How to Dry Out a Crawl Space and Keep it Dry, avoiding crawl space mold, insect damage, rot, & allergens
InspectAPedia® -
Ten steps to drying out the crawl space and keeping it dry: how to remove moisture and water from crawlspaces and how to keep the crawl area dry
How to deal with a crawl space that has no ready access
How to Inspect & Assess Crawl Space Conditions for Water or Other Problems
How to Stop Ongoing Crawl Space Water Entry
How to Dry Out the Crawl Space Quickly in Three Steps
How to Clean Out the crawl space: get rid of unsafe debris, junk, insulation
How Clean Up the crawl space: remove asbestos, mold, rodent droppings, unsanitary soils, insulation
How to Sanitize and/or Seal Crawl Space Surfaces including the floor: is it needed?
How to Keep Out Crawl Space Water & Moisture - Interior Measures for Crawl Space Moisture Control
How to Keep Out Crawl Space Water & Moisture - Exterior Measures for Crawl Space Moisture Control
Crawl space de-watering, crawl space sealants, and crawl space insulation & ventilation advice
Questions & answers about how to dry out a wet crawl space & prevent future crawl space water entry
This article describes the steps needed to get into, inspect, clean, and then dry out a building crawl space. We add advice on how to keep the crawl space dry and clean so that this process doesn't have to be repeated. This 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.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers nor with topics or services discussed at this website.
How to Dry Out a Problem Crawl Space, Remove Mold, Rodent Debris Unsafe Materials, & Then Keep the Crawl Space Clean & Dry
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. Separately our series of basement dry out, clean up and leak prevention articles begins at BASEMENT LEAKS, INSPECT FOR. Also see MOLD CLEANUP - MISTAKES to AVOID for a master list of the principal ways that people foul up mold cleanup projects.
Damp or wet crawl spaces or basements are often a source of health and structural problems in buildings. Wet areas beneath the occupied space invite mold contamination, insect attack, and structural rot and may also contribute to bacterial hazards. Keeping these spaces dry and clean is not difficult if we address the steps needed in the right order.
The crawl space shown in our page top photo was in our opinion not a readily accessible area because of flooding. This decision is made by the inspector on the scene, not by anyone else. The crawl space shown at left was tight and so junk-filled it could not be entered either.
We break down a thorough crawl space dryout and cleanup process into these steps:
Get into the crawl space. If there is no crawl space access door, make an entry if necessary; if there is no space to get into the crawl space, outside clues might justify making inspection portals through the foundation wall or through an interior floor.
Stop water from entering the crawl space: find and fix the sources of water entry or high moisture
Dry out the crawl space; get rid of crawl space water & high moisture levels; remove crawl space water, moisture, dampness that is already there. At Dry out we describe the steps necessary for rapid crawl space dryout.
Clean-Out the crawl space: get rid of crawl space debris. Junk left in the crawl space makes inspection and cleaning difficult and can support mold, rodents, hold moisture, etc.
Clean-Up the crawl space: clean or remove mold from crawl space surfaces; remove unsanitary dirt, rodent droppings, dead animals, etc.
Sanitize and Seal Crawl Space Surfaces including the floor. When should we use spray disinfectants, sanitizers on crawl space surfaces? What about sealing wood surfaces?
Keep Out Crawl Space Water & Moisture - Crawl Space Moisture Control. Crawl space interior measures to keep water out: sump pumps, dehumidifiers, drainage, plastic or poly moisture barriers on the floor and walls
Prevent future outside water from entering the crawl space: check roof drainage, gutters, downspouts, surface grading
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.
How to Get Into a Crawl Space That Has No Ready Access
Watch out: Before entering a crawl space to inspect its condition or to clean up the crawl space or make repairs in the crawl area, first please see CRAWL SPACE SAFETY ADVICE. While some crawl areas under buildings are safe, roomy, dry, and easy to move around in, other crawl spaces can be really ugly or hard or even impossible to enter without making an access opening and wearing protective gear.
Entering a hard-to-access crawl space: if necessary, make an access opening and while there fix everything you can
At a home where there was no crawl space entry whatsoever, and where inspection was necessary, we pulled back carpeting, found the floor joists by noticing the nail pattern in the subfloor, cut a removable subfloor panel at the center of joists (so it would be easy to replace), and made our own crawl space entry door.
