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I'm refinishing my basement bathroom and installing a vent. I can easily vent out to this wall, but the thing I'm worried about is it's semi-enclosed. In the winter we put up tarps and store wood here, in the summer we keep garden tools here. Should I have anything to worry about in this? Is the moisture hittig the cold air there going to condense and leak back inside or anything? Am I going to get unexpected results that are going to make me hate myself for thinking it could be so easy? Additionally, I was wondering if I could just continue the duct work along the floor joists of this attachment and put the vent all the way on the exterior wall? We've been using that space to store 2x4s as you can see in the photo, but I can spare some for a duct. I'm just concerned that the pipe will get cold and when hot wet air is pumped through it, I'll have water buildup in the duct. Another option I have is venting it under the outside stairs, but that's very similar to letting it out under the attachment

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The warmer air exhausted from the bath entering a cold environment will condense and create moisture. It will create high humidity which will lead to mold growth. this will become a health issue as well as rot the wood. You want to vent it to outdoors, never indoors.

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At first I was going to question you on what your definition of "indoors" and "outdoors" was, since I'm not sure what this space qualifies as.. but regardless, your concerns are the same as mine are - moisture. –  kavisiegel Apr 6 '12 at 15:57
    
The problem is it is an enclosed area, even if it is somewhat open. The vented air will tend to linger in the area, rather than dissipate in open air. That trapped air, from the bath, if the temperature differential is big enough and there is enough humidity in the air will cause condensation to form. Weather or not you have sufficient air flow in that room is questionable. You may get away with it, or not. You could try venting it there, and watch it closely, if it is a problem you will need to move it to outdoors. If it were me, I would just go for venting outdoors, and be done with it. –  SteveR Apr 7 '12 at 12:31
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I concur with SteveR. You have access to the ceiling joists in this area, so I would just buy an extra 10-15' of 4" rigid vent line and extend the vent out to the eaves beyond this enclosed porch area. The reason is exactly as SteveR said; regardless of your definition of "inside" vs "outside" with regards to this space, it is an enclosed "cold zone" which has more limited airflow, and so the exhausted air will not have much opportunity to exit this area before becoming too cold to hold its moisture, which will cause condensation onto various surfaces in the room. Some are designed to handle that (like your siding), others aren't (like the firewood, joists and sheathing).

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