How a Proper Fan Can Create a Moisture Free Bathroom

image of bathroom.Is your bathroom suffering from steaming mirrors? Maybe a bit of mildew in the corners? And of course, those unpleasant odors!?

The answer to moisture and odor problems in the bathroom is a good exhaust fan that's properly sized and properly vented.

Unfortunately, many “builder-grade” fans are noisy and barely adequate to the task, and often are vented into the attic. They often don't get used or they don't help as much as they should.

How big a fan do you need?

Ventilation fans are required by code in any bathroom that doesn't have an operable window. But window or not, you need one in every bathroom, because how often do you open the window while you're showering — especially in the winter?

Step one in selecting a fan is to determine the size you need, and you're going to find some conflicting information about this. The fan should be equal to the size of the room in cubic feet per minute (CFM) of air volume, with a minimum size of 50 CFM.

To determine the amount of air your bathroom contains, you just need to do a bit of simple math. 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). You would want to move that air at least 8 times per hour, meaning the fan has to move 5,120 cubic feet per hour (640 x 8). Divide that by 60, and you'll arrive at 85.33. So for this bathroom, you'd need a fan of somewhere between 85 and 90 CFM.

Make it quiet

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 1.0 of 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, and if yours has those, be sure and use them. Also, fill up all the mounting holes as recommended by the manufacturer.

The fan housing will have a damper control, 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.

Duct it outside

I can't emphasize this enough! 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 create mold and 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 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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