In a different building investigation, the crawl space shown in our photos (below) turned out to be clean and dry. But access to the crawl space very tight and like the home in our photo at left, the second building originally it had no entry opening whatsoever.
We needed to get in to the crawl area to inspect (and repair) floor framing support (shown in our photo at below left) as well as to understand house conditions.
We took advantage of a bathroom renovation project to make an opening through the flooring above.
An alternative crawl space entry could have been made from outside with a bit of digging and cutting an opening through the foundation wall. We left a removable panel to enter the crawl area. If an outside entry had been made through the foundation we'd have made sure that the entry itself didn't become a water entry pathway by protecting it from surface and roof water.
Once going to the trouble of making an access to get into the crawl space we don't waste the effort. We reinforced existing floor supports that had not been checked since the home was built in 1920 and we added a few improved posts ourselves (above left). We then added insulation (above right) under the floors and even though this crawl area was dry, before leaving we put down 6-mil poly on the dirt, making crawling (slithering in this case) easier for the next person who would have to work in the area.
What to do if a crawl space is literally inaccessible
If a crawl area is literally inaccessible because there are only a few inches between the floor framing and the crawl space surface, no one can enter it.
If there are crawl space vents it may be possible to remove the vent screen to permit a partial view into the crawl space, but it's not easy.
Crawl space vent covers were typically installed and secured from inside the foundation wall during construction. I'd consider destroying the vent cover if necessary, replacing it from outside later.
For such areas we have made inspection openings through the floor above. If on inspection we find conditions that absolutely need repair such as toxic mold, sewage, rot, insect damage, then the floor will have to be removed to permit repairs.
What if there are no crawl space entry openings and no holes to even peek into the area?
Our pro-bono inspection of a church crawl space (you couldn't pay anyone enough to do this) began at the floor opening shown at above left. Our second photograph (above right) shows how little space there was between the floor joists and the soil below. This crawl space is impossible to enter.
Inaccessible Crawl Spaces: using secondary clues about building condition
When a crawl area is impossible to enter and when view is quite limited we have to rely on secondary clues of crawl space condition. Examples of such clues that are collected from above include:
Outside: drip lines below roof eaves indicating a history of gutter overflow and water spillage by the foundation - high risk of water entry, rot, insect damage.
Indoors: sagging floors, crunchy flooring (termite damage), odors, evidence of insects coming up through the floor, evidence of recent structural repairs, evidence of insect pest treatment;
When in the judgment of an experienced building diagnostician the accumulation of clues suggesting hidden trouble is sufficient, it becomes cost justified and appropriate to begin further invasive inspection by making test openings and where those inspections confirm trouble, more flooring will need to be removed.
How to Inspect & Assess Crawl Space Conditions for Water or Other Problems
Hazards in some crawl spaces include breathing moldy or unsanitary air, getting poked by a rusty nail, stirring up a hornets nest, getting shocked or electrocuted by unsafe wiring while crawling over wet ground, crawling through unsanitary water from burst waste piping, kneeling in unsafe pesticide chemicals left by an ignoramus, and the occasional spider, rodent, snake, or even trapped raccoon.
Wear appropriate protective clothing, use a good light, and don't work alone.
Take a thorough look in all areas the crawl space itself for water and dampness and for unsafe or unhealthy conditions such as
Water: puddles, water stains, signs of prior leaks or actual crawl space flooding, as well as odors that indicate mold or dampness
Moldy crawl spaces: start with a careful visual inspection for mold or mold suspect materials. Keep in mind there may be hard to see or even "invisible mold" such as moldy insulation
Wet insulation or inappropriate insulation materials (we don't use fiberglass or other fibrous insulation materials in wet or damp areas)
Rodent or other animal feces or droppings or urine, or smells from dead animals
Unsafe building materials such as falling or loose asbestos pipe insulation, construction debris, rusty nails
How to Stop Ongoing Crawl Space Water Entry
Stop water from entering the crawl space from outside: this means attend to roof gutters and downspouts - the most common cause of crawl space water entry or high moisture in most areas.
Our photo shows frozen water in a wet and cold crawl space below a condominium in Poughkeepsie, New York. The very wet masonry blocks along the rear wall of the crawl area and water stains higher on these walls tell us where the water is coming from.
