaust our stirred-up air outside, that may be ok - if we run one or multiple dehumidifiers whose condensate is collected and taken by gravity or pump to a drain. I've seen very good dryout success using the combination of extra circulating fans and a constant-running dehumidifier. The additional fans significantly increase the efficacy of the dehumidifier, increasing the rate at which it removes water from crawl space air.

Following an initial dryout, if it was not already addressed by implication in your question, we must make darn sure we've stopped water from entering the crawl area. This means making sure that roof drainage is away from the building, that there are no other leaks into the crawl area (such as from plumbing, or even a nearby spring), and that we have adequate moisture barrier (6 mil poly) on the crawl space (dirt?) floor. Articles above include addressing the crawl space ground cover.

Question: what about drying out a crawl space over a radiant heat floor?

Thanks for all the info, very helpful.

Question. What about a crawl space with in floor radiant heat? Built in the 1940s in Flagstaff, AZ. So very cold winters and mild summers. The crawl space is damp and I have found rotting wood at the foundation. I can do the above repairs as noted, but does the in-floor radiant heat change any suggestions with regards to venting or vapor barrier. Also, I have read elsewhere that just a vapor barrier on the ground, not necessarily up the foundation walls, will stop 80% of the moisture. Is this adequate? Thanks for your help.
- Robert 11/4/2012

Reply:

Robert, it's an interesting question that I'll think about further, but my initial thought is that presuming you're talking about tubular radiant heat - tubes stapled up under the subfloor over a crawl area, I don't think that material's presence changes our normal recommendations for drying out a crawl space

- remove the sources of water entry

- put down poly

- inspect and clean if moldy

- convert to a heated, conditioned space

You are right that in your climate, most of the benefit of poly will be from what you place on the ground, though in winter there might be condensation on the interior surface of the block foundation around the crawl space - moisture you can keep out of the crawl area by running poly up the walls.

Question: Hurricane Sandy flooded our Ocean City NY House - what do I do in the crawl space?

Hurricane Sandy hit my old Sears House (1930's)in Ocean City, NJ so the water was 1 foot deep on the first floor (the floor being 3 feet off the ground!) Sometime within the last 25 years the water was deep enough to get the insulation wet and it was removed at that time and not replaced (The house is not heated in the winter).

What should I do in the crawl space area? If I would encapsulate the crawl space, it would be like a swimming pool next time it floods. Ideas on a good solution? Is a spray insulating foam worthwhile for protecting flooring from underneath? Thanks!
Rick - 11/12/2012

I have also a home in Ocean City, NJ that got hit with Sandy. I have just pulled out the mostly wet insulation. I got water damaged in my first floor from the water seeping up from the craw space. Electric wires are down now and I am worried about structural damage.
Can anyone recommend someone in South Jersey who could do work of reinforcing existing floor supports?

These past week I am worried that those who are hired to do work are being extremely careless and doing other damage. THanks - Roberta 11/13/2012

Reply: tips for improving the resistance of crawl spaces to flood damage

Rick,

Closed-cell foam products are somewhat resistant to wetting from flooding or other water intrusion in a building crawl space or anywhere else, but if a building area is actually inundated with floodwaters again after such an installation, I'd be concerned about the difficulty and cost of disinfecting or addressing the risks of sewage-contaminated floodwaters.

For this reason, just taking up some closed cell foam board can be problematic - how will you clean the space that was soaked with sewage waters between the foam boards and framing or subfloor above?

Talk with spray foam insulation contractors in your area about the water resistance of sprayed-in-place closed cell foam insulation. That product actually adheres to wood surfaces, a step that may resist sewage-contaminated water from entering the space between the insulation itself and wood surfaces that otherwise would need cleaning.

See

• CRAWL SPACE INSULATION RETROFIT
• Open-celled vs. Closed-cell Foam Insulation and take a look at and compare
• Foam Insulation Types - Visual Id of different types of foam insulation products
• ICYNENE FOAM SPRAY INSULATION
• POLYISOCYANURATE FOAM and IAQ
• URETHANE FOAM DETERIORATION & OUTGASSING where we discuss polyurethane as well as urethane closed cell foam insulation

In addition, sealing the exposed wood and interior foundation surfaces with a sanitizing or fungicidal sealant (after they are thoroughly dry) will also reduce the moisture uptake (and sewage-contaminated water uptake) of those surfaces in future flooding, making surface cleaning and area dryout a bit faster after the next flood.

• See MOLD SANITIZER, SPRAY, BIOCIDE USE GUIDE for details. Our photo above shows a crawl space after an extensive cleanup; the remediator installed 6 mil or heavier plastic on the crawl space floor, and all surfaces were coated with a sealant.
• Also take a look at FLOOD VENTS & FLOOD PORTS - a product installed to deliberately allow floodwaters into a basement or crawl space to reduce the risk of foundation collapse due to outside floodwater pressures.

Frankly, if the home is likely to be flooded to a depth that submerges the first floor or higher, no crawl space encapsulation is going to completely protect the building; if you cannot afford to raise the building on a taller, flood-damage-resistant foundation or pier system, I'd be troubled about the prospect of recurrent, perhaps even increasingly frequent significant cleaning and repair costs from future area flooding.

...

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

Or see these

Recommended Articles

• CRAWL SPACE ACCESS

• CRAWL SPACE CLEAN UP
• CRAWL SPACE DEBRIS
• CRAWL SPACE DISINFECTANTS & SANITIZERS, SOURCES
• CRAWL SPACE DRYOUT - home
  • CRAWL SPACE DEHUMIDIFICATION
  • CRAWL SPACE DRY-OUT PROCEDURE
  • CRAWL SPACE GROUND COVERS
  • CRAWL SPACE MOISTURE BARRIERS
  • CRAWL SPACE SEAL & SANITIZE
  • CRAWL SPACE WATER ENTRY STOP
  • CRAWL SPACE WATERPROOFING
• CRAWL SPACE INSULATION RETROFIT
• CRAWLSPACE MOLD ADVICE
• CRAWL SPACE SAFETY ADVICE
• CRAWL SPACE SEWAGE CLEANUP
• CRAWL SPACE VAPOR BARRIER LOCATION
• CRAWL SPACE VENTILATION CODES
• CRAWLSPACE MOLD ADVICE
• HUMIDITY CONTROL INDOORS - home

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