Basic Concepts: Moisture Transport
Where Does the Moisture in a Crawlspace Come From?
There are four primary modes of moisture migration into a home: capillary action; bulk moisture transport; air transport; and vapor diffusion. Each of these modes, described in the following list, must be controlled to preserve comfort, health, and building durability.
(see Figure 2):
Figure 2. Capillary action
Bulk moisture transport
Water wicks through porous materials or through small cracks.
- Primary sources are rain and groundwater.
(see Figure 3):
Figure 3. Bulk moisture transport
Moisture flows through holes, cracks and gaps.
- Primary sources are rain and groundwater.
(see Figure 4):
Figure 4. Air transport
Unsealed penetrations and joints between conditioned and unconditioned areas allow air containing water vapor to flow into enclosed areas.
- The primary sources are water vapor in the air and soil gas.
(see Figure 5):
Figure 5. Vapor diffusion
Water vapor in the air moves through permeable materials.
The first step in developing a plan for a crawlspace conversion is to perform a thorough inspection of both the exterior and interior conditions. The following areas and specific details must be attended to before starting the sealing and insulating of any crawlspace:
- Immediate health and safety issues:
- electrical system;
- mechanical systems (for the threat of carbon monoxide);
- framing (for potential mold growth);
- hazardous materials storage (including the presence of asbestos); and
Exterior moisture sources:
- roof drainage away from the house;
- grading around the house;
- lawn sprinklers;
- foundation waterproofing and drainage; and
- access door.
Interior moisture sources:
- leaking drains or supply plumbing;
- condensate drain;
- standing water on the floor;
- crawlspace floor;
- vent openings;
- sump pump crock pit; and
- water softener discharge.
Immediate Health and Safety Issues
Other contaminants and issues:
- feces, carcasses, and other animal waste
- animal infestation
- termites and carpenter ants
- thermal breaks;
- insulation strategy; and
- space conditioning.
Assessing the current electrical system within the crawlspace is one of the first steps to take. A qualified electrician might need to be consulted if there is exposed or damaged wiring within the crawlspace. If there is a risk of shock or electrocution from faulty wiring, postpone the inspection of the interior systems until any issues have been resolved and the space is safe.
Mechanical Systems/Carbon Monoxide
Establish the type of space conditioning equipment and water heaters. If fuel-fired combustion appliances are located in the crawlspace, it is important to determine their venting and combustion air sources. These life-safety strategies are a critical first step in the overall renovation efforts of the crawlspace. If natural draft-vented appliances that depend on combustion air from the open vents in the foundation are present, equipment upgrades and replacement or alterations to the makeup air methods will be necessary. The best strategy is to replace natural draft-vented appliances with sealed combustion, direct-vent furnaces or all-electric heat pumps. The direct-vent gas furnaces draw combustion air directly from outside through piping to the unit. All combustion byproducts are vented through sealed piping to the outdoors, minimizing the possibility of any carbon monoxide spillage. Verify that an operational carbon monoxide detector is installed in the crawlspace.
If there are any gas line regulators within the crawlspace, they will need to be moved so they vent to the outdoors.
Finding some traces of mold or mildew is common in a poorly detailed and ventilated crawlspace. The floor deck and all framing in the crawlspace are vulnerable to some level of fungal attack if moisture levels within the crawlspace go unchecked. Once high levels of moisture are present, moisture will tend to condense on the cooler surfaces of the wood framing and floor deck. These areas are easy to observe and are an indicator of high moisture content. Any surface mold or fungal growth will require a certified mold inspector to conduct an inspection and determine an appropriate remediation protocol to clean all the components before work can proceed. Structural repairs to framing components might be necessary if there is extensive damage or wood rot.
Exposure to asbestos increases a person’s risk of developing lung disease. Old crawlspaces might have asbestos-based insulation wraps on ductwork and plumbing pipes. If asbestos is discovered, a certified asbestos abatement firm will have to be contracted to inspect and determine the best treatment strategy for the particular situation. General movement and unintentional contact with asbestos-based products can stir the fibers, which can then become airborne within the confined space, posing a health risk to anyone in the immediate vicinity.
Hazardous Materials Storage
If any hazardous materials have been stored in the crawlspace (e.g., creosote-treated lumber, pesticides, etc.), they must be completely removed and properly disposed of.
