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How-To Size A Bathroom Exhaust Fan

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Try this. Close the bathroom door and take a steamy shower. Does the mirror fog up? Of course!

If you leave the bathroom door open the adjoining room will fill with moist “shower air”. Now you have two rooms filled with moist air. What’s the long term effect on your home? The short answer is: it’s not good.

You want that moisture out of the building. Here’s why bath fan sizing is important.

  • Ventilation is essential to improve indoor air quality – IAQ
  • Bugs and mold love humidity
  • Humidity can leave painted walls streaked
  • Humidity can be controlled – READ ON!
What happens when your fan isn’t sized correctly? The walls sweat.

There is a rule in the “good book” (the code book, for you 4 letter word users) that says if you have a window in the bathroom, you can open it and ventilate the room. That’s right – if you have a window in the bathroom, the good book says, the window can be a substitute for a fan.

Really? I say – no way – Jose! You need a fan.

More importantly…who on earth is going to open a window when the temp is below freezing, or even near it? It’s not happening!

Here’s how to size a bathroom exhaust fan.

Where Do You Start?

The first step is to understand that you want to measure the air movement. If you don’t measure it – you can’t control it.

You’ll want to measure the volume of air for a specific period of time. It’s called ACH or air changes per hour.

ASHRAE, the American Society of Heating, Refrigerating and Air-Conditioning Engineers sets the standard for indoor air quality. They’ve created some basic principles to follow and I’ve outlined them for you in this handy FAQ for Bathroom Exhaust Fans.

What is the industry standard air change rate for a bathroom?

When you operate the bathroom exhaust fan, air is pulled from the room through the fan to the outside at a specific rate or speed. That speed is usually measured in terms of cubic feet per minute (CFM). According to ASHRAE 62.1, the recognized standard for indoor air quality and ventilation, the standard ACH for residential bathrooms is 8. Your fan should have the capacity to change the air 8 times in one hour. 8 x 7.5 = 60 minutes.

What is the formula for rating my bathroom fan size?

Measure the height x width x length.

I’ll answer your question with a question: How big is the room?

First step – measure your room. You want to know how many cubic feet of air you’re trying to exchange.

Multiply the room’s width X the length X the height.

Example: If your room is 10 feet wide and 10 feet long and has an 8 foot tall ceiling, the total volume will be 800 cubic feet; 10 X 10 X 8. Divide 800 by 7.5 = 106.67. That means every 7.5 minutes the air in the room is replaced with fresh air.

Install a fan with a flow rate of about 107 CFM and the air volume will be change 8 times in one hour. Amazon will deliver one to your home for about $100.

Here’s another way to back into the same number. Let’s use a larger bathroom size.

Multiply the width x length x height of the room, divide by 60 (minutes in an hour) then multiply by 8 (number of air exchanges per hour).

10 X 15 X 10 ÷ 60 X 8

For example, a 10′ wide by 15′ long bathroom with a 10′ ceiling would need:

10x15x10 = 1500
1500 ÷ 60 = 25
25×8 = 200 CFM rated vent fan

Can I close my bathroom door with the exhaust fan running?

Yes, but make sure you have balanced air flow. Here’s what I mean.

This isn’t the proper way to terminate the vent pipe. See the video below. Choose a quality roof or wall vent cap.

When you operate the fan, typically air is pulled from the bathroom through a 4 inch exhaust pipe. The space under the door should be, at a minimum, the size of the opening in the pipe. What’s the area of a 4 inch pipe (circle).

The formula for area of a circle is Area = pi(radius²). 3.14 X 2²= 3.14 X 4 = 12.56.

To be able to close the door and not restrict the air flow out the pipe you’ll need 12.5 square inches of space under the door. A typical bathroom door measures 24 inches wide. 24 X .75 = 18 You’ll want to confirm you have at least a 3/4 inch gap/space (or larger) at the bottom of a 24 inch door to allow enough air to enter the room and keep the air flow balanced.

When I turn the fan on, where does the air go?

The exhaust fan pulls humid air from the bathroom and sends it through the vent pipe that’s routed above your ceiling. It works in the same way that a kitchen exhaust hood does except moist air is being moved through the pipe instead of smoke. Here in our area (Central Mississippi), almost all builders terminate the bath exhaust pipe (and the moist air) into the attic.

Wait a minute – is it proper (or smart) to dump moist humid air into the attic?

It depends. In very cold climates, the answer is a resounding NO. Humid air blown into the attic will freeze in layers on the roof and framing lumber. Later, when the temperature rises, the ice will thaw and drip water into the ceiling. In some cases mold and mildew can form on the moist wood and drywall. However – in our subtropical climate (Zone 3 – Mississippi), the humid air dissipates into the warm attic and very seldom presents a problem.

How do I terminate the pipe at the roof line?

The proper termination includes a dampered vent cap. FAMCO makes an excellent product. The 4″ dampered cap cost less than $35.00. Designed to use with bathroom or kitchen exhaust venting systems the roof mounted vent comes with:

  • a damper to prevent back draft
  • an insect screen
  • 3″ stem for flex pipe connections
  • 28 gauge pre-painted galvanized steel in black or brown
  • sizes: 4 inch, 5 inch, 6 inch, 8 inch, and 10 inch
  • 1-year warranty against product defects and workmanship
  • not intended for rooftop dryer termination.

Most bathroom exhaust fans come with a rating of between 50 CFM – 110 CFM. Larger commercial fans offer higher capacities. The rating should be listed somewhere on the packaging.

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Home Inspector – Home Builder and Building Consultant/Coach

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