How Does a Crawl Space Vapor Barrier Help?

How Does a Crawl Space Vapor Barrier Affect the Living Area of My Home?

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. 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. The crawl space often houses the HVAC system, which is the single most expensive and complex component of a home. The entire home is built off the foundation and piers which enclose the crawl space. 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. 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.

Typical Crawl Spaces Have Moisture Issues

Crawl 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. 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. 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. 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. When warm, humid air enters the crawl space, it quickly cools, which causes it to become more humid. 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. It is common to find crawl spaces dripping condensation from insulation and ducts, with moist framing taking on more and more moisture.

Moisture Issues Can Cause Other Problems

Over the long term, high humidity in a crawl space leads to many problems:

  • Relative humidity over 60% breeds mold and mildew, which is harmful to breathe. 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.
  • 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. 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.
  • HVAC equipment is damaged by long-term exposure to high humidity. Corrosion forms more rapidly on metal duct components, flue pipes, and other external parts of the HVAC system. Also, when return ducts pull in humid air, this humidity can damage internal system components such as the blower fan and heat exchanger. Maintaining appropriate humidity in the crawl space is critical to the longevity of the furnace, air conditioner coil, and other HVAC system components.
  • Vapor Diffusion occurs when humidity in air (but not the actual air!) moves through vapor-permeable materials, such as wood. When evaporation from soil and air movement through crawl space vents cause air in the crawl space to become excessively humid, the humid vapor migrates into the wood subfloor and framing. Over the medium term, this causes the interior surface of the floor to grow mold under furniture and cabinets, as well as under carpeted areas.

Vapor Barriers Are a Great Solution

Crawl Space Vapor Barrier Installed

Properly Installed, a Crawl Space Barrier Can Help with Moisture Issues in Your Home.

Installing a crawlspace vapor barrier virtually eliminates the ability of moisture to evaporate from the soil into the crawl space. In fact, crawl space vapor barriers are so effective that most building codes allow for major reductions in the amount of ventilation required for a crawl space. Instead of one square foot of vent per 150 square feet of crawl space, a crawl space with a vapor barrier only needs one square foot of ventilation per 1,500 square feet of crawl space – a 90% reduction! By reducing the air movement between outside and the crawl space, the moisture gained by outside warm air cooling in the crawl space further reduces overall moisture. In addition, during the winter, having less ventilation in the crawl space keeps the crawl space warmer, which can decrease the home’s heating load and heating energy consumption, especially if the floor of the home has not been insulated.

Every dirt-floored crawl space will benefit from the installation of a vapor barrier, but if you notice any of these signs in your home, installing a vapor barrier system is especially beneficial:

  • Moist soil in the crawl space
  • White or gray streaks or deposits on the masonry walls of the foundation
  • Mold or mildew present in the crawl space
  • Crawl space feels clammy
  • Carpet inside feels clammy to the touch
  • Mildew or condensation buildup under and behind furniture
  • Wood in crawl space feels soft or damp
  • Musty smell coming from HVAC vents

Make Sure Your Vapor Barrier is Installed Right

Once you have decided that installing a crawl space vapor barrier is a good idea, ensure that the professional or company you work with to complete the project will ensure the following requirements are met:

  • The vapor barrier is continuous throughout the crawl space floor
  • Vapor barrier material is at least 6 millimeters thick, and 10-15 millimeters is even better
  • All seams in the vapor barrier material are taped. In order for the tape to be durable, it is recommended to use a barrier material and a tape that are designed to be used together, such as Viper CS
  • The perimeter of the vapor barrier should be fastened to the foundation walls and support piers, and taped or caulked to ensure a continuous, effective seal
  • If gas appliances, such as a furnace or water heater, are present in the crawl space, a Combustion Ventilation Air (CVA) calculation should be performed to ensure that closing off vents will not create safety problems
  • Crawl space ventilation should be capped from inside to meet the 1 square foot per 1,500 square foot guideline, assuming additional ventilation is not needed for combustion.

Have Your Home Evaluated as a System

Since so many factors are in play – moisture mitigation, indoor air quality, energy efficiency, and gas appliance safety, it is important to work with someone who understands how the different components of the home interact with one another. A home performance contractor is typically more knowledgeable of these system interactions than a crawl space specialist or a general contractor. Choosing a system that meets all the above criteria and an installer who understands the many factors at play will ensure that your crawl space vapor barrier is effective in reducing moisture in the crawl space and inside the home for years to come.


Copyright September 2014 by Advanced Home Energy

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