If a building site is unusually wet or springs are adjacent to the building foundation, additional steps such as the installation of a drainage system and sump pump inside or exterior foundation drainage and waterproofing could be necessary.
But before pursuing those costly measures, be absolutely certain that roof runoff and surface runoff have been directed well away from the building.
Inspect the building exterior, from roof to ground, for sources of water that need to be directed away from the structure.
Also see BASEMENT WATERPROOFING and see FOUNDATION WATERPROOFING that discuss various methods used to keep water and excessive moisture out of buildings or to get rid of water that has leaked into a building.
Stop water from entering the crawl space from inside: this means:
Inspect the building interior, attic to basement and crawl spaces, also looking for sources of leaks, moisture, improper venting, that could affect the structure. In the basement and crawl spaces, look for visual evidence of the pattern of water entry - where has water been entering the building? Trace that water to its source and cause.
Repair any leaks in building drains over or in the crawl space. Check below bathrooms, kitchens, and also check hot water heating lines or steam condensate return lines in the crawl space.
Repair any leaks in supply piping in the crawl space.
Insulate cold water pipes in the crawl space to stop dripping of condensation that forms on the cold water lines in humid weather. It's a good idea to do this in the basement too. See CONDENSATION or SWEATING PIPES, TANKS
Crawl space venting – this is a debated topic as in humid weather venting outside air into a cool crawl space might increase crawl space moisture.
We 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 it to a dry, conditioned space.
That means we close off crawl space vents, dry out the area, and add some heat to it.
Crawl space venting practices and building codes specifying crawl space vent areas are discussed further at CRAWL SPACE GROUND COVERS.
How to Dry Out the Crawl Space Quickly in Three Steps
Here is a Step by Step Guide to Drying out the Crawl Area and Keeping it Dry
Step 1: Remove crawl space debris and wet materials - without this step the dry-out will be more difficult, take more time, and be more dangerous. Do not try to "dry out" wet or damp or suspect fiberglass crawl space insulation. Just remove and discard it. But don't insulate the crawl space yet. There is more cleaning, drying, and sealing work needed first.
We describe first Clean Out the Crawl Space - that means get rid of insulation, junk, debris, then we describe Clean Up the Crawl Space - that means dealing with mold or other surface contaminants.
Step 2: Remove standing water in the crawl space. This process can go on more or less simultaneously with debris removal.
At How to Use Sump Pumps in Crawl Spaces we describe that procedure. But depending on the shape of the crawl space floor surface you may need to use one or more portable sump pumps of the type that pick up water from a wet "floor" surface.
If the crawl space floor were flat and drained to one single low point, just one sump pump would usually be enough. But if there are significant puddles or multiple low areas, use one or more portable sump pumps to remove that standing water. Later you'll want to level the crawl space surface so that should water enter it will drain to a single pump location.
Step 3: Remove moisture from the crawl space materials and surfaces. This means dry out wood framing, subfloor overhead, and the crawl space floor surface.
The rate at which a dehumidifier removes water from the air depends on the size of the unit (how many quarts of water it can remove from air per hour) which in turn depends on the cooling capacity of the dehumidifier, ambient temperatures, the moisture in ambient air, and, very important: the rate at which moist air moves from the dehumidified area (crawl space in this case) to and through the unit.
By placing multiple fans around the crawl area you can increase airflow across wet surfaces (crawl space floor, walls, ceiling).
Connect the dehumidifier output directly to a floor drain or direct the output outside of the building to a location at least 10 feet away and where water will continue to drain away from the building. Obviously in freezing weather you'll need to direct dehumidifier output to an indoor drain. Use a condensate pump and plastic tubing if your dehumidifier does not itself include a lift pump drainage system.
Watch out: if you start running air moving equipment in a contaminated crawl space (mold, asbestos etc) before those contaminants have been removed, the air in that space is likely to be unsafe.
Do we need to blow crawl space air to outdoors during the rapid dryout phase?
If the crawl space air temperatures are lower than those in the occupied space above, the thermal updraft that might move moisture (and possibly contaminants) into the occupied space may not be significant.
