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.
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.
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
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.
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.
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
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.
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
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