Basement Humidity

I am currently finishing 75 % of my basement area. The remainder of the area will remain unifinished (all the mechanicals and some storage). The foundation is block and I do have a sump pump so things are dry. As a last step in the project I plan to install carpet. I do have a decent size dehumidifier to keep the moisture level down. Is there a humidity threshold that I should stay at or below to avoid the musty smell that could develop over time? My house is 7 years old and I do have Central Air. However, there are no cold air return vents in the basement.
Thanks, -a12vman
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Sealing walls and floors with oil paint will help keep moisture from entering. But any leaks or dampness will develop mold behind walls. This is where a moisture meter is necessary to pinpoint bad areas. I keep my basement around 50-60%. If you smell mustiness it is mold even if you don`t see it. If so then killing it with bleach is a good first step, even spaying the walls with a grden sprayer. Test the floor tape plastic to it maybe several ft square if moisture collects you have to fix it first or the carpet will mold. Under carpet consider plastic sheet, it is better than ruining carpet. Cutting in an adjustable-closeable return is good for summer to move moist air out and a closeable supply for winter. It is easier to find and fix problems now instead of later.
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agreed, you need a vapor barrier on the floor of some kind
either waterproof paint or plastic
Mark
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You say that you are finishing your basement, and there are no cold air returns. I assume you have or are adding vents for A/C and heat, right? If they're at floor level, as opposed to in the ceiling, and if the air can return upstairs (no closed doors), that should help.
Since your mechanicals are there, it shouldn't be hard to add an air return.
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Buck
The method depends on your local conditions. It would help if you said where you are located. Mold starts to grow at about 80%. 75% RH is too high. If there is a cold spot in the room, it will grow mold because the RH will be higher there. Keep the Relative Humidity (RH) 40 to 60 %. Mold is more likely to grow on surfaces where AC blows directly on it.
Stretch
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a12vman wrote:

I would want that 75% (how are you measuring this?) down to no more than 60% at any time. I would suggest you likely need more ventilation. I suggest you have a HVAC professional come out and do the numbers (Manual D) to see what should be done.
--
Joseph Meehan

Dia\'s Muire duit
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Is furnace on basement floor, it is a upflow unit with return on bottom? Just cut in a return at the lowest point. mine is 4" from the floor. A 10 min job.
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On Mon, 11 Apr 2005 13:54:10 -0700, Mark wrote:

Here in Wisconsin, most carpet in basements are installed _without_ waterproof paint or plastic. I called many many places for recommendations and they all said that carpet directly over the slab is the best way to go because any vapor emission is vented through the carpet instead of getting trapped behind plastic causing mold to grow under the plastic.
I was surprised too. My brother-in-law paid lots of money for a professional contractor to finish his basement and carpet was laid directly on the slab.
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I currently have 4 Ceiling Mount supply vents and no returns. The Returns runs right alongside the supply trunk(right down the middle of the house) so does it make sense to have the cold air returns that close to the supply vents?

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