Maintaining appropriate humidity in the crawl space is critical to the longevity of the furnace, air conditioner coil, and other HVAC system components. Also, when return ducts pull in humid air, this humidity can damage internal system components such as the blower fan and heat exchanger. Corrosion forms more rapidly on metal duct components, flue pipes, and other external parts of the HVAC system. HVAC equipment is damaged by long-term exposure to high humidity.Besides harboring mold and mildew components within this air, accumulation of this high humidity air inside the home can cause additional mold and mildew growth in its own right. Leaky return ducts (as of 2008, the average US duct system leaked about 30% of its air into crawl spaces, basements, and attics) can suck in moisture-laden air and transmit it into the house.Microscopic, airborne mold and mildew particles can migrate into the living area of the home through a variety of sources, including holes in the subfloor of the living area around plumbing, electrical, and HVAC components. Relative humidity over 60% breeds mold and mildew, which is harmful to breathe.Over the long term, high humidity in a crawl space leads to many problems: It is common to find crawl spaces dripping condensation from insulation and ducts, with moist framing taking on more and more moisture. 85 degree air at 70% RH reaches dew point at 73 degrees – a common summer temperature in a below grade crawl space that is shaded from the sun.

When warm, humid air enters the crawl space, it quickly cools, which causes it to become more humid. However, due to the cooler temperatures of the crawlspace, which is perpetually shaded and dark, this air exchange often adds to the moisture load, rather than mitigating it. Because so much moisture enters the crawl from the dirt floor, most building codes mandate that crawl spaces must have 1 square foot of venting for every 150 square feet of crawl space. More homes in the US are built in mixed/humid climates than in any other type of climate, and a 1,000 square foot crawl space in a mixed/humid climate typically evaporates about 10 gallons of water vapor into the crawl space every day.

Typical Crawl Spaces Have Moisture IssuesĬrawl spaces are characterized by having dirt floors, and this exposed, permeable soil is one of the biggest sources of moisture in most homes with a crawl space. Ignoring problems in the crawl space can cause premature HVAC failure, allow indoor air quality pollutants to enter the living area of the home, and contribute – more than any other factor – to moisture problems. And, whether it’s pleasant to think about or not, in most homes a significant portion of incoming “fresh” air actually comes into the home from the crawl space. The entire home is built off the foundation and piers which enclose the crawl space.

The crawl space often houses the HVAC system, which is the single most expensive and complex component of a home. However, the crawl space is an integral component of the inter-related systems that work together to make the home a comfortable, healthy, and energy efficient place to live. Most homeowners don’t pay much attention to what’s going on down in their crawl spaces, and few, if any, are excited by the prospect of spending money to improve their crawl spaces. How Does a Crawl Space Vapor Barrier Affect the Living Area of My Home?

Crawl space vapor barrier