But if temperatures in the building above and the crawl space are within 10 degF. of one another, or if the crawl space is visibly moldy or suspected of containing other contaminants (rodent debris, asbestos), then you want the crawl space air pressure to be negative with respect to the building above - so we don't push contaminated air upstairs. That means using exhaust fans or if appropriate, professional negative air equipment.
Do we also need to add heat to help dry out the crawl area quickly?
It depends. Warm air carries more moisture than cool air and thus will deliver moisture to the dehumidifier more rapidly. The answer is ... it depends on the relative humidity in the area and ambient temperatures. If the relative humidity (RH) is high, say over 60%, and temperatures are over 60 degF. (this is speculative opinion) you probably won't gain much by adding heat to the crawl space during dryout.
We don't want the crawl area so warm that we start sending moist, possibly moldy crawlspace air riding thermal currents into the occupied space of the building. But if crawl space temperatures are low, say below 50F, you will probably speed up drying by adding some warmth, beyond that already provided by the exhaust side of your dehumidifier fan motor.
How to Clean Out the crawl space: get rid of unsafe debris, junk, insulation
Remove other Hazardous Materials Such as Asbestos in Poor Condition
Remove crawl space debris from the area.
Not only are nails, splinters, and possibly rodents that may be in the debris a hazard to workers entering the area, these materials also invite termites, carpenter ants, or rodents which in turn damage the building.
Old cardboard boxes and stored junk invite mold growth and rodents.
Do not bring mold-suspect boxes of stored items out of the crawl space (or basement) back into the living area of the building. Take them outside for inspection and if appropriate, cleaning and salvage.
Is there wet or falling or rodent-infested fiberglass insulation in the crawl area? If so there is a high risk of mold or rodent contaminants that could present a fungal, bacterial, or viral airborne hazard. We recommend completely removing all fiberglass insulation from crawl spaces.
Fiberglass is an effective and cost-effective building insulation product that performs very well in many applications. And from the factory you can expect fiberglass building insulation products to be nice and clean. But we do not recommend this material for use in crawl spaces or other tight, limited-access under-building areas nor for use against foundation walls.
If the floors above a crawl area or the rim joist or perimeter of the building need to be insulated we prefer to use spray foam or solid foam insulation in such areas because they resist moisture uptake. Here is a photo of a crawl space that was insulated with spray foam insulation.
How Clean Up the crawl space: remove asbestos, mold, rodent droppings, unsanitary soils, insulation
Is there asbestos insulation in the crawl space?
Often we find a crawl area in which the asbestos pipe insulation is not just hanging (photo at left) but has fallen onto the crawl space floor.
Asbestos pipe or HVAC duct insulation that has been disturbed, damaged, or deteriorated, including asbestos that has fallen to the floor should be removed and cleaned up by a professional. Asbestos pipe insulation that is in good condition, that is not damaged, might be left in place or perhaps left in place and encapsulated.
Do not enter such an area without protective equipment; take care that you do not track hazardous materials out of the crawlspace and into other building areas.
Do not track asbestos debris back into other building areas.
Is there evidence of mold contamination such as areas of wood, paper, or other material covered with mold or mold-suspect material.
If the crawl area has more than 30 sq.ft. of contiguous toxic or allergenic mold it should be cleaned by a professional mold remediation company.
Beware: often the most serious mold we find in crawl spaces is not the mold you see on wood framing but rather the mold you don't see in fiberglass insulation in a damp or wet area. See Mold in Fiberglass Insulation for details.
Watch out: If the crawl space has a large area of mold (more than 30 sq ft) or if you suspect it may be contaminated with rodent droppings, chemicals, pesticides, (or electrical hazards) see CRAWL SPACE SAFETY ADVICE and CRAWLSPACE MOLD ADVICE. Do not enter such an area without protective gear.
Beware also of rodent-infested crawl areas; there may be bacterial or viral hazards from these animals, or from their urine and feces, and in some areas there may be a Hantavirus hazard.
Hantavirus pulmonary syndrome (HPS) is a deadly disease from rodents. Humans can contract the disease when they come into contact with infected rodents or their urine and droppings.
