Dehumidifying a crawl space (C) Daniel Friedman Crawl Space Dehumidification: cures for damp crawl areas
Using heat, dehumidifiers, & exhaust vent fans to dehumidify a crawl space

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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.

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

Dehumidifying a crawl space (C) Daniel Friedman

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.)

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.

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.


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




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