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Sunday, October 17 2021
Bathroom Fan Venting Tips & Guidance



Energy auditor and window specials

Written by Eric Gans

I have completed over 2000 home energy audits in Maryland.  BGE and Pepco standards require me to measure exhaust ventilation rates and determine where each exhaust fan in a residential house terminates.  I must also manage new fan installs and airflow at the end of home performance projects to maintain indoor air quality.  So, I am around this stuff every day.

Did you know that your bathroom exhaust fan could be the number one reason for your high energy bills and lack of comfort?  

Finding the right guidance can be overwhelming.

Read this article to avoid several pitfalls that could come along with a replacement exhaust fan project. 

You will get answers to the following questions:

----  5 ways to get a head start on your bathroom fan venting project?
----  How can a bathroom exhaust fan waste energy?
----  Should your bathroom exhaust fan vent to the attic or outside?
----  How do you properly vent a new bathroom exhaust fan through the roof?
----  What bathroom exhaust fan model provides good performance?


5 Bath Fan Things You Need to Know

Do Bath Fans Need Love?

  1. Most exhaust fans installed in Maryland homes vent moisture directly into the attic
           --This causes mold
           --This accelerates the age of the roof
           --This degrades existing insulation
  2. Bathroom exhaust fan ducts running through an attic can "sweat" during a Maryland winter if not insulated properly
  3. Different fan models exhaust different amounts of air per minute
  4. If an exhaust fan is making noise, that does not mean it is working right
  5. Incorrectly installed fans can act as gateways to the attic, increasing BGE and Pepco energy bills

A bathroom exhaust fan is essential for indoor air quality and should be installed correctly.



Bathroom Exhaust Fans are Major Energy Waste Offenders

Stack Effect - A Building Science Principle

bathroom exhaust fan replacementThe stack effect is a building science principle that says your house acts like a chimney in winter. When a chimney is in use, the fire is at the base, pulling in oxygen to keep the fire alive (negative pressure). As it heats, the hot smoke rises and billows out of the opening at the top (positive pressure).

You do not have to have a chimney in your house for the stack effect to work against you.

When the heat is on during cold days, the warmer air you're paying for rises like the heat and smoke in the chimney.  The smoke leaves at the opening at the top of the chimney, and so does your precious warm air - at the ceilings below any unsealed attic space.

When I mention this to people during audits, I can see the sparks in the brain saying, "Where is my air going?  I don't see any holes in my ceiling.  What do you mean sealed?"  

For starters, the bathroom exhaust fan is an excellent example of a location where this happens.


This older bath fan in the attic does not have a duct running to the "outdoors."  It also lacks a damper, which creates a gateway between the inside and outside of the house.


Look below at this diagram of a modern high-performance ceiling-mounted exhaust fan. 


bathroom exhaust fan installation







Notice where the air flows from the fan in the photo above.  There is a little flap there known as a damper.  When the fan is not in use, the damper closes to prevent air communication between the two parts of the home (bathroom and the attic). 

If the damper is missing or not working properly, the fan becomes a gateway to the attic, allowing the stack effect to overwork your heating system. This leads to a shorter life, high bills, and comfort issues.


bath fan installation price

This IR photo shows a leaky damper.  As a result, this exhaust fan is a liability and a big communication point between the attic and the inside of the house.

I have one tip for you. Those who report cold bathrooms in the winter typically have an issue with the damper, so you don't need an expensive IR camera to figure this one out!



Is it Okay if Your Bathroom Exhaust Vents Directly Into the Attic?

Your Home is a System of Many Working Parts

Most Maryland housing stock has bathrooms with exhaust fans installed that vent moisture directly into the attic.

As the photo below shows, leaving exhaust fans unchecked for long periods can lead to more significant sustainability-related problems. 

bathroom fan replacement
The upstairs bath fans in this attic have ducts connected to the actual fan; however, they are not properly vented to the "outdoors."  The builder ran the duct to the ridge of the roof. but stopped short of sending them to the "outdoors." The result is compromised roof sheathing.

If you are experiencing comfort problems in your home, addressing the ventilation could help ensure warmer winters and cooler summers.

