Moisture & Fungi

When, Where, How, & Why to Install Moisture Barriers, Heat & Dehumidification Equipment in the Crawl Space

Provide a moisture barrier over any dirt floors and extending up foundation walls but not up to nor in contact with wood framing or sills. /p>

Moisture barriers may be 6-mil poly or other special basement sealer products such as special moisture-sealing concrete coatings.

The crawl space moisture barrier system shown here uses mechanical fasteners and adhesive to secure the plastic barrier to the foundation walls near the tops (but short of wood framing to reduce insect attack risk).

We install moisture barriers to stop the "pumping action" that draws moisture into a building through the foundation walls or through the crawl space dirt or concrete floor. Moisture pumping occurs as moisture evaporates off of the indoor surface into building air. This process invites movement of additional water molecules through the outside surfaces and into the building.

Choose your crawl space plastic vapor barrier material carefully. Some plastics can give off an obnoxious odor themselves - an effect which may become suddenly noticeable and more significant than you anticipated once you've spread a smelly plastic out over a large crawl space area below a building.

It may take some time for such odors to dissipate. (We don't have data on possible health effects of the volatiles and gases comprising the odor from smelly plastic vapor barriers. Contact us if you do.) The white reinforced plastic material shown in the photos here was not producing any noticeable smells at the time of our inspection.

Our photo shows a successful cleanup of a very moldy crawl area that had a gravel and dirt floor. The framing was cleaned, dried, then sealed with a Fosters™ clear fungicidal encapsulant, and a heavy poly vapor barrier was placed on the crawl area floor and sealed neatly throughout. Even high quality workmanship such as this job may not last however, if the building owners do not take the outside steps to keep water out of the crawl area.

Crawl Space Moisture Control

Moisture warning when installing concrete in a crawl space: Poured concrete or similar dense coating on crawl space floors and lower walls is a crawl space sealing and cleaning process offered by some mold and flood damage remediators as an alternative to the plastic vapor barrier.

Before pouring concrete or similar coatings into a confined space under a building you should determine if the surge of moisture that will be generated under the building is going to be a problem for other building areas.

If so, you'll need to take steps to direct that moisture outside rather than up through the building.

In hot humid climates, venting a crawl space is actually asking for trouble: moisture-laden warm air entering a crawl space leads to accumulated condensation and water and to a building moisture and often building mold problem.

Be sure that the dirt floor moisture barrier slopes to a drain.

Great many installations we see, even in company product literature, show multiple low spots in a crawl space, each of which is subject to ponding and water retention from leaks or other unanticipated water entry into a crawl space or basement.

How to install a continuous dehumidification system in a crawl space: (with a permanent drain hookup so the 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.

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 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.)

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.

Key building crawl space water entry diagnosis and cure articles

Mold and mildew growth: A damp environment is very unhealthy and destructive. Mold thrives and reproduces by producing airborne spores by the millions, some of which are toxic.

High humidity: High humidity is absorbed into the wood structure and moves upward into the hardwood floors which causes the floors to "cup". Cupping is when the hardwood floor absorbs so much moisture the wood swells and pushes upward at its edges.

Structural damage: Rot and decay occur in damp environments causing structural damage.

Higher energy costs: Heating and cooling costs are higher in a home with a vented dirt crawl space.

Insects and critters: Insects and critters of all kinds love damp environments and wet materials.

Repair and Replace

Dirt crawl spaces will always have humidity that creates a foundation for wood rot. Before outfitting the crawl space with a vapor barrier and dehumidifier, take the time to repair water-related issues. Inspect the roof of the house for leaks. Fix all downspouts and gutters. Look for plumbing leaks in the house in the bathrooms, the kitchen and the laundry areas. Check that water drains away from the foundation. You may need to grade the property to allow water to drain away from the crawl space. Remove and replace any damaged wood in the crawl space.

Encapsulate

Install a vapor barrier in the crawl space. The vapor barrier will lower your utility costs and reduce mold and insect infestation. The floors in the house will be warmer in the winter. Check local building codes before installing a vapor barrier. Use at minimum a 6-mil plastic. Overlap the seams and use construction tape to seal the seams. The plastic should run 6 inches up the walls and be taped. Moisture proof and insulate the walls with a fire-resistant barrier. Insulate the joists. Place a termite barrier on the inside of the foundation.

Dehumidify

Install a dehumidifier that is meant for crawl-space use. This type of dehumidifier does not have a bucket that requires emptying and will only run when necessary. Place the humidifier in the middle of the crawl space on a solid surface that is higher than the crawl space to allow for gravity drainage. The dehumidifier will reduce moisture and eliminate mold, dust mites and wood rot. The dehumidifier will need to be plugged into a standard three-prong outlet that is GFCI protected and out of the water. The dehumidifier can drain through a sump pump, an existing HVAC drainage system or a garden hose.

Crawl-space dehumidifiers are sized based on cubic feet in the crawl space and how much moisture needs to be handled on a daily a basis. Additionally, the dehumidifier must be able to fit in the crawl space. Larger crawl spaces might need more than one dehumidifier installed.