Is Venting A Crawl Space Needed?

Traditional crawl spaces are typically built over a dirt floor with ventilation air from foundation perimeter vents.  Recent building science studies conducted by various institutions including the University of Tennessee challenge this old way of thinking and indicate that we need to rethink traditional building methods related to venting a crawl space.

As a Home Inspector, Home Energy Rater, and Building Scientist I have witnessed many problems that moisture in crawl spaces can cause including condensation, wood rot, mold, buckling hardwood floors, smelly moldy carpets, mold in the attic, sticking doors and windows, dust mites and wood destroying organism/insect activity. I have often thought that crawl spaces are the most neglected areas of homes and for a good reason. After all, who wants to crawl into a moist, insect and rodent-filled area?  Most homeowners don’t go there, shut it out of their minds, and never think too much about it.  But, if you care about your family’s health, overall reduced property maintenance, and how much you pay to heat and cool your home, you should care about what is in your crawl space. 

I’ve heard this myth repeated many times, even by some home builders: “the house needs to breathe”. My response is: “the house doesn’t need to breathe -you do!”  Consider that building science now tells us that 1/3 to 1/2 of the air you breathe on the first floor of your house comes through the crawl space.  If you have HVAC ducts in your crawl space, then there is even more air from the crawl space potentially mixing with the house air. So, if there is mold in your crawl space, there is a good chance you are breathing the mold spores. Also, consider radon gas, a naturally occurring radioactive gas that comes from the ground and is the second known leading cause of lung cancer behind smoking; it can be in your crawl space. In eastern Tennessee where I live, higher-than-average rainfall makes the air very moist and humid much of the year. Studies have proven that open foundation perimeter vents can sometimes make condensation problems in crawl spaces worse than if the perimeter vents were closed.  

So what is the solution? Back when energy was cheap, and we didn’t know any better, more ventilation in the crawl space was the answer.  But that was 100 years ago when we did not insulate houses and drafty, uncomfortable houses were the norm.  But, if you don’t want drafts and want a more comfortable and energy-efficient home, more open venting of the crawl space to the outside is not the answer. The best solution is an enclosed crawl space system that makes the “housework a system”.  This is easier to accomplish while building a home and more difficult to accomplish as a retrofit to an existing house. This is why so many Crawl Space and Basement Specialty contractors have started recently to help homeowners. In order to understand the “house as a system” approach, we must understand a little building science. 

As warm air rises in a home, it leaks out of the upper levels. New air must replace the escaped air. The replacement air comes from the area with the least resistance to air movement which in most cases is partially the crawl space.  This “stack effect” causes the airflow which results in you breathing in crawl space air.  Therefore, whatever is in your crawl space air affects you, whether you realize it or not. 

Sealing off the perimeter vents reduces air movement from the crawl space and accomplishes one of our “houses as a system” criteria.  Other criteria should be taken into account that are too lengthy to discuss in this blog including roof and surface drainage around the foundation,  groundwater leakage, installation of sump pumps and dehumidifiers, mechanical ventilation of the crawl space, and ducting in the crawl space.  

One of the conditions that cause mold is an enclosed space with no air changes.  When enclosing crawl spaces, they must either be treated as part of the living space by “mechanically venting” these areas with HVAC ducting supply and return air, or, they must be otherwise “dehumidified”.  Dehumidification is a substitute for achieving proper air changes in crawl spaces when impractical.    

So, when encapsulating a crawl space, venting a crawl space the traditional way is out and a new way of venting a crawl space (mechanical) is in.  Some would argue this mechanical ventilation uses energy and is, therefore, more expensive.  Studies show this is not true and the exact opposite occurs.  Homes with sealed and insulated crawl spaces saved about 20% in natural gas during the winter and an added 15% in annual energy savings compared to the control group.

If you need a thorough evaluation of your house to identify all your “house as a system” correction criteria before considering an enclosed crawl space and living in East TN, call us at 865-658-1050 to schedule an inspection.