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Old 08-03-2009, 03:57 AM   #1
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Finished attic humidity?

I live on the Tidewater Peninsula in Virginia, where humidity levels can get pretty high--primarily in the summer months. This past spring, I had two solar powered attic fans installed to help ventilate our unfinished attic space. The fans seem like they're removing a lot of super heated air and reducing the humidity levels in that space. This has enabled us to increase our first and second floor thermostats by a few degrees, effectively reducing energy costs.

The downside is our finished attic space seems to be more uncomfortable than ever due to the installation of these fans. Please note that our attic is cooled by a separate Mitsubishi mini split system. While the temperature in the finished attic space is fine, the humidity is often pretty high. I'm trying to figure out what forces are contributing to the increase in humidity and how I solve this newly created problem.

One thing that may be happening is since the attic fans have lowered the unfinished attic temperatures, the air conditioner in the finished attic space may not be operating as often. This would reduce the system's natural ability to to remove humidity in this space.

The second theory I have is since the solar fans don't operate at night, the humidity levels in the unfinished space is increasing considerably until the fans resume their venting at dawn. The excess humidity in the unfinished attic then permeates through the walls and any open spaces, which increases the humidity in the finished space.

Having said that, and not really knowing what forces are at work here, what can I do to lower the humidity in the finished attic space. This week it's been hovering from 60 to 65%, with outside humidity somewhere between 75-85%.


Last edited by kcrossley2; 08-03-2009 at 04:02 AM.
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Old 08-10-2009, 09:14 PM   #2
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Old 08-11-2009, 05:06 AM   #3
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Your fans are probably pulling your conditioned air from the finnished area.
And the finnished area, is probably drawing in fresh unconditioned air.

Seal the 2 areas from each other.

This is one of the problems with powered exhaust fans. If the attic doesn't have enough intake venting. And its not sealed from the occupied area. It draws conditioned air from the occupied area.
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