HPS was first recognized in 1993 and has since been identified throughout the United States. Although rare, HPS is potentially deadly. Rodent control in and around the home remains the primary strategy for preventing hantavirus infection. - [3] US CDC
If it is necessary to remove rodent feces or urine-stinking soil (try using a "black light" to spot urine stains) your cleaning company may decide to actually remove the top few inches of contaminated soil in the most offensive areas, followed by sanitizing, sealing, and poly vapor barriers on crawl space floors and lower walls.
Watch out: don't remove so much soil that footings are undermined at foundation walls or supporting columns. If necessary, bring in and compact clean fill and gravel to fill low areas.
Should we Use a Vacuum Cleaner to Clean up the Crawl Space?
Do not use a household vacuum cleaner nor an ordinary shop-vac to clean up crawl space dust and debris. Those machines will temporarily but significantly increase the level of airborne dust and debris. Since that dust and debris may contain harmful particles, tossing it into the air increases the risk of cross contamination from the crawl space into other building areas such as a basement or even upper floors in the building.
To vacuum and clean surfaces in a crawl area use a HEPA-rated vacuum cleaner that will trap very fine particulates, and select a vacuum cleaner model that does not have air bypass leaks that escape from the equipment when they should be passing through the filter.
Do we need to Set up Dust Containment and Negative Air to Clean Up the Crawl Space?
Negative air machines and dust barriers are procedures used by professional mold remediation companies, asbestos removal experts, and building flood damage restoration companies. These measures reduce the risk of blowing contaminated dust and debris into other building areas - an event which could be a health risk, especially to occupants who are in fragile health. Photo courtesy of Anabec, Inc.
Even an amateur, do-it-yourself cleanup project would benefit from these methods.
But you should recognize when your cleanup project needs the help of a professional. Here are some examples of conditions that mean you should hire a professional to clean up a crawl area:
There is asbestos material that needs to be removed
The area contains a large reservoir of harmful mold, more than 30 square feet of contiguous moldy material
The area has been contaminated with sewage and sewer backups
The people who plan to do the cleanup are not fit, healthy, and properly dressed and equipped
Other building occupants are at particular risk of health effects from stirred-up building dust that may contain mold, fiberglass insulation fragments, bacterial, viral, or other hazards. Examples are the elderly, infants, asthmatics, people suffering from COPD, or people suffering from MCS.
Should we use a Power Washer to Clean up the Crawl Space?
If there is room to work, a power washer is a very effective way to clean surfaces, but the increase in water and moisture in the crawl space can make mold or other building-related moisture conditions worse.
In some circumstances such as a crawl area or basement that can be well ventilated, in dry weather, that is well drained, and where other measures are taken to avoid sending a surge of moisture upstairs, power washing might be possible.
Alternative dry-spray surface cleaning methods called media blasting include use of dry-ice spray and baking soda or other abrasive particles, even sand or walnut shell fragments. See MOLD CLEANUP by MEDIA BLASTING. Our photo shows the ceiling over a basement after cleaning by media blasting.
How to Sanitize and/or Seal Crawl Space Surfaces including the floor: is it needed?
When Is it Good Practice to Use Biocides, Sanitizers, or Fungal sprays in a Crawl Space or in Other Building Areas?
First it is important to physically remove contaminated or moldy materials and debris or materials and debris that have been contaminated with sewage or flood water sludge and mud.
Second it is important to physically clean the contaminated surfaces in a crawl space such as moldy joists or subflooring.
Third, use of a biocide, disinfectant, or surface encapsulating spray as a final step makes people feel more confident about the mold or sewage cleanup, but although this is a widespread practice, the use of biocides is questioned by experts.
One could certainly question the usefulness, effectiveness, and wisdom of spraying carpeting such as shown in our photo. If the carpeting is so infected as to "need" spraying with a disinfectant, it should be removed and the surfaces cleaned instead.
Anabec NG2000™ and Bioshield BST Protectant RTU75™ are products used in mold remediation projects. These companies produce fungicides and fungicidal sealants as well as related items.
CalBrite™ (Caltex International) was sold to one of our clients as a biocidal spray but the client's product's package indicated that it was a deck cleaner. And while online access to product literature described the product as a deck cleaner, that document was later removed from public view. We were left confused. The company makes a variety of products used in or on buildings.
In general, deck spray cleaning products are intended for outdoor use to clean wood-framed decks and deck surfaces. A representative of the company stated that information here is incorrect, but was unwilling to provide specifics.
