Home Cleaning Tips Bathroom Ventilation: Everything You Ever Wanted and Needed to Know

Bathroom Ventilation: Everything You Ever Wanted and Needed to Know

by Catherine

I recently wrote a blog post detailing how to clean your bathroom ventilation fans properly for mold. (HERE is a link to that post on the blog.) Because ventilation, and especially bathroom ventilation (think an area with large amounts of moisture, warmth, and residue), are crucial to preventing mold growth in your home, I thought I would bring you a supplemental post to add to that one.

Today, I will share some helpful insights with you on how to pick the best fan for your bathroom and ventilation needs. To help me do that, I’ve enlisted some professional help. Let’s face it, I may know a thing or two about mold, cleaning for mold, and mold prevention, but when it comes to the intricacies of bath fans, I only really know what I have read. With that said, I would like to introduce you to Paul St. Pierre, founder of Bathfanreview.com.

A little background on Paul:

Paul is a practicing electrician who owns his own electrical business. Most recently, he founded bathfanreview.com.

Paul has been a licensed electrician for over 25 years and owns his own electrical service company.

After installing 100’s of bath fans—every make, size and model you can imagine—in 100’s of people’s homes, he felt the need to educate his customers a little more on this little-considered, but very important appliance. To deliver his experience, expertise and professional opinions to a broader audience, he started his website. On his site, he reviews various makes and models of bath fans, tells homeowner’s how to properly install them, and offers information on how to calculate type and what power of fan a bathroom requires, based on the size of the space.  Additionally, his site is easy to navigate and offers honest, easy-to-understand opinions about the latest and greatest ventilation fans on the market.

In order to bring as much of Paul’s helpful knowledge to my readers as possible, I thought I’d present this post to you in interview fashion.  I did my best to ask Paul any and all questions I could think of, in terms of bathroom ventilation and keeping moisture out of your home. If I left anything out, or if you have questions that I failed to ask, Paul’s contact email will be included at the end of this post.

Now, without further ado, here is my interview with Paul:

Me: Hi, Paul, thank you so much for agreeing to be my “in-house” bathroom fan expert. I definitely have a new respect for bath fans and their necessity in a home. Many of the interior humidity problems we were having in our “new” (new to us) home hinged on broken or poorly functioning bathroom fans. We also had to fix ventilation pathways, because both upstairs bath fans were vented directly into our attic insulation. It was a mess. So, starting with that, what do you tell or advise folks to do when looking into a home they are thinking to purchase, in terms of bathroom ventilation? Should they be going up into the attic to see where the fans vent? Will a home inspector be looking for this? If the fans don’t vent to the outside of the home, how important is having that fixed?

Paul: Hi, Catherine, and thanks for reaching out. Yes, when looking to purchase a home it is very important to make sure the bathroom fans are vented properly. This can be done by looking into the attic, or from the outside, by looking for vents or grills located on the exterior walls of the house, where the bathrooms are located. Reputable home inspectors should also be looking for this. If the fans do not vent to the outdoors, this should be fixed ASAP to prevent further moisture and mold problems.

Me: What about if you purchase a home that doesn’t have a bathroom fan? Can a fan always be installed?

Paul: Yes, a fan can always be installed. If the bathroom is located in an area where there is access to the ceiling above it, we can usually install a fan without any damage to the drywall. In cases such as a 1st floor bathroom with a 2nd floor above it, drywall repairs to the ceiling may be necessary.

Me: I learned in our debacle that the distance that the air has to travel to the outside from the fan is also an important consideration. As I understand it, if the air has to travel too great a distance, it almost renders the fan ineffective. The moisture can sit in the ductwork and cause a host of additional problems with mold and other things. How important is this to think about? How should homeowner’s calculate the ideal distance from fan to fresh air, and what things can be done when choosing a fan to help cover the distance issue?

Paul: That’s a tough one because it’s not just the distance of the duct but also how many elbows or bends there are. Ideally you would like the shortest possible distance without any elbows. When this is not possible, further calculations must be made to compensate for the duct length:

Equivalent Duct Length

To calculate the equivalent duct length, you’ll have to consider not only the length of the duct, but you’ll also have to know the duct construction, and number of elbows. The basic rules that apply to both 3″ and 4″ ducts are as follows.

