I had the drywall behind my stove disintegrating. When I replaced the bad section, I found a draft behind the wall. There was a vent ducted though the attic and out the roof. Intended to be used as a range hood exhaust. Funny thing was, the range hood was never set up to exhaust, it just recirculates back into the kitchen. Seems that the inside of the wall was seeing conditions a liitle too close to outdoors. On a cold damp day, I could see the drywall absorbing moisture, but I am pretty sure there was no water getting in. I removed the section of round duct in the attic. I stapled screening under the roof exhaust fitting, and left it as additional attic venting, and I sealed the lower portion into the wall studs with expanding foam. Two questions --- was this type of venting improper, or why did my paint and drywall disintegrate ? And was my solution OK, or is there a better way.