I'm trying to add bathroom exhaust fans to a two-story condominium. One bathroom is upstairs with access to the attic. The other is downstairs with access to the dryer exhaust but little else. The dryer exhaust runs between the first and second floors and vents through the wall. I don't have permission to add a vent in the roof or an outside wall. Upstairs, I could vent to the attic but I've heard this is not a good idea. Downstairs I could connect to the dryer vent and use baffles to keep the dryer and bathroom exhausts from flowing in the wrong direction. Or should I try room-to-room ventilation to increase air circulation and get the bathroom moisture closer to the air-conditioning intake?
Let's start by recognizing that both of your bathrooms should have exhaust fans that are vented directly outside. That may be required by your local building code, and it's good-building practice whether required or not. There's simply no reasonable excuse for skipping this relatively minor cost during construction. All the warm, moist air generated in a bathroom can lead to a host of problems if it's not ducted out of the building.
Never vent a fan into the attic or basement.
Exhausting moisture-laden air into an attic or basement is never a good idea. True, it's sometimes the easiest thing to do, but it's still wrong. The reason is that moisture will condense into liquid water on cooler surfaces - for example, roof sheathing or framing in a cold climate - and increase the chance of mold and decay.
Builders sometimes drape flexible duct across an attic floor and direct it toward a soffit vent at the eaves, but this is just laziness or ignorance. So for your upstairs bathroom, the answer is to vent the exhaust air through a gable-end sidewall or the roof (we'll deal with the permission problem in a minute).
As for your downstairs bath, I think you're better off with a new exhaust duct rather than tapping into an existing dryer vent for several reasons. First, your dryer vent may not be installed correctly, or it may have become damaged over time. And second, it sounds as if you'll have to do some demolition to make the connection anyway - so why not go all the way in and install a new line?
Plead common sense.
I've never lived in a condo so I'm not sure what hoops you have to jump through to make changes. But why would a well-meaning condo association object to something that should have been done in the first place? If you offer to correct a flaw in the original plan, wouldn't they thank you?
Mechanical ventilation is such a basic consideration for good building these days that I can't imagine any other reaction. A variety of wall and roof vents are available and none of them has to be an eyesore.
Size your fans correctly.
Once you've won your condo board over, be sure to buy a good quality fan that moves enough air. The Home Ventilation Institute recommends a fan that will provide eight complete air changes per hour. For detailed recommendations on fan sizes, try the institute's Web site: hvi.org.
Cheap fans are noisy. Look for one with a low sone rating. Some models made by Panasonic and Broan-NuTone (and possibly others) are rated for well under 1 sone, so quiet they are essentially inaudible.
In the end, both you and your condo association will be happy you did the right thing. Your condo will be healthier, and so will you.
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