Expert: Bruce E. Johnson - 5/4/2010

Question
Back in 2005 you spoke of a ventilated crawl space as the answer for humidity. Everything I have read for NC says that is the old theory. Should be unvented with a dryspace membrane and a dehumidifier. I have done this but can not get an answer on what is a safe maximum percentage of humidity. One company down here recommends no higher than 55 because mold grows at 60. Doesn't sound right and the unit will probable run forever. Do u have a recommendation. Thanks

Answer
Hi Bob, yes the thinking has changed over the years as to ventilation of confined space.  Over the last few years we have been building all of our buildings without ventilated attic space. I live in Florida so we don't have a lot of crawl space foundations down here, mostly slab on grade.  The key to non ventilated space is that it becomes incorporated into the "conditioned" space.  In other words the vapor barrier and the insulation create the envelope and the air conditioning turns the entire building envelope into conditioned space.  In your case you have created the envelope underneath the house and are conditioning that separately using a dehudifier.  It's possible that the dehumidifier isn't large enough to handle the cubic feet involved.  It is also possible that the vapor barrier and insulation used to creat this envelope have failed somewhere, allowing more moisture into the space than it should.  Is your vapor barrier sealed properly with all the joints taped with the proper type of tape?  Are there any penetrations that are not sealed?  Are the walls of the crawlspace properly insulated? In order for a non ventilated space to work you cannot have any air leaks so is your crawlspace sealed at all the places air can get in? This may involve some serious caulking and foaming of foundation cracks and other places of air intrusion? Those are questions I can't answer.  However to get back to your question..45-55% humidity is considered the proper range.  Over 60% and moisture related problems may begin to show their nasty head.  I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com