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Crawlspace Moisture Problems

Many homes built on crawl space foundations in the United States suffer from poor moisture management. Excess moisture is often noticed in the humid spring and summer seasons but can occur at any time of the year. Often, the heating and air conditioning contractor is the first person the residents call to deal with the problem. Typically though, the problem is not due to a failure of the air conditioning system; it results from poor moisture control in the crawl space.

Health Risks & Hazards:

Some of the common symptoms of a crawl space moisture problem are:
  • Mold or moisture damage in the crawl space or living area
  • Musty odors in the living area
  • Condensation (sweating) on air conditioning ductwork or equipment
  • Condensation on insulation, water pipes or truss plates in the crawl space
  • Buckled hardwood floors
  • High humidity in the living area
  • Insect infestations
  • Rot in wooden framing members
These symptoms are most often noticed in the humid spring and summer seasons but can occur at any time of the year. Often, the heating and air conditioning contractor is the first person the residents call to deal with the problem. Typically though, the problem is not due to a failure of the air conditioning system; it results from poor moisture control in the crawl space.
For many decades, building codes and conventional wisdom have prescribed ventilation with outside air as the primary method of moisture control in crawl spaces.  Ventilation with outside air only makes moisture problems worse. Research indicates that a conditioned type of crawl space system, with NO vents to the outside, can provide greatly improved moisture control and significant energy savings when properly installed.


In most areas of the U.S., sealed crawl spaces work much better than vented crawl spaces.  Most building codes permit the construction of unvented crawl spaces. In the 2006 International Residential Code, requirements for unvented crawl spaces can be found in Section R408.3. If an unvented crawl spaces has a dirt floor, the code requires exposed earth to be covered with a continuous vapor retarder with taped seams: “The edges of the vapor retarder shall extend at least 6 inches up the stem wall and shall be attached and sealed to the stem wall.”
The code lists two options for conditioning unvented crawl spaces; both options require the installation of a duct or transfer grille connecting the crawl space with the conditioned space upstairs.
  • Option 1 requires “continuously operated mechanical exhaust ventilation at a rate equal to 1 cfm for each 50 square feet of crawl space floor area.” In other words, install an exhaust fan in the crawl space that blows through a hole in the rim joist or an exterior wall; make sure that the fan isn't too powerful. (The makeup air entering the crawl space is conditioned air from the house upstairs; since this conditioned air is drier than outdoor air, it doesn't lead to condensation problems.)
  • Option 2 requires that the crawl space have a forced-air register delivering 1 cfm of supply air from the furnace or air handler for each 50 square feet of crawl space area. (Assuming the house has air conditioning, this introduction of cool, dry air into the crawl space during the summer keeps the crawl space dry.)


Water is our most prevalent natural resource that we all rely on every day in our existence.  Water has properties that effect how we exist and live in harmony with nature.  Case in point: we need water to survive; we just don’t want to live with it in our home or business.  Left for periods of time, undisturbed water will destroy anything.  In fact most building materials are made with water or contain some level of water as sits in your home or business right now (i.e. concrete and wood).  So often the statement applies to many things in life, we can’t stop the water; we can only hope to contain it.  Waters effects to our structures can be truly devastating in acute situations: flooding, typhoons, rising tides, hurricanes, and breaching dams/rivers just to name a few, but often it is not those unexpected and extreme natural disasters that affect our everyday lives.  The chronic water intrusions that lie beneath our undetected eye, often caused by human error, are the ones that create havoc on our structural buildings we occupy and rely on to protect us from the elements of nature.  Structurally, water introduces problems that can create an array of issues, such as, but not limited to, foundation shifting, foundation settling, hydrostatic pressure, foundation cracking, foundation deterioration, wood rot, fungus rot, mold growth, termite/insect damage, and framing deterioration.  On a health prospective, we need water to live, but so does other organic matter, microbial activity needs water to thrive, put with the right temperatures and food sources (i.e. the building materials in your building) and you have just created one the most deadly combination to comprise your indoor air quality.  And it is not just water leaking or flowing into our buildings, but the problem also deals with water in the air that can go undetected causing a slow and painful result.  Building or modifying a structure to be water resistant has always been a challenge that began with our forefathers and will continue long after we are gone, but the technology on new building and existing renovation activities against water have progressed so rapidly over the past ten to fifteen years, that water resistant buildings are truly a reality, no longer a hope and a prayer.

