Elevated Radon Warning for Unvented Crawl Spaces!

Home Inspector with Certified Inspections, PC - Residential & Commercial Property Inspections TN HI #334

Unvented crawl spaces have been gaining popularity primarily due to the push for energy conservation.  One of the significant adverse side affects to the earlier types of unvented crawl spaces was a concentration of Radon gas.  Scientifically installed and monitored studies from North Carolina, Louisiana, Arizona and Washington state all registered a noted rise in Radon levels with an unvented crawl space.  The model homes in Arizona had to have the crawl spaces re-vented because the Radon levels rose to such a high level.

It is important to know that 2003 was the last model code that allowed a "positive pressure sealed air crawl space", where an A/C duct was vented into the crawl space to provide conditioned air.  Now all unvented crawl spaces must circulate air, either from the interior of the home through the crawl space to the outside or through the interior of the home and the crawl space in a circular motion.  The crawl space is required to be neutral pressure or negative pressure compared to the inside of the home.

Lots of home owners, specially in the South are switching to unvented crawl spaces.  Any home with an unvented crawl space should be checked for appropriate Radon gas levels for the safety of the occupants, even if the home has been previously monitored. 

Beware!  Unvented crawl spaces will raise Radon Gas Levels in the home!

Comments (5)

Paul Campbell Realtor Lexington
Rector Hayden, Lexington, Ky - Lexington, KY
Kentucky Homes for Sale

Hi Paul,

I recently heard a home inspector here in Lexington promote the unvented crawl space without any mention of Radon.  What you write makes a lot of sense though.  Thanks for sharing.  Paul

Feb 16, 2010 01:40 PM
Eric J
Eric J - Dream Home Financing - Freehold, NJ
Dream Home Financing

I have a radon mitigation system.....and I also sealed off the vents in my crawl space. Afterwards, I tested the radon levels (sent out to a lab) and there were not problems. I am in an area where radon can be an issue.

Feb 16, 2010 01:42 PM
Paul A. Perry
Certified Inspections, PC - Residential & Commercial Property Inspections - Crossville, TN
Home Inspector - Crossville & Cookeville, TN

Paul, Unvented crawl spaces are still in the experimental, "research & development stage".  People sometimes have a tendency to jump on the band wagon of something new if it hits one of their "important" buttons, like energy efficiency.  I think it will end up being a good viable alternative to a vented crawl space but at this point, everyone should do plenty of research on the subject and be cautious.

Eric, The mitigation system is the key!  If installed properly the system will evacuate and/or dilute the Radon gas concentration before it enters your crawl space or home. 

Feb 17, 2010 12:33 AM
Gene Allen
Fathom Realty - Cary, NC
Realty Consultant for Cary Real Estate

Unvented sounds like a good idea but with new technology there are always problems.

Feb 22, 2010 11:30 AM

Stumbled across this blog when researching my own issue.  Live in Atlanta on a crawl that was 0.8pCi/L radon reading when we purchased (vented at the time), then had the crawl encapsulated due to humid air causing condensation on ductwork/furnace and cupping of hardwoods.  Had radon tested subsequent and it rose to 6.8pCi/L!  Have now put in a radon mitigation system, utilizing the now existing 20mil encapsulation membrane.  In retrospect I guess I could have left it as is, but at least now I have comfort that radon will be low AND my crawl will be even drying with the additional sucking of moist air under the membrane.

Nov 03, 2011 03:08 AM