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Residential architects who specialize in the hot, humid, southern climate

Components of a Closed Crawl Space

Until around 75 years ago, houses in the Coastal South were built out of solid lumber components of stud walls,  diagonal sheathing, wood siding, and the interior was finished with plaster. When the walls got wet they were able to dry because there was no insulation and plentiful leaks to let air circulate in and out of the walls. There was not a problem with mold because solid lumber is not  eaten by mold. Without air-conditioning, high ceilings, porches and cross ventilation contributed to making  the house bearable in a hot, humid climate. This sounds like a good system except thermal comfort is almost impossible to achieve in a leaky, un-insulated structure . With the advent of air-conditioning, the homebuilding industry pursued the goal of thermal comfort  to the detriment of the indoor air quality and structural integrity. By 1970, houses still had wood stud walls but almost everything else had changed. The interior was finished with paper faced drywall. The exterior sheathing was oriented-strand board (OSB).  Thermal comfort was achieved with insulation and air-conditioning. Insulation was placed between the floor and the vented crawl space that was full of humid air. Likewise, insulation was placed between the ceiling and vented attic. The heating, ventilation, and air-conditioning (HVAC) equipment and ducts were located in the vented crawl space and/or vented attic.  The openings where ducts and plumbing pipes penetrated through the ceiling and floor were typically not sealed  and the introduction of the hot, humid air into the cooled interior spaces caused condensation and the growth of mold. Similarly, when the cold air in the ducts passed through the extremely hot attic it created condensation in the ducts;  this in turn caused mold and mildew to grow because the house could not dry.  By tightly sealing houses and using products full of adhesives, houses were turned into mold and rot factories. Through research, building methods for hot, humid climates have been transformed.  It wasn't until 1996 that the International Residential Code allowed modification for hot, humid climates and many people are unaware of these advances.

One of the most significant changes is enclosing the crawl space.  The research company Advanced Energy "has confirmed over the long term...that outside air contains more water vapor than the air in the crawl space during the warm seasons, and has no potential to dry the crawl space. Instead, the outside air ends up contributing water vapor to the crawl space." Thereby, vented crawl spaces support mold growth in a hot, humid climate. Advanced Energy's research on closed crawl spaces compared to vented crawl spaces showed that "closed crawl spaces consistently outperformed the wall-vented crawl spaces in terms of both moisture control and energy use."

The components of a closed crawl space include the following:

  • A ground vapor barrier installed on the entire floor . It wraps up the walls and is mechanically fastened and sealed to the top of the wall. A 3" termite inspection gap is located between the top of the wall vapor barrier and the top of the masonry wall. The vapor barrier should be at a minimum 6- mil. with seams sealed with fiberglass mesh tape and mastic.
  • All penetrations through the foundation walls and first floor to be sealed with non-porous materials, caulks or sealants. The access door should be weather-stripped.
  • Insulation is applied either on the exterior walls or between the floor joists.
  • A mechanical method of removing moisture must be installed, options include; a dehumidifier, air ducted from the house air-conditioner with a back-flow damper to prevent  crawl space air from entering the house, or  an exhaust fan.
  • If the house is located in a Special Flood Hazard Area and required hydrostatic vents for the management of flood waters they must be  engineered flood vents that seal closed when not in use.
  • The rain washing off of the roof needs to be controlled, either by gutters or a foundation drain system. The ground needs to slope away from the house at a minimum of six inches in ten feet.

For more information on closed crawl spaces go to http://www.crawlspaces.org.