Q. I am stumped. I have a question regarding a bathroom exhaust fan in the second story of our colonial home. We have two bathrooms on the second floor and both have bath fans that vent through the roof. The master bath vents straight up through the roof and functions perfectly. This arrangement was added several years after the house was built. The hall bath, however, has a vent fan with a long angled path (approximately 12-15 feet). The bathroom is toward the front of the house and the roof vent is on the rear roof of the house. My guess is that the builder did not want anyone to see the vent on the front of the house. The hall bath vent occasionally will have damp drywall around the fan.

At first I thought it may have been due to moist bathroom air that is being vented and condensing in the exhaust duct. Now I am not sure that is the true story. We also have a whole house humidifier, the kind that mounts on the furnace and uses a water panel to introduce humidity into the house. Now that it is very cold out we had a little condensation on the inside of the windows and I noticed that the ceiling in the hall bathroom is damp even without anyone taking a shower or bath. I turned down t he humidifier to see if that will help. This seems to be a continual problem with the hall bathroom exhaust fan. I have resisted repairing the drywall as the problem is recurring. Do you have any suggestions? Would it be wise to vent the hall bathroo m fan straight up through the roof?

A. You have diagnosed your problem very well. You have two things going on and the solution to your basic problem will be a combination of efforts. It’s hard for me to argue against the use of whole house humidifiers especially now that my largest compl aint has been remedied by the industry with the addition of ultra-violet (UVC) light sanitizers into the system to keep the production and dispersal of mold, mildew and bacteria down. You probably don’t have such a device on your system but you haven’t mentioned that anyone is chronically sick in the house during the season of humidifier use.

I won’t bother to go into explanations about sensible heat of heated air augmented by latent heat of moisture introduced by the humidifier creating a sense of greater body comfort at lower temperatures or the avoidance of those annoying little static ele ctricity shocks you get at light switches and such when the air’s really dry because you know that-- that’s why you bought the humidifier in the first place.

But I do see humidifiers gone wild pumping excessive moisture into houses through malfunctioning controls or sticky valves gummed up by minerals deposited from evaporating water or even leaking humidifiers that drip water onto the ductwork ultimately rus ting things out. Go take a hard look at your humidifier and make sure it’s not doing anything like that. If you are seeing condensation on your windows then your interior humidity levels are pretty high, assuming you have insulated (double pane) glass in your windows. Don’t believe those settings on the humidifier controls. They are guesses at best. Buy yourself a cheap hygrometer at the hardware store and monitor the humidity levels in your house. You don’t want readings above about 45 percent re lative humidity (RH). 35 percent RH or so at the maximum should do just fine. If things are really out of whack with your humidifier, go take a look at the underside of your roof on a cold morning on the side of the house that gets the least sun. I’ve seen attics dripping wet with humidifier moisture and owners thinking the roof was bad. Well, not yet, but it’ll rot soon if that condition isn’t brought under control.

Now that you’ve got the humidity under control, keep an eye on that hall bath vent. It’s not uncommon for the vent grille to become cold due to air leakage and that can provide the cold condensation surface that will collect moisture and wet the drywall . In some cases the cold ducts leading from the bath will condense warm moist bathing exhaust air and if the duct is angled just right, drain the condensate back to the bath. I wouldn’t entertain the idea of cutting a roof vent just for that bath. Any h ole you cut into a roof can give you trouble, so the fewer the better. Frequently I see baths on the second level vented with ductwork that lays over the insulation and dumps out at a vented soffit. You might consider that but I’ll bet things will strai ghten themselves out when you get the humidity under control.