From mildew in the corners to mists on the mirrors — not to mention those less-than-pleasant odors — it’s not tough to figure out when a bathroom doesn’t have adequate ventilation. Sizing and installing the right bathroom ventilation fan can make a big difference, and it’s not a tough undertaking.

How big a fan do you need?
Ventilation fans are required by code in any bathroom that doesn’t have an operable window. But realistically, how often do you open the window while you’re showering — especially in the winter? So as a practical matter, required or not, your bathroom really needs to have a fan in it.

The first step in the process is to determine the fan’s size. You could install the typical small, noisy and generally ineffective "builder’s grade" fan, but what you really want to do is take the time to size the fan properly to the room.

The fan should be able to change the air in the room about 10 times per hour, so first you need to know the volume of air that the bathroom contains. To do that, measure the length, width and height of the room, and multiply the three numbers.

For example, if the bathroom is 8 feet by 10 feet with an 8-foot ceiling, it would contain 640 cubic feet of air (8 x 10 x 8). If you want a fan that will change that air 10 times in one hour, you would need a fan that will move 6,400 cubic feet per hour (640 cubic feet of air x 10 air changes).

However, ventilation fans are actually rated in cubic feet per minute (CFM). To make the conversion, divide your 6,400 cubic feet per hour by 60, and you’ll arrive at 106.67 CFM. You won’t find one with that exact rating, so you’ll probably end up shopping for one in the 100 to 110 CFM range.

Let’s quiet things down a bit
There are two general rules of thumb when it comes to fans — cheap fans are noisy, and noisy fans don’t get used as often. So when you’re shopping for a new ventilation fan, plan on spending a little bit more and get one that’s quiet.

In addition to the CFM rating, the other important ventilation fan rating is noise level. This will be listed in "sones," which is a universally accepted measurement of how we recognize and perceive sound. The lower the sone rating a fan has, the quieter it is. Ideally, look for one with a rating of 1 or less. Larger fans for large rooms may have sone ratings closer to 1.5, but don’t get above that.

The final thing you’ll be looking at on your fan shopping trip is the overall appearance of the fan, which is actually just the outer trim piece. This is obviously an important consideration, but it should come second after selecting the proper size and noise levels.

Install it right
Now that you’ve gone through the trouble and the expense to select the best fan for your application, you want to be sure it’s installed correctly. Ventilation fans are not difficult to install, but there are definitely some things to pay attention to if you want to do the job right.

First of all, be sure the fan housing is attached securely to the ceiling joists, using screws. If you don’t install the housing correctly, or if you rely on nails for fastening, the housing can eventually work loose and begin to vibrate against the joist, adding unnecessary noise. Some larger fans come with braces that are designed to extend between two adjacent joists for additional support. If yours has those, be sure to use them. Also, fill up all the mounting holes as recommended by the manufacturer.

The fan housing will have a damper control in it, right where the exhaust duct attaches to the housing. The damper is designed to swing open when the fan’s in use, and a spring pulls it closed when the fan’s off to prevent cold air from coming back down the duct and into the room.

Be sure that the damper is installed correctly, and that the flaps are opening and closing properly, without any interference. Better fans have a gasket to minimize air leaks and to quiet down the closing of the damper, so be sure that’s in place as well.

Finally, there’s the ducting itself. First and foremost, be sure you duct the fan all the way to the outside of the house, not just into the attic. Remember, the primary job of the ventilation fan is to remove moisture, not odors, so if you don’t duct it outside, you’ll be taking all that warm, moist air and pumping it into the attic where it can severely damage framing and insulation.

Use the proper type of ducting as recommended by the manufacturer, and in compliance with local building codes. For most installations, the best choice is smooth-wall, 4-inch sheet metal ducting, which allows for quiet passage of the air through it with little or no buildup of moisture inside.

To keep the fan moving as much air as possible, and to minimize both noise and potential moisture traps, keep the duct run as straight as possible. Where turns are necessary, use adjustable elbows, and make the turns as gradual as you can.

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