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sheslostcontrol 05-21-2010 08:22 PM

Cement crawl space ventilation
Hi -- new to the forum..

My 20'x30'x6' crawl space has cement floors, and stays very humid. Even with a large dehumidifier running the humidity is around 70%. The crawl area is where my furnace, hot water heater, and washer/dryer are located.

I ordered a 200 CFM powered crawl space vent which has a built-in thermostat and humidistat.. It will be installed on one side of the space and a standard non-powered vent on the other side.

My question for the forum is whether I should run a fan inside the crawl area to circulate the air, or just let the vent fan draw air from the standard vent?

beenthere 05-21-2010 09:48 PM

Depending where your located at. The power venting could make the crawlspace worse. Since it already has fresh air coming into it.

sheslostcontrol 05-22-2010 06:56 AM

sorry -- I'm in Georgia.

The humidistat will turn the fan & standard vents off if the humidity rises above a set level (like during a rain storm).

As of today there is no air coming into the crawlspace. No ventilation whatsoever.

beenthere 05-22-2010 07:02 AM

IS your water heater connected to a chimney? If it is, then air is being drawn into the crawl.

If it isn't, then the water heater is causing your humidity problem.

sheslostcontrol 05-22-2010 07:18 AM

It may be, but before the water heater was filled up & turned on I still measured 70% humidity. I'm currently at 70-75% with the water heater operational.

beenthere 05-22-2010 07:23 AM

A chimney drafts/draws air up through it, weather or not the water heater is filled or not, and weather the burner is on or not.

sheslostcontrol 05-22-2010 07:25 AM

I do not have a chimney -- there is a flue for the furnace, but that is closed off to crawlspace air as best I can tell.

I was planning to place the powered vent near the top of the wall, to pull the most humid air.

beenthere 05-22-2010 07:39 AM


Where does the water heater vent to. Does it have a chimney. Or does it just vent into the crawlspace.

High humidity in a crawlspace, often comes from the crawlspace being vented.

sheslostcontrol 05-22-2010 07:48 AM

The house sat without power and water for several months. During this time, with no water in the heater (drained), and no ventilation, the humidity was around 80%. The previous owner had zero ventilation in this crawl space. I haven't added anything other than a dehumidifier (which feeds into a bucket I empty about once a day).

beenthere 05-22-2010 08:32 AM


Since you don't answer my questions.

I'll leave someone else help you. Good luck.

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