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denemante 05-07-2010 10:24 AM

Finished basement humidity at 80% for unknown reasons
Hey all,

We just bought a house that's 12 years old. It has 1/2 of the basement finished, and this was done by the previous owner just two years ago.

The humidity in the finished part is 80%.

The finished part is not daylight. The unfinished part has a door to outside and windows. This finished area has finished doors leading to the unfinished part.

Visually, the finished area is of very high quality. It would appear the contractor did a fine job - but I cannot speak for what's behind the wall. I can see behind them in one section - they build a new insulated wall about 4 inches away from the concrete wall - so there is a 4 inch void.

The downspouts and grading of my property are correct. There are no leaks or any sign of water intrusion or mold. Everything looks 100%. The single only fabric material in the basement is the carpeted steps (again, 2 years old). They smell musty. They must be sucking up the moisture. I pulled the carpet back and there are no signs of mold or any moisture.

Worth noting is that this finished side of the basement shares HVAC with our first floor. And - all the vents in the basement were shut. I know that air needs to cycle, so I opened them all. There are two 10 inch feeds and one return for this finished side (which is about 600-700 square feet). I've left the thermostat fan on, and confirmed that air is cycling through the basement. I also turned on the bathroom exhaust fan in the basement, set up other fans, and opened the doors to the unfinished side (which strangely always feels far less muggy). Humidity dropped to only about 76% on the finished side.

I've heard dehumidifyers will work - but then you're effectively sucking moisture inward from where ever it's coming from originally. I'm trying to locate the reason the humidity is in the high 70s.

So I value all your opinions. I'm trying to figure out the source of this humidity and stop it. But if there is no source, and it just happens that this room is muggy natually - then perhaps a dehumidifyer is the way to go. Finally perhaps I just need s few more days of the system running and fans blowing down there to level stuff out. Perhaps the previous owner just had the vents all shut for too long.

Any thoughts/ideas?


Dave Carney 05-07-2010 11:01 AM

I don't think it's that uncommon for a basement to have that humidity level, unless you run a dehumidifier. Get a dehumidifier that is capable of at lower temperatures (they are rated at 80 degrees or so which is deceiving). What's the humidity upstairs? What I did was install a dehumidifier upstairs, where it's warmer and easier to wring out the air, it runs 4 hours a night on a timer, and it still dropped the humidity in the basement from 70's to high 50's after a week. Upstairs it's 49% (morning) to 53% (bedtime).

denemante 05-07-2010 11:49 AM

The humidity on my upstairs (second floor) is about 55%, and the first floor about 57%....with the basement again around 80%.

From what you've implied - reduced humidity doesn't happen overnight. Perhaps I need to circulate and internally air out the basement with the HVAC for another week? Perhaps that would solve the problem.

But if not, I'm all for trying a dehumidifyer, but still a little worried about what is causing the 80% in the basement to start with.

I'd still like to sort of find out what the humidity is like down there under natural (without dehumidifyer) conditions and cycling air from HVAC.

Dave Carney 05-07-2010 12:02 PM

I'm not sure what you mean by what I've implied. What I'm saying is, for me, it works as good or better to run the unit for a few hours upstairs overnight, rather than running it 24/7 downstairs (where the temp is in the 60's and it's much slower pulling water out of the air). If you buy a portable humidifier (I have a 70 pint) you can experiment and see what I mean. For every 1% I pull the upstairs down, I see a 2% drop in the basement. That drier upstairs air has a great affinity for water vapor and mixes with the basement air (by virtue of shooting the dehumidifier exhaust air right down the basement stairs and by virtue of a return air vent in the basement for the HVAC)

You can try your way first, doesn't cost anything. But I bet at best you'll will still be in the very high 60's to low 70's in the basement and still end up getting a dehumidifier.

denemante 05-07-2010 01:39 PM

I meant you said you were able to drop your humidity after a week with a dehumidifyer. If I'm just cycling air, it might take that long or longer until I start to see reduced results...

beenthere 05-07-2010 02:48 PM

What temp is/was your basement when you had 80%RH down there. And what temp did you have your thermostat set for.
Are you currently heating or cooling your house.

Dave Carney 05-07-2010 03:33 PM


Originally Posted by denemante (Post 438839)
I meant you said you were able to drop your humidity after a week with a dehumidifyer. If I'm just cycling air, it might take that long or longer until I start to see reduced results...

A week but only running 4 hours a day...

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