Building Homes or Extensions/Crawl Space moisture


Hi Bruce,
I live in east Tn. and have a dirt crawl space (clay soil). The crawl space is around 18" at the lowest point and around 48" at the highest point. No water issue but I felt the RH was running too hight. It was running around 80-90% during the summer. The builder only put vent on one end of the house. I had Orkin to install a new moisture barrier and had them to run it up the walls, leaving a few inches at top for inspections. They also sealed the 4 vents on the side of the house (all 4 vents are on the same side of the house). The RH came down to about 70%. I purchased a dehumidifier and I can now control the RH down to around 45%. The electric heating unit is located in the crawl space. I don't like running the dehumidifier because of the additional expense of electricity. If I open up the vents that the builder put in and add a couple of vents on the opposite side do you think with the cross venation this would be acceptable and that I would not have a mold problem develop. I am open to other suggestions.
Thank you,
East Tennessee

Hi Eddie, try sealing off that last few inches of vapor barrier so that no moisture can enter the crawl space and see what your moisture content becomes. A vented crawl space usually needs a square foot of vent per 150-200 sq. feet of floor space and are normally located within 3 feet of the corners and every 12-16 feet after that.  In warm wet  climates,  having vents can cause moisture to be drawn into the space thus causing the very problem they were meant to control.  Unvented spaces are becoming the norm but require a good sealing vapor barrier.  Some people go so far as to cut vent holes between the floor and the living space to actually condition the space the same as the house..try sealing off the top edge of the vapor barrier and check your humidity level in a week and see if this helps..sincerely Bruce Johnson  

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Bruce E. Johnson


I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.


Custom Commercial and residential buildings. Churches, theaters, schools and auditoriums. Most recently I am working with the Catholic Church on several design build committees. I have a website related to scheduling and project supervision. Although my expertise is more related to multimillion dollar commercial, educational and theatrical projects my generous credentials in residential and remodelling construction make me a viable source of information regarding all forms of building questions.

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