In a singularly unpleasant response to our telephone request for clarifying information, the Caltex president declined to provide any information whatsoever, though a later letter from the company's lawyers explained that the company describes its product as a "sanitizer" not a biocide.
At MOLD SPRAYS, SEALANTS, PAINTS we sort through these terms and help clear up consumer confusion among these terms including biocide, disinfect, sanitize, and sterilizer and we include recommendations from an expert on the use or non-use of chemicals to try to kill mold.
Microban Nuocide™ ( Microban antimicrobial products) (produced by a company whose contact was more pleasant) is an example of a broad spectrum biocidal spray that is in wide popular use by mold and flood damage remediation contractors.
Also see Protex™ 63, a sealant and coating formulated for exterior use.
When is it Good Practice to use Fungicidal Sealants and Encapsulant Sprays in a Crawl Space or in Other Building Areas?
Clean the crawl space, don't just spray it: Do not, however, permit the use of biocides, disinfectants, sprays, or encapsulants as a substitute for the physical cleaning that must come first.
Otherwise there is risk that you will leave harmful contaminants and particles in the building, and it is likely that cleanup will be inadequate. Look at the thick debris sprayed-over in this building. Simply stirring the debris shows that this approach was ineffective.
Fiberlok IAQ 6000 HD™ (above, left, misapplied right over a high volume of loose debris), Anabec X70™ waterborne sealant, and Fosters 4051™ (clear coating shown at above right) produce sealants frequently used by mold and flood damage remediation companies.
Once the crawl area has been cleaned of debris and moldy materials, and crawl space water has removed, and after we've eliminated the sources of crawl space water entry, we are ready to take the next steps to keep the crawl area clean and dry.
How to Keep Out Crawl Space Water & Moisture - Interior Measures for Crawl Space Moisture Control
Below we focus on measures to take inside the crawl space to help keep that area dry. Things to do outside the building to keep water out of a basement or crawl space are outlined later at Exterior Measures for Crawl Space Moisture Control.
Crawl Space Surface Slope & Drainage Specifications
Be sure that the dirt floor moisture barrier slopes to a drain.
Before putting down plastic to keep moisture down in a crawl space, be sure that the crawl space floor surface is smooth - we don't want puddles of water below nor above a poly vapor barrier that we may be planning to put down.
In addition it's important that the smooth crawl space floor surface also slope gently towards one or more drain points where, if necessary, you'll be able to install sump pumps to remove any water that might enter in the future. Slope on the crawl space floor can be as little as 1/8" per foot.
A 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.
Crawl Space Floor & Wall Moisture Barriers: When, Where, How, & Why to Install Moisture Barriers, Heat & Dehumidification Equipment in the Crawl Space
Provide a moisture barrier over any dirt or gravel crawl space floors and extending up foundation walls but not up to nor in contact with wood framing or sills.
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.
Crawl space 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.
Should the crawl space plastic go under or on top of the gravel in a crawl space?
Some installers like to put down the plastic then cover it with gravel, arguing that the gravel will "protect" the plastic from damage. That approach is shown at left.
In our opinion, placing gravel on top of the crawl space plastic is a bad idea.
First, gravel on top of the plastic won't work well. The gravel, especially sharp gravel, is more likely to puncture the plastic ground cover.
Second, if gravel is on top of the plastic we can't see that the plastic was placed over an adequately smooth sloped crawl space floor - the risk is future puddles of water that collect on top of the plastic but hidden from view under the gravel - an ongoing crawl space water and moisture trap.
Third, we can't even see the plastic to know that it was properly placed and is continuous.
As you may have noticed in our first photo at the start of this section (Keep Out Crawl Space Water & Moisture), if you can see the plastic you will see any puddles that are forming on top of it.
Use heavy 6-mil poly or stronger plastic on top of whatever else is on the crawl space floor, and walk or crawl with care. Also see CRAWL SPACE GROUND COVER.
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. See CRAWL SPACE GROUND COVER.
Advice About Pouring (Placing) Concrete on Crawl Space Floors
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.
The sketch of the role of a dirt floor crawl space in building moisture and mold problems (above) is courtesy of Carson Dunlop. 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.
