What Type Of Ducting For Bathroom Fan?

In old times, the bathrooms had no fans but an open window for ventilation. But nowadays, ventilation fans are an important part of the bathroom. While installing a bathroom, choose the right ducting type for venting.

You can use three types of ducting for the bathroom- rigid, flexible, and PVC ducts. A common way to duct the bathroom fan is by using rigid ducts for the main duct and a short length of flexible duct to connect the bathroom fan and the main duct. Generally, Round ducting is the ideal choice.

Venting is important for bathrooms. So, knowing the different duct types will help you choose the right one. This article will guide you about the different ductwork designs and their benefits and drawbacks. So let’s begin without further delay. 

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Flexible duct

Flexible ducts are a good way to reduce the cost of ducting. 

Flexible ducts are easy to install and have a longer lifespan. 

Most people consider using flexible ducts over rigid ones. 

Though this duct has pros and cons, the benefits overpower the drawbacks, which is why they are preferred the most. 

The building code approves flexible ducts for the bathroom. 

Chapter 15 of the International Residential Code, or IRC, Section M1504, mentions that one can use ‘flex ducts’ in their table for the allowed duct lengths.

Flex ducts are flexible ducts.

Flexible duct materials

Flexible ducts are made up of plastic, fiberglass, or metal. 

Avoid using cheap aluminum ducting materials, as they can tear easily. 

It is common for the aluminum ducts to tear at the installation time only. 

After a few years, you can find duct tearing while investigating the moisture damage in the attic. 

Find ducts will have good and protective layers on the top of the aluminum, for example, PVC-covered aluminum ducts. 

They are tear-resistant and easy to insulate and install. 

One good option is Heavy-duty AC Infinity Flexible Aluminum Ducting, heavy-duty 4-layer protection, 25 feet long for Heating and Cooling, Ventilation, and Exhaust

The product contains a 4-layer design and is more resistant to leakages, ripping, and cuts. 

You can couple this duct with a spring-loaded and rubber-sealed backdraft damper for long-lasting and quiet ventilation, for example, AC Infinity 6” Backdraft Damper with Spring-loaded Folding Blades for 6” ducting in Range Hoods and Bathroom Fans.

Duct length

The length of the flexible duct has to be around 1.5 meters or 5 feet. 

A lot of people are seen using very long flexible ducts. 

They start using it from the bathroom to the exterior walls. 

Over time, the duct will squeeze and become saggy. 

Do not use very long flexible ducts for the bathroom fan. 

If the duct becomes saggy, it prevents airflow and drops the local code requirement rates. 

Size the flexible ducts based on your bathroom ventilation requirements for enough ventilation. 

You can use the flexible duct from the bathroom to the exterior walls if the bathroom is near the exterior wall and the distance is below 5 feet. 

Benefits of flexible ducts for bathroom fans

One of the most visible benefits of flexible ducts is their flexibility. 

  • You can place the duct over a more confined space easily. 
  • You can bend the duct and follow the angles which the rigid ducts cannot follow without using joints and extra attachments. The curve of the flexible duct is better than the rigid’s sharp corner.
  • Another great benefit of using flexible ducting is its cost. Even if you buy high-quality flexible ducts, the cost will be much less than the rigid metal ducting. 
  • You can also save on the labor cost of installing flexible ducts because it is easier and takes less time. 

Drawbacks of flexible ducting

The ducting in the bathroom should carry the warm moist air out and let fresh air enter. 

It will further prevent moisture buildup, mold, and mildew. 

Despite having so many benefits, there are some disadvantages of using flexible ducts:

Increased friction

The ridges on the inner corner of the flexible ducts can cause airflow turbulence. 

This turbulence will increase so much that the air particles will crash with the duct walls and increase friction. 

Friction further increases the airflow pressure through the duct. 

The airflow pressure must be enough so that the air moving through the duct can carry the air and moisture until the air is released outside. 

The airflow pressure and duct efficiency will reduce when the friction increases. 

