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Eric Gans
Certified Energy Auditor


Residential Comfort & Energy Efficiency

Sunday, October 17 2021
Bath Fan Replacement and Proper Venting (with Videos)



Energy auditor and window specials

Written by Eric Gans

I have completed over 1500 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 am also required to manage new fan installs and air flow at the end of home performance projects in order to maintain proper indoor air quality.  So, I am around this stuff every day.

Did you know that your bathroom exhaust fan is basically a pathway from inside your house to the outside?  If it is not managed properly it can become a source of problems ranging from fogged mirrors and high indoor humidity in the summer to a drafty house in winter.  

Finding the right guidance can be overwhelming.

In this article, I want to help you avoid several pitfalls that could come along with a replacement exhaust fan project. 

You will get answers to the following questions:

----  What are the 5 most important bath exhaust fan details that will give you a good head start?
----  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 Actually 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 important 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 basically says that your house acts like a chimney in the winter months.  When a chimney is in use, the fire is at the base pulling in oxygen to keep the fire alive (negative pressure) and 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 be in play inside the building you live.

When the heat is on during cold days, the warmer air that 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 a great example of a location where this happens.


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


Take a look below at this diagram of a modern high performance ceiling mounted exhaust fan. 

bathroom exhaust fan installation






Notice where the air is flowing out of 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 the communication of air 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 which leads to a shorter life, high bills and comfort issues.

bath fan installation price

This IR photos 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.

One tip that I can give you.  I typically find that those that report really 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

The majority of the Maryland housing stock have bathrooms with exhaust fans installed that vent moisture directly into the attic.

As you can see in the photo below, leaving exhaust fans unchecked for long periods of time can lead to bigger problems related to sustainability. 

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 simply 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 then addressing the ventilation could go a long way towards warmer winters and cooler summers.

For example, in summer extra humidity can build up inside your home when moisture from the shower is unable to 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 have a profound effect on your comfort and energy bill.

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

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


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)

The way to evaluate which is best for your home is to consider the location of the fan and which option creates the shortest path with the least obstacles for the duct to take. Sometimes the style of house or the features in a house 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 volume of air from the space.  The shorter the duct run the more effective the fan will be.  For longer duct runs that are unavoidable, it is recommended that flexible ducts are pulled nice and tight.

In many cases, the shortest point to the "outdoors" is through the roof.

proper bathroom fan venting

If you are not sure if your fan vents to the outside, one easy way to see is to take a look at your roof to see if you have any vents that look like the ones below.  If you see these, there is a good chance your exhaust is venting properly.

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

1. Drill a 4" diameter hole in the roof sheathing (plywood) from the attic with a drill and the proper hole cutter.
2. Get up on the roof, prep the area, remove the top layer of shingles for the new fitting, seal the new fitting in place with roofing cement, 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 for some reason, 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, there are many details that go into proper installation of a new bathroom exhaust fan.  One important step is to seal around the fan once it has been installed.  This will help to improve energy efficiency and comfort by making sure that 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 all of the installation nuances, at the end of the day it is important for the fan to remove the moisture effectively and efficiently.  The Panasonic Whisperlite has several models that are up for the challenge.  With a flawless installation, the fans can be adjusted to pull between 50-110 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 be pulling air, but the mirror is still fogging up then it may be that the cfm is not high enough to get the job done.

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

In addition to a fan that really gets out the moisture, our customers seem to get really 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 be used to improve indoor air quality?  With speciality switches and timers, properly installed exhaust fans not only make for a more healthy bathroom, but bath fans can also improve a Maryland home's overall indoor air quality.




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