Watch out: It is almost always preferable to keep water from entering a building rather than allowing it to enter and then working to get rid of it. But if you are going to do something permanent like pour a crawl space slab, and if your building has had chronic crawlspace water entry, consider including the measures described at PERIMETER DRAIN SYSTEMS. It is much less costly to install a perimeter drain system before a concrete slab has been poured than afterwards.
How to Use Heat and/or Dehumidifiers in Crawl Spaces to Keep Moisture Levels Down
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.
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.
How to Install & Use Sump Pumps in Crawl Spaces
It's always better to keep water out of buildings than to let it come in and then try to get rid of it. But some building sites and conditions may still justify one or more sump pumps in the crawl space. Earlier we stressed the importance of making sure that the crawl space floor drains to one or more points where as sump pump can be installed if needed.
Not like this! We cannot show all of the ways to foul up a sump pump installation in one article, but our photo at left is particularly disgusting. Don't just throw a sump pump into a low area in the floor. The resulting lake will continue to damage the rest of the building.
At SUMP PUMPS GUIDE we discuss types of sump pumps and how they should be connected to electrical wiring and to drainage destinations. Keep in mind that the time you are most likely to need a crawl space sump pump is during hurricanes or tropical storms or in northern climates during times of heavy snow melt.
During a storm is just when electrical power may be lost. If your electrical power is not reliable you should consider a battery-operated backup sump pump system with enough capacity to keep the pump(s) running until power is restored.
How to Keep Out Crawl Space Water & Moisture - Exterior Measures for Crawl Space Moisture Control
We noted earlier that it is almost always preferable to keep water from entering a building rather than allowing it to enter and then working to get rid of it. Here we refer to articles giving more detail on measures to keep unwanted roof runoff or surface or even subsurface water from entering a basement or crawl space.
Inspect the roof drainage system, gutters and downspouts to be certain that roof spillage is not ending up by the building foundation. Defects in handling roof runoff is the number one source of basement and crawl space dampness and water entry.
Key building crawl space water entry diagnosis and cure articles:
Frequently Asked Questions (FAQs) about crawlspace de-watering: how to dryout the crawl space and how to keep that area dry
Question: crawl space insulation and vapor barrier retrofit questions
I live in Syracuse, NY in a 1920 colonial with full basement. I recently put on an 18'x 20' addition for my mom. Bedroom and bath. The crawl space is open to the full cellar via the former cellar window opening. The crawl space has water lines and p-trap for shower as well as heat/cold air runs. The contractor installed 1 small vent on each side of the addition. Should I permanently close off the vents and turn the crawl space into somewhat heated and conditioned space? The contractor was going to install batt insulation on the underside of the floor- I told him to hold off. Should I install 10mil vapor barrier on the dirt floor and use 2-part closed cell spray foam insulation on the interior of the block? Or is it a better method to pour a concrete slab and use the 2-part closed cell foam? I understand I may need to cut a small register in the heat run as well as run a dehumidifier in the warmer months. I will also be installing a radon mitigation system soon. Thank-you very much for your response. - J.R. - Syracuse
Reply:
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with air and moisture control for a structure that combines crawl and basement areas. That said, here are some things to consider:
First, kudos for packing so many good questions into a small (crawl) space:
The crawl space has water lines and p-trap for shower as well as heat/cold air runs.
Leaving the crawl area open to the basement will make it easy to access, easy to inspect on occasion, and it'll share air (and thus anything else) with the basement. Your comment that there is plumbing and A/C ducting there means there are potential leaks and condensation sources - you don't want them hidden and you want to prevent condensation - at least by insulating the cold water lines.
The contractor installed 1 small vent on each side of the addition. Should I permanently close off the vents and turn the crawl space into somewhat heated and conditioned space?
Your suggestion is conventional wisdom. The problem is that at times blowing air into the crawl adds unwanted moisture, and 2 small vents isn't going to dry anything out anyway, even in the best of conditions. There is just not enough dry air movement into the space - during the part of the year that such an approach might work. So I agree with you.
The contractor was going to install batt insulation on the underside of the floor- I told him to hold off.