For example, in summer, extra humidity can build up inside your home when moisture from the shower cannot escape due to a poorly performing exhaust fan.  High humidity levels in summer lead to less comfort and the tendency to create the urge to lower the thermostat.  Controlling humidity in summer can profoundly affect your comfort and energy bills.

The extra humidity injected into the already super-heated attic ends up, in most cases, right back in the house, creating a cycle that does not help to improve the entire system within the home. So, the result is less comfort and higher bills.

Properly venting bathroom exhaust fans to the "outdoors" is not a high-cost improvement, and it can be done in a few different ways.


Where is the Best Place to Vent a Bathroom Exhaust Fan?

3 Ways to "Term" a Bathroom Exhaust Fan Duct

1. Through the roof (best)
2. Through the gable wall (preferred over soffit)
3. Through the soffit overhang (least desired)

To evaluate which is best for your home, consider the fan's location and which option creates the shortest path with the fewest minor obstacles for the duct. Sometimes, the style of the house or its features will require special techniques that will weigh into the decision. Cost-effectiveness should also be factored into the overall decision. 

Shorter duct lengths will help the fan pull the listed air volume from the space.  For longer duct runs that are unavoidable, it is recommended that flexible ducts are pulled nice and tight.

The shortest point to the "outdoors" is often through the roof.

proper bathroom fan venting

Suppose you are unsure if your fan vents to the outside. One easy way to find out is to look at your roof. If you see vents like the ones below, there is a good chance your exhaust is venting correctly.

There are three steps in venting a bathroom exhaust fan through the roof for the first time.

1. Using a drill and the proper hole cutter, drill a 4" diameter hole in the roof sheathing (plywood) from the attic.
2. Get up on the roof, prep the area, remove the top layer of shingles for the new fitting, seal the new fitting with roofing cement, and secure the roof jack and the shingles around the disturbed area.
3. Connect the fan to the fitting from inside the attic.


Watch how it is done in 2 minutes...

If venting your bathroom exhaust fan through the roof is not an option, then going through the exterior gable wall is the next best option.

Watch how it is done in less than one minute...




Which Bathroom Exhaust Fan does Hometrust Recommend?

With So Many Fans Out There - Which One Should You Get?

Much like most other home improvement projects, many details go into installing a new bathroom exhaust fan properly.  One crucial step is to seal around the fan once installed.  This will help improve energy efficiency and comfort by ensuring that the gap around the fan does not allow air to flow between the bathroom and the attic.

Watch us air-sealing a newly installed bathroom exhaust fan during a home performance project in Maryland.

Despite the installation nuances, the fan must remove the moisture effectively and efficiently. The Panasonic Whisperlite has several models that are up for the challenge. A flawless installation can adjust the fans to pull between 50 and 110 cfm (cubic feet per minute). The minimum recommended setting for a bathroom with a shower/tub is 50 cfm.

If the fan in your bathroom has been confirmed to pull air, but the mirror is still fogging up, the CFM may not be high enough to do the job.

Watch a properly installed and vented Panasonic Whisperlite get the job done in this Laurel, MD bathroom.

In addition to a fan that gets out the moisture, our customers seem excited when reporting that the Panasonic Whisperlite is much quieter than their previous bathroom exhaust fan.

Another advantage to the Panasonic Whisperlite is that it is versatile and works well with outside components like moisture sensors and third-party switches. 

Did you know bathroom exhaust fans can also improve indoor air quality? With specialty switches and timers, properly installed exhaust fans make for a healthier bathroom, and bath fans can also improve a Maryland home's overall indoor air quality.





Not interested in poking around in your attic?  

Get a home energy audit & get questions answered such as:

Is my current exhaust fan working?
Does my home have adequate ventilation?
Should I use blown-in insulation or rolled insulation?

How much insulation do I need in my attic?
How do I properly vent my crawl space?
Should I remove old insulation from my attic?
What is the best way to seal my crawl space?
Is spray foam insulation the best solution for my home?
Why are my new windows feeling drafty?
Should I insulate the walls in my home?
What areas should I seal to reduce drafts?


For only $100 through the Home Performance with ENERGY STAR Program - I am qualified to get all of the answers for you!

maryland attic insulation expert

Questions are welcomed and I would really love to know if this article has helped you or if there is any suggestions you may have to simplify the process.



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