How to solve new condensation in bathroom exhaust fan ducts?

Question

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.

Answer 1

Given all the experts that have looked at this first hand, I am hesitant to suggest a hypothesis based on other situations I have seen. Also I live in a dry climate, so forgive me if I miss something obvious to everyone who is actually there in New Orleans.

As you know, condensation is the result of warm humid air contacting a cooler surface which is below the dew point. From your description, you probably have slightly negative static pressure in your house and warm humid make-up air is being sucked through particular exhaust vents that connect to the outside. That the problem became less severe when additional dampers and insulation were installed supports this hypothesis. Unfortunately, short of a completely air-tight seal, warm humid air will continue to infiltrate through these paths as long as there is negative static pressure in the house. Even a slow rate of warm humid air infiltration will eventually make every cool surface in its path wet.

The central blower on a system naturally makes slightly negative pressure in a house due to imperfect ducts that leak in spaces that are not open to the return(s) of the system (under the house, for example). Any air that leaks out in such locations must be made up somewhere else, in this case by pulling a little outside air in through your bathroom exhausts. The same situation occurs if the structure is somewhat leaky and the bathroom exhaust ducts are relatively close to the system return. Return air flows via the path of least resistance and it may be easier to draw some air from a nearby bathroom exhaust than to draw 100% of the air from the far reaches of a house if remote supply air can more easily leak to the outdoors.

A relatively easy fix could be to give the return on your central air handler a controlled path for make-up air. For example, you could duct a small outside air inlet directly to the return of your central unit, adjacent to the main return with a balancing damper so you can make final adjustments. The outside air duct should probably be taped or sealed as well as insulated from the cooler temperature of the house. It would also need to be small enough not overwhelm the main return or affect your energy consumption too adversely. With a little make-up air provided directly to the unit from outside, your blower should stop sucking make-up air through your bathroom and dryer ducts.

Answer 2

You have my sympathy for your situation.

Is it possible that exterior air is traveling upstream the vent pipe to the affected exhausts? If so, a well functioning flapper valve at the exterior port should stop it.

I looked briefly and found this which is intended for marine motor exhaust, but it certainly should do the trick.

Answer 3

Just a thought that maybe the ducts could be replaced with PVC instead of metal. Moisture seems to condense on cold steel much easier than plastic.

The other thought is to make sure exhaust fan ducts run vertical for a short distance and then run downhill to their terminus at the eave. Try to keep the vertical to a minimum.

Exhausting into the attic is against the code. But it does eliminate the problem. The moisture is then dispersed throughout the attic but this reduces the effectiveness of your insulation.