Water is created of atoms that join together to form molecules.  A water molecule has three atoms: two hydrogen (H) atoms and one oxygen (O) atom. Water takes form in three distinct phases or forms: solid, liquid, or gas.  The water cycle or hydrologic is a continuous cycle where water evaporates, travels into the air and becomes part of a cloud, falls down to earth as precipitation, and then evaporates again. This repeats again and again in a never-ending cycle. Water keeps moving and changing from a solid to a liquid to a gas, over and over again.
Precipitation creates runoff that travels over the ground surface and helps to fill lakes and rivers. It also percolates or moves downward through openings in the soil to replenish aquifers under the ground. Some places receive more precipitation than others do. These areas are usually close to oceans or large bodies of water that allow more water to evaporate and form clouds. Other areas receive less precipitation. Often these areas are far from water or near mountains. As clouds move up and over mountains, the water vapor condenses to form precipitation and freezes.
Water activity or aw was developed to account for the intensity with which water associates with various non-aqueous constituents and solids. Simply stated, it is a measure of the energy status of the water in a system. It is defined as the vapor pressure of a liquid divided by that of pure water at the same temperature; therefore, pure distilled water has a water activity of exactly one.


The aw scale starts from 0.0 (dry) and goes to 1.0 (pure water). Microbial growth can start as low as 0.6 aw. However, environmental molds have genera/species specific variability regarding how much water is required for their growth.  Fungi that require high amounts of available water are called hydrophilic and grow at water activities above 0.90. Stachybotrys sp., Chaetomium sp., Trichoderma sp., Memnoniella sp., Acremonium sp., and Fusarium sp. are examples of hydrophilic molds. They colonize only in moisture rich, chronically moist environments. If building materials do not contain >90% (Aw >0.9) moisture content then their growth is impaired. The largest group of fungi falls in the mesophilic range. Mesophilic fungi include common indoor molds such as Cladosporium sp. and Alternaria sp. These fungi typically grow on continuously damp building materials with water activities between 0.80 and 0.90. Xerotolerant molds include Aspergillus sydowii, A. versicolor and some species of Penicillium are able to grow at water activities below 0.80 but grow optimally above this value. They are common on water-damaged materials. The final group of fungi, known as xerophilic, actually grows best at water activity ratios below 0.80. A common xerophilic fungus is Aspergillus restrictus.  Building materials such as drywall and wood contain cellulose-based materials chocked full of nutrients that serve as suitable food sources for mold growth. However, unless water is present at levels presented above, mold growth will not occur. Water impact into a structure then provides the enriched moisture environment fungi need to grow. Once a building product is soaked with water without immediate drying, mold spores present on the water-laden building material, germinate and grow.
Relative Humidity & Dew Point - Not only are we dealing with water activity in direct liquid form, but as pointed out previously, water in gas form can create enough water vapor to begin mold growth or dust mites.  Relative humidity is the ratio of the partial pressure of water vapor in an air-water mixture to the saturated vapor pressure of water at a prescribed temperature. The relative humidity of air depends not only on temperature but also on the pressure of the system of interest.  A good range is between 30% and 60% relative humidity. You can determine humidity levels with a relative humidity sensor typically referred to as a hygrometer or psychrometer. This level of humidity minimizes the indoor growth of allergenic or pathogenic organisms such as dust mites and molds. 
The dew point is the temperature below which the water vapor in a volume of humid air at a constant barometric pressure will condense into liquid water. Condensed water is called dew when it forms on a solid surface.  The dew point is a water-to-air saturation temperature. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. Relative humidity of 100% indicates the dew point is equal to the current temperature and that the air is maximally saturated with water. When the dew point remains constant and temperature increases, relative humidity decreases.

Advantages of Unvented (Sealed) Crawl Spaces:

  • Stay dryer than vented crawl spaces
  • Protect pipes from freezing
  • Require less insulation than vented crawl spaces (since the area of the perimeter walls is less than the area of the crawl space ceiling)
  • Bring ducts within the conditioned envelope of the home - an improvement that usually results in energy savings compared to vented crawl spaces
  • Promotes better indoor air quality and moisture controls

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