Condensation Between Floors


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Old 01-25-16, 09:25 AM
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Condensation Between Floors

So what was supposed to be a quick project this weekend turned into a much bigger project...

I planned on replacing the exhaust fan in my first floor master bathroom with a larger quieter unit (110CFM for 60sqft or so). Upon removal of the old fan, I noticed frost on the underside of the subfloor above, about 1" wide running along the edge of the floor joist for the second floor... This band of frost/condensation starts just forward of where the exhaust fan was located, and continues to within a foot or so of the outside wall.

This bay is insulated from the outside wall about 4' back to where the exhaust fan is located. About 3/4 of it is completely insulated, the only portion which is not completely insulated is that which the exhaust duct runs. The house is a cape with doghouse dormers, so it just so happens that the area above the frosty flooring is a cold space between an upstairs inside wall and the eaves. There is no evidence of condensation on the opposite side of the bay where the duct runs. The wall of the dormer upstairs, about cuts the bay such that about 3/4 of the length of it is cold (frosty part), 1/4 heated (above fan housing), and 1/2 of the width is cold (fully insulated part), 1/2 heated (partially insulated where duct runs).

The area directly above the fan housing is heated, and nothing around, or beyond the fan farther into the house is insulated or frosty, or condensated.

I suspect what was happening was that moist air escaping through the fan housing and/or fan housing/duct connection was escaping into that bay and condensing on the colder Advantech sheathing above which would explain why all condensation is below the cold space only. I was able to reach my hand in and fluff the insulation up to better seal the cavity prior to installing the new fan which was a major ordeal...

Upon attempting to install the new fan, I had a ducting elbow fail at the far end where the duct work turns up to go through the second floor and proceeds to route outside. I was able to gain access from above enough to pull the section the failed elbow was connected to out of the hole in the floor and repair the elbow. Then came the issue of blindly mating the 3' length running through the bathroom ceiling to the now repaired elbow... Obviously I am not able to tape this connection, and had to just slide it together with the help of the wife holding the elbow assembly upstairs and me sliding the pipe assembly into it from the bathroom. Then came the issue of getting the new fan into the hole... I had to straighten out the bent lower edges in order to get it to slide completely into the enlarged hole I made so that I could slide the fan between the top of the sheetrock and subfloor, then slide it forward to connect the duct adapter all while lowering the assembly back through the hole.

Anyways, after a solid 6 or 7 hrs, I have everything back together. I am still not happy about that condensation I found, which is what prompted this post.

Is there anything I can do to stop that from happening? Should I bust through the wall on the 2nd floor in order to maybe lay down some foam insulation on the unheated portion of that subflooring to try and keep it warmer?

Also, I have a small gap around the bottom edge of the fan housing and the sheetrock hole. Should I seal this up? The old fan did not have the gap sealed.
 
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Old 01-25-16, 12:02 PM
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Another thing I am considering doing is replacing the rigid duct that is in the bathroom ceiling with flex from the fan to the point where I have access on the second floor, then running rigid the rest of the way. In doing that, I would be able to pull it tight to remove most of the "flex" and also ensure that all my connections in the bathroom ceiling are leak free.
 
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Old 02-01-16, 06:24 AM
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So I took everything back apart yesterday and replaced the ridged duct in the ceiling with flex duct.
 
 

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