In areas where radon is a risk or where the local residential code requires control of radon or other soil gases, houses with closed crawlspace foundations must be tested and monitored by a certified radon tester. If necessary, an approved mitigation system must be installed. The U.S. Environmental Protection Agency (EPA) and the U.S. Surgeon General recommend testing all homes for radon.
Installing a passive radon mitigation system before undertaking all sealing work could be necessary in radon-prone areas. After the crawlspace is closed and sealed, final testing for radon should be conducted. Converting the passive system to an active system is easily accomplished if readings exceed the EPA’s acceptable levels.
Exterior Moisture Sources
Roof Drainage Away from the House
Inspect the roof’s bulk water discharge strategy, including all gutters and downspouts, to ensure bulk water runoff is not allowed to collect next to or near the foundation. Downspouts should discharge into drains that are directed to a storm sewer system, retention pond, daylighting (if the land topography permits), or yard bubblers that are a minimum of 10 feet away from the foundation.
Grading Around the House
Evaluate the grading at the perimeter of the house. Ideally, the ground should slope a minimum of 5% away from the foundation walls for at least the first 10 feet in order to direct groundwater away from the structure. If the slope cannot be established because of the home’s elevation and surrounding grade, a surface drainage system should be installed. This type of system collects water and diverts it away from the foundation.
Before any excavation work starts, the homeowner should identify any underground utilities that enter the house and notify the respective utility companies, as required.
Turn on all sprinklers installed near the foundation to observe their flow pattern. The sprinklers must be positioned so that they don't subject the foundation or the immediate vicinity to water.
Foundation Waterproofing and Drainage
Implementing foundation waterproofing and a perimeter drain strategy on the exterior walls will be required on exceptionally problematic sites. Such sites typically have high groundwater or water flow against the foundation that cannot be remediated in any other way.
The foundation vents can be another source of bulk water entry. Make sure that they are not too low in relation to the exterior grade. If the vents fall near or below grade, they will need to be filled in with block, brick or concrete and sealed to prevent water entry.
Inspect the general location and overall weather seal of the access door. The access door should be located high enough off of grade to prevent groundwater runoff and snowmelt from entering the crawlspace. If the door is located too low and there is not enough clearance to adjust the entry height, remove the door, seal the opening, and install a new entry at a different location at the exterior that will accommodate the clearance. If no suitable location meets these criteria, cutting an access way through the floor system within the house will be required.
Interior Moisture Sources
Standing Water on the Floor
Any standing water must be eliminated, and the source of the water identified and repaired to prevent a recurrence that would raise the moisture levels within the space. Standing water typically indicates a leaking pipe from above or groundwater entering the space from outside the building or under the floor.
Leaking Drains or Supply Plumbing
Conduct a thorough inspection of all drains and supply plumbing located within the crawlspace to make sure that no leaks are adding moisture to the space.
Mechanical System Condensate Drain
Evaluate the entire length of the condensate drain, from the air handler to the discharge point. Because of the potentially corrosive discharge from sealed combustion furnaces and air conditioners, condensate drains must be installed as required by local jurisdictions. They must be properly supported and routed directly to a sealed sump crock lid, a floor drain fitting, or a sanitary sewer.
In addition to the primary drain (and as a safety precaution if the primary drain fails), an auxiliary drain pan with a separate drain beneath any cooling or evaporator coil is recommended to prevent condensate from flooding the crawlspace. Inspect the current condition of the crawlspace floor because it is a critical factor in directing any surface water toward collector drains and the sump. It also minimizes moisture movement from the soil into the crawlspace environment. Poor sealing practices of the polyethylene vapor barrier allow ground moisture to infiltrate through gaps or open seams in the barrier, increasing the relative humidity of the crawlspace. Grading the base to a minimum 3% slope toward low-spot collector drains that can move water directly to the sump is necessary to eliminate ground moisture from seeping into the space.
All intentional, operable or fixed foundation vents will need to be sealed to prevent warm moisture-laden air from being drawn into the crawlspace. If the vents are too low and appear likely to permit bulk water intrusion into the crawlspace, they will need to be sealed with concrete block, brick, concrete, or glass block windows to prevent water entry.
Inspect all sump pump crock pits installed in the crawlspace, looking at the detailing of their lids and any drainpipe penetrations. Missing lids and an absence of sealing will allow moisture to enter and increase the relative humidity of the crawlspace.