K.L., Wrentham

A. No, no, a thousand times no. Well, maybe, but even with a breezy attic, all that moist air could hit a specific portion of the inside roof, enough to condense into water. And with that wet roof, mildew growth is virtually inevitable. And with mildew, decay is sure to follow. It's just not a good idea; don't tempt fate. So, vent the fan through a wall or through the roof.

Q. I am venting a bathroom exhaust fan into a soffit in the overhang of my roof. I know that I have to make a tight connection between the vent pipe and the soffit outlet to prevent warm, moist air from filling the soffit space, but I can't find a proper cover for the outlet. What can I use in place of such a cover?

HARVEY SCHMUCK, Watertown

A. Good point; you probably can't find a cover for this outlet because they don't exist, although I can't imagine that any round louvured cover cannot be found. If the vent pipe is 4 inches in diameter, buy a 4-inch, round, louvered aluminum vent, the kind installed in soffits for attic ventilation. If the vent pipe is an odd size and/or shape, use your imagination: Install the pipe securely in the soffit opening, then staple a screen over it and secure the screen with a border of wood molding.

Q. My contractor wants to vent the bath exhaust fan through the attic ridge vent. I have a funny feeling about that. Should it be done?

LORI HARVEY, Acton

A. Your instincts are good. No, it should not be done. It would be difficult if not impossible to make a tight seal where the duct goes up to the ridge vent. Without such a seal, moist air could escape from the duct before it goes through the vent, resulting in a wet attic ceiling and possible mildew.

There are two better ways: 1. Run the duct along the attic floor and guide it into a soffit (overhang). The connection between duct and soffit must also be tight. 2. Run the duct straight up and through the roof. Use PVC pipe for the duct going through and above roof, and make sure the pipe is well flashed. You can buy rubber boots that slip over the pipe; the boots are self-flashed for a tight seal.

Q. I have an exhaust fan directly over the tub/shower. When it is on, I get dust that looks like lint on the vanity top. How can I prevent that linty dust from building up on surfaces?

MARY GUERRIERO, Westwood

A. Clean the duct from the fan, where it goes through the wall or soffit to the outdoors. If you find lint there, you know the source of your problem. Also, if there is a door at the outside end of the duct, make sure it is fully open when the fan is on and, of course, fully closed when it's off.

It is my guess that there is some sort of backdraft in the duct when the fan is on, bringing some lint from the duct itself back into the room.

Q. The outside flap of my exhaust fan does not close when the fan goes off. The fan is housed in a hood, so it opens on a horizontal plane rather than a vertical plane as on a fan installed in a wall. My neighbors have similar ones that work well. It is a Nu-Tone.

JOHN DONNELLY, Needham

A. It sounds as if the spring or other device that closed the flap when the fan stops is broken or gone. I would contact Nu-Tone, which has a good name in ventilation and which should be happy to fix it, even for a fee, to keep up its good name.

Q. have a Panasonic exhaust fan (exhausting to the outside) in the master bath of my new house. Water is leaking out of the fan. It is clearly condensation. The fan was originally 70 CFM (or 70 cubic feet per minute exhausted), so we increased it to 90, and insulation was added to the duct. There is less dripping, but still enough to be a nuisance. Would a 110 CFM fan stop the dripping, which is hitting the fan motor? I would not want the fan to short out.

R.S., Newton

A. If 90 CFM did better than 70, then 110 might do it all, and the 110 would not be too big for a master bath, which in a new house I suspect is pretty big. And, keep the fan on for as long as necessary after bathing or showering, to exhaust all that humid air, which is what is condensing in the cool duct.

Another trick that might work is to make the duct coming from the motor as short as possible before it goes horizontally to the outdoors, and put a slight sag in the horizontal duct, so any condensation will stay in the sag and stay away from the motor. There will not be enough water in the sag to cause any problems in winter or any other season.

If the duct goes vertically to the roof, even hnsation. In that case, more use of the fan might help.

eavy insulation on the vertical duct in the attic will not stop the conde