Proper Crawl Space Ventilation

[Earle Kubat]

Can someone give me a qualified response to how much ventilation is needed in my 1500 sq ft crawl space to prevent or cure the unpleasant odor that results from dampness under the house in the summer months? Over the last several cooling seasons here in South Jersey, it gets very damp under my house from condensation dripping from my central air conditioner duct work.
When you first enter the house when you have been out for the day, your can detect a very strong "musty" wet odor. Any advice?
Earle

[jadnashua]

First, if the ducts aren't insullated, you need to do that. Unfortuneately, the more ventillation you have down there, the more condensation you'll get if the duct's surface is below the dew point, so insullation is critical.

Then, if you don't have some plastic on the ground to stop it coming up through there, try that.

[Cass]

The insulation needs to be air tight. Any areas that allow air to contact the duct work will sweat and dampen / wet the insulation and will defeat the purpose of it.

[BigLou]

None adding ventilation will just add more moisure to the crawl space. instead like the others have suggested add insulation to the duct work or better yet move the duct work inside the conditioned space

[Bob NH]

Condensation on the duct work is costing you electricity. Fix that first and you will also increase the cooling capacity of the A/C. Also, insulate between the crawl space and the occupied space.

Then open up the crawl space so the air can move through it. If you have bare ground under the crawl space, lay down a layer of plastic.

[Mike Swearingen]

I'm not sure how it works in Jersey, but down here in NC on the Albemarle Sound where we have very high moisture in the ground and in the air, I finally resolved a similar dripping crawlspace issue with the following.
I built a pressure-treated plywood and 2X4 crawlspace door and installed a 10" louvered dairy barn exhaust fan in it. It only has a 1/30 hp motor, and it runs 24/7 in all but a few months of the year here when it freezes and the moisture is low.
I don't have a ground moisture barrier and the constant circulation really helps stop the dripping.
I ordered the fan from W.W. Grainger. You need to know your crawlspace cubic footage for their Tech Support to advise you on the size CFM fan you need. (Cubic footage: crawlspace heightXwidthXlength.) I ordered a fan withmore than twice the CFM that they recommended.
Mike

[Earle Kubat]

Thanks Mike,
Your weather and climate is very similar to ours here in Cape May New Jersey, which is the extreme southern tip of the state. It can get very humid here during the summer months.
My crawl space has an earth floor. I am adding additional vents to place in the cinderblock foundation walls to give me additional cross ventilation. As an added measure I am going to go with your idea of a framed exhaust fan at the access opening under my Bilco Steel Door entrance to my crawl space that is outside the rear of my home. Crawl space is approx 1500 sq ft and 3-1/2 ft high which gives me 5250 cu ft of space to ventilate. My access panel under the Bilco Door is a 30 X 30 inch opening.
What do you think?
Earle

[jadnashua]

If you don't cover the earth floor, you'll have a constant source of moisture...a nice thick layer of plastic should work.

[Cass]

If you do not insulate the duct work you will decrease the efficiency of your A/C as you will be losing cooling to the crawl space.

Even if you add circulation, and it stops moisture from dripping off the duct work it will only be because it is evaporating before it has a chance to drip. The energy loss will maybe be even greater in that situation.

[cybervex]

Why not look into a sealed crawlspace? Plastic coat the floor, spray foam the walls and rim boards, and seal up the vents. Once sealed you will need to open a vent to let in a bit of conditioned air. Treat it like a mini basement rather than a extremely inefficient crawlspace.

Its nice to not have cold floors in the winter.