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Joined: 9/1/2009
Posts: 16
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My home is a 1981 build rancher at 1400 square feet. The crawl space is about two and a half feet high with a concrete floor and cinder block walls. It was inspected by a Basement company who found it to be dry, a bit dusty and no signs of water leakage. They spray foamed the walls and joist ends and covered the six former vents. No plastic was laid on the floor. Three days after this was done, a hygrometer read the relative humidity at 79% so I put a dehumidifier in the space and set it to 45%. I called the Basement company and asked them for a quote on the vapor barrier, Their reply was $7280.00 for 40 mill plastic over 1400 sq. ft. My reply was colorful but was basically a no. I can put in 6 mill vapor barrier with acoustic sealant and tuck tape but I'd like some help with ventilation as well. The ventilation would be during the summer only but what do I use in fans and controllers?
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Joined: 7/17/2007
Posts: 248
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I'm not positive but I think I would start with the vapor barrier on the floor joists, install a small blower(50 cfm should be plenty) and turn it on with a humidistat and install or uncover two vents at the other end of the crawl space.
Do it right the first time, not counting all the trial and errors of course
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Joined: 6/30/2007
Posts: 115
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If they have insulated the end joists and outside walls as well as closed vents, I would just ventilate the space to the whole house system as if it was a basement. The load to the outside has been taken care of so it will not add a lot of load to the home. Ventilating the space by opening up vents will only bring in the humidity once again. Locking it in and ventilation to the home would be my choice, as long as the space is tested for radon and mold I do not think I would use a vapor barrier. If either of those is positive, I would install the vapor barrier and then ventilate to the whole house system.
Greetings from St. Louis MO.
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Joined: 10/1/2009
Posts: 6
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I read what you have written, and the replies. The reply from Swiley is the best with the exception of the humidistat. You do not have to worry about the humidity in the air, but the humidity in the wood supporting your house. Use a timer instead of the humidistat, set it at about 8-10 hours of on time per day. Use an inexpensive wood moisture meter and check the moisture in the support beams. It should be showing a decrease over time, check it weekly until you get a handle on your problem. Increase the "on" time of the blower if the moisture doesn’t come down. Likewise, you can decrease the "on" time if the moisture content drops too low. By the way, if the crawl space was dry before you started, why did you insulate the walls and seal the crawl space?
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Joined: 9/1/2009
Posts: 16
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RE:
"By the way, if the crawl space was dry before you started, why did you insulate the walls and seal the crawl space?"
One of the driving forces was a Hydro Bill for $460 for two months. Another was really cold floors since there were 6 vents to the outside in the crawl space. The third was a nagging feeling that this whole thing is just plain stupid; why put two inches on foam board up against the crawl space walls and then leave vents to the outside wide open. Why don't I just take the foam board outside and insulate the fence? The effectiveness would be the same. I wanted a warmer house and sealing off the crawl space to effectively turn it into a basement was the right thing to do. It's just not completely finished without a vapor barrier.
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Joined: 9/1/2009
Posts: 16
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I bought a crawler that lets me "drive" around the crawl space and in my travels and paying very close attention, I found something strange: I found the remains of a form rotting away in the floor located about six inches away from a wall. Covered in dust, it's hard to see and it looks dry. Underneath, it's half gone and under that is wet earth. The cure will be to add concrete to the place where the wooden form used to be and then put a vapor barrier on top. (I hope) The contractor chosen to do the job had never seen forms left in place until he saw this one. Expect the unexpected.
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