We live in New Orleans in a two story, 95 year old stucco house. When we renovated 10 years ago, we put over 30 recessed can lights in the upstairs ceiling. Those openings in an older leaky house—coupled with a poorly insulated attic—caused us to have terrible condensation problems upstairs in the hot humid months between May and October.
Every ceiling opening had moisture and mildew. Our smoke alarms filled with water, and our hallway walls and ceiling would drip water in the early morning.
It took us some time (and the input of some smart people) to figure out the issue, but in the last year we have made several changes: our HVAC system has been completely redesigned and we made the decision to spray foam the attic (a fairly new option for Gulf coast states). Our condensation problems have been resolved and we have a comfortable, dry house. Going from consistent 65-75% relative humidity in the summer to the low 50s—which is low for New Orleans! The attic is now pleasant as well, just a few degrees warmer than the house.
Just as we were ready to move back in, we discovered both bathroom fan exhaust ducts, as well as the gas dryer duct—all upstairs—have been filling with condensation. (The exhaust vent in the powder room downstairs is fine.) This is a new thing.
The guest bathroom duct and the dryer duct (both about 3-5' runs) travel horizontally and exhaust out the side of the house. The bath goes through the attic, the dryer through an inside closet. The master bath exhausts go upward diagonally through the attic to the roof about 5-7'. We then installed an additional damper within each bathroom duct (in addition to the damper that is part of the fan unit) and insulated the ducts. The problem reduced somewhat (the bathroom ceiling no longer has condensation around the grill), but when the fans are turned on, water comes pouring out.
We have talked to a few HVAC companies and mechanical engineers and all acknowledge they are seeing more of this recently and are not sure what is causing it, nor how to solve it beyond what we have already done. We definitely have a tighter, drier house. But we have not really been able to achieve positive pressure. Not without spray foaming the crawlspace under the house, an option with a lot of potential problems for us.
Our HVAC company thinks the bathrooms could exhaust into the HVAC system or even the attic itself without problems, but given our history with moisture issues, I am leery. Another idea has been to install an electrically controlled damper that works in sync with the exhaust fan and is not dependent on the air flow. Any ideas would be appreciated.