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A bathroom exhaust fan is an inexpensive upgrade that packs a value punch. The shoe-box-size fan clears obnoxious bathroom odors (priceless!) and removes moisture, which protects your home and health, and reduces maintenance costs.

And, it turns out, everyone wants one. Exhaust fans are the No. 1 feature homebuyers want in a bathroom, says a National Association of Home Builders report. Ninety percent ranked exhaust fan as No. 1, with linen closet second, and a separate tub and shower as third. Who knew?

Still, many homes don’t have a bathroom fan. Although the fans are required by building code in many places, older homes — pre-1960s — didn’t routinely install them. And homeowners today may be reluctant to retrofit bathrooms with an appliance that requires venting to snake through attics, joists, soffits, and ultimately punctures an exterior wall or roof.

We feel your fear, and we’re here to help. Below, we break down everything you need to know about selecting and installing a bathroom exhaust fan.

What Does a Bathroom Exhaust Fan Do?

A bathroom exhaust fan is a small, ceiling- or wall-mounted fan that pulls air from the bathroom, sends it through venting (4-inch is preferable), and deposits it outside.

This helps you and your home by:

  • Improving indoor air quality, especially by removing bathroom smells.
  • Removing shower and bath humidity.
  • De-fogging mirrors.
  • Thwarting mold growth.
  • Preventing door and window warp.
  • Slowing fixture rust.
  • Retarding paint blister and wallpaper peel.

Related: How To Kill and Prevent Household Mold

How Are Fans Rated?

Exhaust fans are measured by two factors found on the fan’s box:

  • CFM (cubic feet per minute): Indicates the strength of the fan’s draw. CFM’s can range from 50 to 1,000-plus, although most bathrooms typically require fans with less than 200 CFM.
  • Sone: Measures of the sound the fan makes, typically from 0.5 (almost silent) to 4.0 (sounds like a normal television) — loud for a fan, but it does provide privacy against toilet sounds, especially nice for powder rooms often located near public areas of your home.

Most people choose a 1- or 2-sone fan — quiet enough keep your teeth from rattling, but not so quiet that you’ll forget it’s on.

CFM and sone are related, because stronger fans — with higher CFMs — usually create more noise; quieter fans — lower sone — often can’t adequately clear air from bigger areas.

The important thing is to pick a fan that’s right for your space, ears, and budget.

Sizing Your Fan

The Home Ventilating Institute, which tests and certifies manufacture claims, suggests that homeowners follow these formulas when sizing a fan:

For bathrooms less than 100 sq. ft.: Calculate your bathroom’s square footage (length x width), and pick a fan with at least that number of CFMs. For example: If your bathroom is 6 feet by 8 feet, you should buy a fan that’s at least 48 CFM. A 50-CFM model comes closest and is the minimum size suggested for small bathrooms.

Size a fan for a ginormous bathroom: If your bathroom is bigger than 100 sq. ft., forget about the square footage figure; instead assign a CFM capacity for each fixture:

  • 50 CFM — toilet.
  • 50 CFM —  bathtub.
  • 100 CFM — jetted whirlpool tub.
  • 50 CFM — shower.

If you have a completely tricked-out bathroom, you may need at least 200 CFM of draw, which you can accomplish with several 50-CFM fans (one fan should be in separate toilet enclosure), or one big, 200-CFM fan.

How To Install Your Fan

Bathroom fan installation isn’t brain surgery — collect air here; exhaust air out there. But it’s not for beginners either, because the project includes removing drywall, perhaps drilling through joists, certainly busting through an exterior wall or roof.

We suggest hiring an HVAC pro, who will charge $150-$700.

If you decide to install a fan yourself, here are some decisions you’ll have to make:

Location: If you have a separate WC, put a small fan there. If your toilet is part of the bathroom, locate the fan between the toilet and tub/shower.

Venting: Exhaust flows through venting attached to the fan and out an exterior wall or roof. Never vent smelly, damp air into an attic or crawl space, which will warp rafters and promote mold growth.

The idea is to run venting the shortest, straightest path from the bathroom to outside. Every extra foot and bend the venting makes increases friction and decreases air draw and fan efficiency.

Appropriate venting runs up into your attic, then along or through floor joists until it reaches the eaves. From there, it can be exhausted out a soffit.

In some instances it may be more practical (and less expensive) to run the vent directly out a wall, or through a vent stack in your roof.

Door clearance: During installation, make sure your bathroom door has at least ¾-inch clearance from the floor, so “makeup air” can easily replace the sucked-out air, putting less stress on the fan.

Related: A Replacement Fan That’s Easy to Install

Fan Options

Bathroom exhaust fans come in custom styles and colors, but most of us would rather spend our decor budget elsewhere and will choose an off-the-rack fan with one or more of the following options:

Fan only: If you’re retrofitting a small bathroom that already has a ceiling fixture, select a basic fan, 50-70 CFM. Cost: $15-$50.

Fan-and-light combo: Good for small bathrooms or WCs. Choose a combo with enough wattage to sufficiently light the area, typically upwards of 60 watts. Cost: $30-$150.

Deluxe combo: All the bells and whistles — fan, light, heater, nightlight, timer (necessary for super-quiet fans you won’t remember are on), humidistat (automatically turns on fan when air moisture rises). Cost: $150-$600.

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