This is the first article in a two-part series on crawl spaces in residential
construction. For more information about DocAir's Proof Positive™
technology designed to solve the crawl space moisture problem, please
contact us.
The Myth
Crawlspaces in building construction have been widely accepted for years. They
have enabled homebuilders to provide a level building surface for sloped lots
cheaply, and they are a convenient place for plumbing, electrical conduits, and
heating, ventilation, and air conditioning (HVAC) systems. However, they are a
frequent source of moisture, mold, and other indoor air quality problems.
Homebuilders have long known that moisture from outside sources can accumulate
in crawlspaces, so nearly all homes built with crawlspaces in the last 75 years
have vents installed in the foundation walls. The idea is that the passive
ventilation will allow humid, musty air to escape to the outside. It sounds like
a good idea on paper, but in reality, vented crawlspaces create many more
moisture problems than they solve. Understanding how vented crawlspaces work
requires little more than grade-school science, but homebuilders and homeowners
alike have failed to grasp the ramifications of the subtle mechanisms at work
underneath their feet.
According to Certified Industrial Hygienist Barry Westbrook, president of DocAir
(www.docair.net), a building systems and
indoor air quality consulting firm, "The concept of relative humidity is the
single most important issue that should be considered by homebuilders when
building a home with a crawlspace." Relative humidity is defined as the amount
of water contained in the air at a specific temperature. As air cools, it loses
its ability to hold moisture. When air contains all the water it can hold, it is
said to be at 100 percent relative humidity, or saturated. The temperature of
air at the saturation point is called the dew point. At the saturation point,
moisture will begin to condense onto any surface whose temperature is at or
below the dew point.
Vented crawlspaces tend to be several degrees cooler than the outside air due to
shading and contact with surfaces cooled by HVAC systems in the living space
above. During humid Southern summers, the relative humidity of outside air is
often 70 percent or more. As this outside air moves into the crawlspace through
the foundation vents, it quickly cools. If the temperature of the crawlspace or
surfaces within the crawlspace is at or below the dew point of the outside air,
condensation will occur. Slowly but surely, this moisture will accumulate inside
the crawlspace, fostering mold growth, wood rot and a host of other problems.
Many crawlspaces have conditions that actually accelerate this process.
Un-insulated floors, torn or damaged ductwork insulation, and leaky supply ducts
lead to colder surfaces, meaning increased condensation. In effect, the
crawlspaces of many homes actually serve as dehumidifiers for the outside air.
The cold sub-floor and/or exposed metal ductwork act like the coils in your
household dehumidifier, and the crawlspace itself becomes the drip pan. By
encouraging the movement of humid air into the crawlspace, foundation vents
actually increase the chances of having unwanted moisture build up underneath
the home.
The Solution
First, all exposed dirt inside the crawlspace must be completely covered with a
vapor barrier made of at least 6-mil polyethylene. Many homes have vapor
barriers, but few are installed properly. The joints must all be taped and
sealed. The barrier should be extended up the foundation walls, covering the
vents. Ideally, the vapor barrier would extend across the floor joists,
resulting in a completely encapsulated crawlspace. Any moisture that percolates
up through the soil moves along the underside of the vapor barrier and can exit
through the foundation vents. Humid outdoor air can no longer reach wooden
surfaces underneath the home. In areas where radon gas is a problem, the
encapsulated crawlspace is also useful in that any soil gases originating
underneath the home are routed harmlessly to the foundation vents without ever
having the chance to enter the living space of the home.
The Myth