07:59AM | 04/16/02
Member Since: 08/14/01
6 lifetime posts
We have an office that is having a moisture problem. In the front of the store the crawl space is about 4 feet deep.

The floor above it is rotting again after being replaced only 4 years ago. There are a couple of small vents to the outside.

We are going to have the floor replaced but I need to fix the moisture problem. I always believed that mositure will build if there is not enough air movement.

Is there a way we can setup a vent with a fan to keep the air moving or is this not correct thinking?

Any suggestions would be helpfull!



05:45AM | 04/17/02
Member Since: 03/13/00
1675 lifetime posts
Actually, in hot dry weather moisture in your crawl space is INCREASED by warm air flow from the outside. This is because the warm air gives up its moisture, condensing in your crawl space, as it cools.
During summer, don't let the outside air move through the crawl space.
One more important thing:
Make sure the ground in the crawl space is covered with plastic. This will make a huge difference in the moisture level year-round.


06:31PM | 08/11/05
Member Since: 08/10/05
2 lifetime posts
I thought I was really making progress on solving the mold problem in my Charlotte NC crawlspace, but I'm back to where I started. I installed a 6mil vapor barrier across the entire floor and opened up all of the vents. Now white mold is growing ON TOP OF THE VAPOR BARRIER!!!! And the windows in my house are all fogging up (on the outside, not inside). Now that I have the vapor barrier, my hardwood floors are getting really tight and starting to buckle a little. The AC unit located in the crawlspace is producing a river of condensation (magnifying the problem) I can't tell if I have too much moisture or too little anymore. Am I missing something here?????


08:13PM | 08/11/05
Member Since: 04/25/05
1915 lifetime posts
"opened up all of the vents. "

NO, NO, NO!!!!

That is where all of your moisture is coming from.

How cold it it in the crawlspace?

Here is a chart of the dewpoint in Charlotte for August.

The dewpoint has been 65 and 75. Any time that air gets below the dewpoint you get condensation. Even when it is just gets near the dewpoint you will still have way too high of a moisture level. A level will support mildew, mold, and rot.


Then check out and do a google on - conditioned crawlspace - and - sealed crawlspace -.


03:20AM | 08/12/05
Member Since: 08/10/05
2 lifetime posts
Thanks, I'll give the closed-vent option a try for a while. I would guess that its the opposite in the winter given that my downstairs heating unit is in the crawl space??? Should I open the vents in the fall. Thanks again. This is a great BB.


05:08AM | 08/12/05
Member Since: 04/25/05
1915 lifetime posts
Yes, you could open then in the fall strickly for mositure control.

But in the winter then you are allow cold air in.

Here is the full drill recommendations for most areas east of the Rocky's. Depending on how bad the problems are and the details of construction you may or may not need to do all of this. But on a new house this is the way to go.

First you need to control ground water. That means sloping the ground away from the house, gutters and downspouts that will discharge the water at least 6 ft away. Have drain tile around the footing and if a high water level install sump and drain tile on the inside. And apply water proofer to the that part of the outside of the foundation that is below ground. Even "dry" soil and "dry" concrete will allow a fair amount of moisture to move through it.

Then on the inside install a vapor barrier over the ground sealed at all laps and at the foundation. A "rat slab" (thin layer of concrete) can be poured over it to protect the plastic if the crawlspace is used frequently for storage.

Install insulation on the inside of the foundation walls (needs to be carefully done as it can be a path for termites).

And no OUTSIDE ventaliation. But you do need a small amount of ventalation to the house.

The crawlspace become a short basment. Not part of the outside.

While I have not read the details NC has admended there code to allow conditioned crawlspace and the IRC is also being admended.

Look at the details of the Houses that Work at buildingscience. Also go to the reports and research section.


04:30PM | 08/13/05
Member Since: 01/28/03
693 lifetime posts

If you have AC and heat in your crawlspace, you have a 'conditioned space' and a conditioned space requires that vents be closed at all times throughout the year.
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