Building Homes or Extensions/crawl space


we are purchasing a 12 year old home in the Charlotte, NC area with about a 3' crawl with venting.   As far as crawl spaces go its not bad but does have some signs of moisture such as some of the floor beams have black fungus and some of the fiberglass floor insulation is stringy.  There is no dry rot and beams are all solid.  It does have the black plastic vapor barrier but it has been kicked around and disturbed a lot so not covering that well.  The dirt underneath is dry throughout and there is good sloping drainage away from the foundation so no signs of water intrusion so I think the moisture is from the humid summer air condensing in the cooler crawl?   A moisture control company recommended encapsulation and when asked about the existing gas furnace in the crawl he said I could just pipe in air.  however a HVAC company said that only works for high efficiency furnaces and one has to be care full with running the intake and it can't be too long, have significant bends, etc and recommended if I encapsulated the crawl to switch from a split system to a heat pump.   In addition, we live in an area where 15-20% of homes have high radon and while the home we were buying was fine I have read that encapsulating a crawl can increase the radon level enough to require remediation (since the radon is no longer escaping out the vents).  So since the crawl is not too bad I was thinking of just fixing the vapor barrier installing a crawl dehumidifier and during the few humid months of the summer close off the vents sort of semi-encapsulating with a dehumidifier during the summer and then do annual checks and/or mold treatment if needed.  Being a layman I just wanted to get your thoughts

In a location like South Carolina, it is my opinion that you should leave the crawl space ventilated year round.  If you try the dehumidifier route, over time you may forget to open or close the vents, and the correct timing of ventilation would be an unknown.  
You should have the soil vapor barrier re-installed as a double-layer 6-mil fire-retardant poly with overlapped taped or glued seams and fully sealed to the foundation walls.  You also may want to increase the number of vents and consider installing an exhaust fan in one of them, which runs off of a humidistat.

Best of luck,

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Stephen Major (Owner--Major Design Group)


I can answer any questions regarding the design and construction of homes and additions. This includes trade-specific questions (how-to) in all major building trades: framing, foundations, site prep, electrical, HVAC, plumbing, water treatment, interior finishing, trim & cabinetwork, exterior finishing, roofing, siding. PLEASE indicate your state or region, so I can provide the best possible answer. PLEASE provide photos whenever possible.


30 years experience in building design and construction, all hands-on, including the construction of dozens of single-family homes and hundreds of remodeling projects in the northeastern US.

Author: "Architectural Woodwork - Details for Construction" published by Van Nostrand Reinhold (now Wiley).

BS Cornell University.

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