The inner walls of the ridges will trap and retain moisture instead of letting them out. 

The lower the airflow pressure, the easier it will be for the moisture to remain in the ducts. 

Sagging duct

A flexible duct will sag if they are not given proper support. 

The sagging will affect the smooth airflow, increase friction, and lower the pressure. 

It can also gather water which the duct should take outside.

Since flexible ducts can retain moisture, you must clear them very often, making them a high-maintenance duct. 

You can’t use range hoods.

It is also not recommended to use range hoods for the flexible ducts. 

According to the IRC chapter 5 Exhaust Systems, Section 505.3 Exhaust Ducts, the domestic cooking exhaust will discharge through the sheet metal ducts made of galvanized steel, stainless steel, and aluminum or copper. 

Such ducts have smooth inner walls.

Flexible ducts are not made of metal sheets; therefore, they do not have smooth inner walls. 

Can I use insulated flexible ducts for the bathroom fan?

Flexible ducts can be both insulated and uninsulated. 

You can either buy a pre-insulated duct or insulate it after buying. 

However, pre-insulated ducting is difficult to cut. 

So, consider the project nature before you select a duct. 

If any extra part is difficult to cut, do not keep the extra ducting part curled in the attic. 

That will reduce the efficiency of the system. 

Make a little effort to remove that extra part. 

A better option would be to buy an uninsulated PVC-covered duct. 

Install it first, then insulate it with a cotton duct wrap for better results. 

Rigid duct

Rigid ducts are also considered the best bathroom fans because of their performance in ventilation. 

The IRC has also not mentioned anything about using rigid ducts.

They have mentioned the term ‘smooth-wall ducts’ in their table for the allowable duct lengths. 

The rigid ducts are considered smooth-wall ducts. 

If your bathroom has range hoods, you should only use rigid ducts made of galvanized or stainless steel ducting. 

This ductwork is good for bathroom ventilation fans with a square or rectangular duct connection. 

The exhaust vent is also connected straight to the rigid duct. 

Benefits of rigid ducting

  • Rigid ducting is made of stainless steel metal sheets and has a smooth inner wall. 
  • The smooth inner walls create less turbulence than the rough inner walls of the flexible ducts. 
  • As a result, there is very less pressure loss inside the ducts. 
  • Since the inner walls are smooth, there is less moisture retention of buildup inside the ducts. 
  • The rigid ducts are low-maintenance, do not require frequent cleaning, and have a longer lifespan. 


Though it is considered better than flexible ducts for bathroom fans, people still prefer flexible ducts over rigid ones. 

Here are the reasons:


Rigid ducts can be very expensive compared to flexible ducts. 

Even high-quality flexible ducts are less expensive than rigid ducts. 

Besides, you need extra joints, adapters, and attachments to install them, which further increases the expenses. 

It is more expensive because installation requires special skills, which increases labor costs. 

Due to this cost, pure rigid ducting is rarely seen. 

Difficult to install

Installing a rigid duct for the bathroom fan is very difficult. 

It will require professionals. 

Moving it is more difficult if you have already installed it and later realize that you did something wrong. 

So, make sure that you have it properly installed. 

Whatever you do must be final. 

Cannot bend them

Unlike flexible ducts, you cannot bend rigid ducts. 

You must use elbows and joints to move them around obstructions and turn corners. 

The more elbows and joints you use, the more turbulence it causes, which is more than the flexible ducts’ turbulence. 


Rigid ducting will make a lot of noise than flexible ducts. 

The noise from other rooms and outside too gets transferred by these ducts. 

That is why the flexible duct is used for some feet of the duct run.

PVC ducts

Polyvinyl chloride or PVC ducts are also good for bathroom vents. 

A PVC duct is generally a PVC pipe. 

If your bathroom is close to the exterior wall, you can use round PVC ducts for the bathroom fans. 

Some bathrooms also use straight PVC pipes to vent the bathroom fans. 