If you are certain that the crawl area is dry and you expect it to remain so, you can use fiberglass insulation under the floor, or you can insulate the perimeter - where I prefer solid closed cell foam that doesn't pick up moisture. Take a look at CRAWL SPACE INSULATION RETROFIT and at CRAWL SPACE VAPOR BARRIER
Should I install 10mil vapor barrier on the dirt floor and use 2-part closed cell spray foam insulation on the interior of the block? Or is it a better method to pour a concrete slab and use the 2-part closed cell foam?
You may find you're paying quite a bit to have the crawl area professionally foam insulated as it's a tight, hard to work-in area. If you go that route and get bids on the job you may find the bidders don't think it's economical unless you're doing other larger insulation work at the same time. But you could use solid foam as I mentioned above.
And similarly, while you could pour a slab (a "rat slab" they call it around here but don't tell your mother I said that) I prefer to avoid doing retrofits that are in tight (expensive) areas and that can pump a lot of moisture up through the building overhead as the concrete cures. If you go that route be sure to ventilate all that moisture outside during the cure period.
Frankly I think the poly vapor barrier would be easier, cheaper, and for a little-used area, effective.. If you can get 10 mil that's more resistant to holes and tears than the thinner stuff, and I never use less than 6 mil. I find that the material is not that precisely uniform and unrolling the thinner poly I sometimes see fragile thin spots.
Before putting down your poly be sure the crawl space floor is clean of debris, smooth, and pitched to a single drain point so that if necessary you can install a sump in the future. Then run the poly as you describe. Tape any joints or overlap them at least 24". Don't run poly all the way up walls to the sills if you're in a termite-risk area.
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
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.
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Questions & Answers regarding this article
Questions & answers about how to dry out a wet crawl space & prevent future crawl space water entry.
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[1] Harriet Burge, Harvard School of Public Health, and EM Laboratory, a private mold and environmental testing lab - email to D.F. August, 2004. Dr. Burge is an educator, writer, and consultant in the field of indoor air quality and mold contamination.
[2] Product literature and MSDS sheets for the biocides and fungicidal sealants listed in this article.
[3] US Centers for Disease Control, CDC: www.cdc.gov/ncidod/diseases/hanta/hps/ describes the risks associated with hantavirus.
Thanks to reader C. Brown for suggesting the need for detail about rapid dryout procedures for a wet crawl space
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
Our recommended books about building & mechanical systems design, inspection, problem diagnosis, and repair, and about indoor environment and IAQ testing, diagnosis, and cleanup are at the InspectAPedia Bookstore. Also see our Book Reviews - InspectAPedia.
The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. InspectAPedia.com ® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.
Asbestos: How to find and recognize asbestos in buildings - visual inspection methods, list of common asbestos-containing materials
Asbestos products and their history and use in various building materials such as asphalt and vinyl flooring includes discussion which draws on Asbestos, Its Industrial Applications, D.V. Rosato, engineering consultant, Newton, MA, Reinhold Publishing, 1959 Library of Congress Catalog Card No.: 59-12535 (out of print).
Asbestos Identification and Testing References
Asbestos Identification, Walter C.McCrone, McCrone Research Institute, Chicago, IL.1987 ISBN 0-904962-11-3. Dr. McCrone literally "wrote the book" on asbestos identification procedures which formed
the basis for current work by asbestos identification laboratories.
Stanton, .F., et al., National Bureau of Standards Special Publication 506: 143-151
Pott, F., Staub-Reinhalf Luft 38, 486-490 (1978) cited by McCrone
Fiberglass: Indoor Air Quality Investigations: Health Concerns About Airborne Fiberglass: Fiberglass in Indoor Air from HVAC ducts, and Building Insulation
Humidity: What indoor humidity should we maintain in order to avoid a mold problem?
How to Clean Moldy Wood Framing & Sheathing How to clean/seal mold from/on exposed lumber or plywood subfloor or roof sheathing indoors - some suggestions based on our field and laboratory research
Stuff that is not mold but is often mistaken for it - things you may not want to test. Also, not all "black mold" is toxic - here are examples of harmless black mold.
Mold-Resistant Building Practices, advice from an expert on how to prevent mold after a building flood and how to prevent mold growth in buildings by selection of building materials and by anti-mold construction details.
"Weather-Resistive Barriers [copy on file as /interiors/Weather_Resistant_Barriers_DOE.pdf ] - ", how to select and install housewrap and other types of weather resistive barriers, U.S. DOE
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