  1. Measure the length of the straight duct.
  2. If the duct is flexible aluminum, multiply that length by 1.25.
  3. OR, if the duct is flexible insulated material, multiply that length by 1.5.
  4. For each elbow, add another 15 feet.
  5. For each terminal (wall or roof louvre), add 30 feet.


Given this duct:

There is 10′ of straight smooth walled duct, plus 2 elbows, and one terminal at the end.

EDL = 10′ + (2 * 15′) + (1 * 30′)
EDL = 10′ + 30′ + 30′
EDL = 10′ + 60′
EDL = 70′

To determine the proper-sized fan:

Once you’ve got all the information, it’s time to select an appropriately-sized fan. To do this, you can use a chart like this:

So, a 10’ by 10’ bathroom with 8’ ceilings (800 cu ft) and an EDL of 70’ would need a 190 cfm fan.

Me: I love how on your site, you go into an in-depth description on how to choose the right CFM (Cubic Feet per Minute) fan for your space. Could you tell my readers a little more about this and about why choosing a fan based on these specs is important? How often do you see bathroom fans installed in homes that are not adequate for the space? What happens when a bath fan that is not powerful enough is installed in a large space—does the air just not dry out? Couldn’t you just run the fan longer?

Paul: Sure, The Home Ventilating Institute recommends a minimum of 50 CFM and at least 1 CFM per square foot. If your bathroom has a ceiling higher than 8′, then you would 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). So a 10×10 room with 10′ ceilings would need a 150 CFM fan:

10x10x10 = 1000

1000 ÷ 60 = 16.66

16.66×8 = 133 CFM rated vent fan and I would round up to 150 CFM

Having the right size fan is important because you need to expel the moisture and humidity as fast as possible. Long-term exposure to excess moisture and humidity can crack and peel paint and wallpaper, ruin wallboard, warp doors and rust cabinets and fixtures.  It also creates a breeding ground for mold, mildew, and bacteria.

In over half of the homes I go to, the fans are undersized, usually because the builder or homeowner was uneducated and just bought the cheapest one. (Most people think a fan is a fan, what’s the difference?).  Running an undersized fan longer doesn’t help much, because the humidity and moisture is there too long.

Me: As you know, this blog is primarily about mold and about educating people and helping them prevent mold growth in their homes and indoor environments. I want to help people understand how important ventilation is to this picture. What can you tell my readers about bathroom ventilation as it relates to mold prevention?

Paul: Mold grows best in warm, damp, and humid areas. This is basically every bathroom environment. Venting is extremely important to prevent this growth. Without it, you will get mold.

Me: I know you read my post on cleaning your bath fans regularly for mold. How important is this? Did I leave anything out that you want to add?

Paul: I think your post on steaming all the tile and grout was great and a really good solution for treating mold. I would also suggest using your steamer with the bathroom fan on and steam right into the fan to kill any mold that may have formed in the duct work.

Me: Now, let’s get to some of the fun stuff. I’ve read on your site about some of the really cool bath fans available nowadays. Can you tell us about some of the coolest styles, features and options out there on the market? Which features are best for mold prevention? Is there a style that you gravitate towards? What brands are the most dependable?

Paul: Another tough question! I like the Panasonic Fans because they are built very well, ultra quiet, and have many options such as automatic humidity controls and timers.  They are the very best fan you can buy. On the other hand, the Broan SPK110 Sensonic™ Speaker Fan is very cool because it has a Bluetooth speaker built in and you can listen to music while you shower, and it also works very well as its 110 cfm.

This is the WhisperLite 110 CFM 0.6 Sone Ceiling Mounted, Energy Star Rated, Bath Fan with a Light and Fully Enclosed DC Motor by Panasonic. It is powerful, but quiet and even has its own light.

Here are the ads for theBroan SPK110 Sensonic™ Speaker Fan.
This fan appeals to the music lover who is also trying to control the moisture and humidity in their bathroom

Me: Thank you, thank you, for your time and knowledge. I hope my readers will check out your site.

Paul: Thank you so much!

To contact Paul directly, you can email him at  paul@bathfanreviews.com, or visit his website bathfanreview.com.


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