Crawlspace Humidity

[golfinator]

My 45 yr old home in Atlanta, GA has an 800(25x32) s.f. crawlspace containing 2600 cu. ft. The dirt portion is covered with 2 layers of plastic sheeting. Furnace, fan and evap are located on a 10’x10’ slab in the crawlspace. Humidity level in crawlspace averaging around 64 percent and temp around 68 degrees. Want to cut down on musty crawlspace odor that circulates thru HVAC system air intake during humid summers. Plan on covering 4 outside air vents in concrete block foundation walls, and hooking up portable humidifier with continuous draining. Estimate 130 CFM's needed to handle 2600 cu. ft. for 3 air changes/hr.
Looks like there’s a Frigidaire Model FDR25 pint-141/177cfm, 4.5 amps, or Model FDR30 pt.-202/240cfm, 4.8 amp. Both rated at 58.1 dB, use R134A refrigerant. Which one will operate more efficiently, or should I be looking for different specs?

[CyFree]

64% humidity is enough moisture to allow mold growth and dry rot.

The vapor barrier in the floor is only addressing one source of moisture: ground moisture. There is the outside air moisture that comes in through the vents and, because the crawl is cooler, condensation happens.

You need to stop that outside air moisture from coming in before you invest in a dehumidifier, otherwise it will be running non stop, waste a lot of energy and might not keep your crawl draw enough to prevent mold, specially in hot and humid summer days, because that outside moisture is consistently pouring in.

To solve your crawl space moisture problems, the best, and US Department of Energy recommended method is to encapsulate the crawl space.

Crawl space encapsulation is a process that involves:
- Correcting crawl space drainage and grading
- Sealing the vents
- Completely isolating the crawl space from ground and outside air moisture by lining the entire space with a think 20mil vapor barrier, and air sealing it. (This needs to be done professionally because you have your furnace running in the crawl)
- Running a dehumidifier or crawl space conditioner system.

Besides protecting your home's structural integrity and indoor air from mold, it will also help you save a bundle in your heating and cooling bills. Around 20% in an average home, much more if, like it seems to be your case, there are ducts running through the crawl.

On another note, the fact that the mold is getting into the house via HVAC ducts, might indicate duct leakage which is not only an energy waster, but also poses a safety hazard: imbalances between return and supply ducts can cause backdrafting in combustion appliances, and increase your family's risk to carbon monoxide exposure.