Humidity in un-finished basement


Old 06-15-05, 09:29 AM
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Humidity in un-finished basement

I have been working up to finishing my basement. I want to ensure I don't have any issues with moisture behind my finished walls and under my floor, though.
My basement isn't wet, but it does feel humid down there, so I have been trying to lower the humidity. I keep a dehumidifier running 24/7. I also keep a couple vents open for the furnace/central air to help circulate air down there. The humidity runs around 45-50% with these things running. If I close the furnace vents, it goes up to around 60-65%.
The exterior of the basement walls are waterproofed and have that blue polystyrene insulation over that.
My first thought was that dampness was somehow getting in through the block walls, so I sealed up the wall/corner interface with hydraulic cement and applied dry-loc to the walls. This didn't seem to help the issue. In fact, it may have made matters worse as I think this is trapping the humidity in the basement now. When I close the vents and let the humidity get up to 60% or so I am gettign a little mildew on the dry-loc'd walls now.
Next I sealed up all the saw cracks in the floor with caulk. This hasn't seemed to help either. If I close the furnace vents, my humidity still shoots up to 60% +. I'm trying to figure out what to try next. Any suggestions?
I just can't figure out what is creating the humidity down there.
Do you think the issue will be there behind the finished walls, or should I just go ahead and finish it?
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Old 06-17-05, 02:31 AM
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This is in North America? Where?

Basement temperature vs. outdoors?

Is the access between basement and main floor left open?

Do you have a lot of new wood and other "green" material down there?
Old 06-17-05, 04:35 AM
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Thanks for the reply
I am in Ohio (Toledo area). The outside temperatures have been fluctuating from the 90/70 range to the 70/50 range lately.
The door to the main floor of the house is kept closed. There are 3 windows in the basement. Not egress windows, just normal sized basement windows.
The house is 3 years old now.
Your question about "green" material makes me wonder. I started framing in the walls to finish the basement in 1 area, using treated lumber for my bottom pieces. The mildew has always appeared on the block wall behind the studs in the partially framed area right above the treated lumber.
Old 06-17-05, 04:53 AM
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I believe dehumidifiers are sized according to how big the room you want to dehumidiy is. Could you have purchased a dehumidifier which just isn't up to the job? I have a dehumidifier which stops itself when the disired humidity level is met.
Old 06-17-05, 08:41 AM
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I'm using a 40 pint "basement" dehumidifier plumbed into my sump drain and I have it set to the dryest setting so it runs 24/7.

One thing that may be of interest is the frequency my sump pump runs. It runs every 2 - 4 minutes on a normal day. During a spring thaw or a big rain storm that can increase to turning on every 30 seconds. The basement never usually gets wet, though, so my perimeter tile must be working. The basement did get about 1/4 inch of water in it when the last pump went out this past winter before I saw it and got it replaced. I have a working backup pump in place now to prevent this from happening again.
Old 06-17-05, 11:55 AM
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Everything you did sealing out the basement probably helps, it'll keep water out, but it won't stop water vapor from entering your basement. There are just too many ways that moist air can penetrate a structure and it'll always find a way to migrate to a dryer area. The only way to control that is to keep that dehumidifier running during the summer. You should not have a mold problem with that thing going. When you build your walls, try to seal any opening that will allow moist air to come in contact with concrete and use a vapor barrier. But like I said, with all that conditioning down there, you'll never see any problems.
Old 06-18-05, 08:28 PM
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I finished my basement last year, and before I did, there was a musty smell. No water problem, just a little humid. I don't know how bad your mildew is, but after I sealed everything up (insulation/drywall/carpet) the "dampness" was immediately gone. I do have to run a humidifier from time to time, but this is mostly a preventative measure. Hope that helps.
Old 06-19-05, 05:35 AM
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That is re-assuring. It sounds like I may be in a similar situation.
How did you insulate in this situation, then?
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