However, these ducts are not seen bending. 

The connection side of these ducts also needs flexible ducts. 

So, you can use PVC ducts instead of pure rigid ducts. 

PVC ducts are cheaper than rigid ducts. 

You may see such venting procedures in most places due to the local authority. 

So, check it to confirm that you can approach this process locally. 

Can I use a rigid and flexible duct combo for bathroom fans?

A combination of rigid and flexible ducts is the most common type used for bathroom fans. 

Both residential and commercial bathrooms have this combination. 

The design is great for bathrooms located far away from the exterior walls of the building, where the distance is around 15 to 25 meters. 

If you have a long rigid duct as the primary duct for the bathroom fan, you can use 1-2 flexible ducts at shorter lengths below 1.5 meters.

It will connect the bathroom fan and the rigid duct and between the rigid duct and the external grille or vent/ louver. 

The combination is quite useful because the rigid ducts have smooth inner walls, which prevents moisture retention. 

Connecting the rigid duct directly to the bathroom fan will cause a rattling noise. 

The flexible ducts will absorb the vibration and reduce the noise. 

The exhaust fan is connected to the flexible duct. 

The flexible duct is then connected to the rigid duct. 

The grille type will be up to you, but use a plenum box as an adapter. 

It will allow flexible duct connection. 

Some people use two flexible duct connections for more exhaust airflow. 

You can use two small ducts for good results. 

The same setup is also used for public bathrooms with more exhaust fans. 

The flexible ducts are always connected to the rigid ducts with the duct collars. 

The duct collar is an adapter that connects the flexible duct with the rigid one. 

You can also use round ducts instead of square or rectangular shapes. 

Since the flexible ducts are not smooth inside and become saggy, you should only use them for distances up to 1.5 meters. 

If your bathroom is 1.5 meters away from the exterior ways, combine the rigid and flexible ducts for your bathroom fan. 

Should I insulate the ducts for the bathroom fans?

It is very important to insulate a duct. 

Otherwise, water will condense inside the duct and drip down from the bathroom fan. 

Insulating the duct will prevent condensation and water dripping. 

It will also create brown stains around the fan. 

An insulated duct also creates excessive heat loss through the bathroom fan.

You can use a cotton duct wrap to insulate your ducts.  

A good product for the cotton wrap is Frost King CF55 Foil Backed “No Itch” Natural Cotton Duct Wrap, 12-Inch x 1-inch x 15 feet.

Also, place the insulation above the bathroom fan to prevent heat loss. 

Bathroom fan duct specifications

Bathroom fans can be humid and corrosive. 

So, the rigid ducts must be made of galvanized iron or better. 

The ducts should be made following the SMACNA (The Sheet Metal and Air Conditioning Contractors’ National Association) standards. 

The wall thickness of the duct should be 0.5 mm.

Additionally, you must choose the duct size according to the bathroom and fan size. 

Check the bathroom fan’s external static pressure to make sure it has enough strength to release the air outside the house. 


The layout for the bathroom fan ducts comes in two types – the side of the house and the roof. 

Most of the time, the ducts are run on the side of the house with weatherproof grilles or vents. 

A bathroom vent system has an exhaust air vent on the ceiling. 

The air vent consists of a plenum box connected to the flexible duct, which is again connected to the rigid duct. 

Before the rigid duct joins the ventilation fan, it remains connected to the flexible duct.

The other end of the ventilation will stay connected to the flexible duct, which will again be attached to the rigid duct. 

The rigid duct will run to the outer walls of your house to meet the external vent for the exhaust air release at the side of the house. 

Add an insect screen to the external vent to prevent pests and insects from entering your house. For example, MVM 125S Metal Vent Grille By Vents-US.

Some people consider using a dryer duct for flexible ducts to duct the bathroom fans. 

For example, Omony Dryer Vent Hose 4-inch 5FT Flexible Dryer Ducting Tube Air Duct Aluminum Insulated Hose Kit Indoor Outdoor. 

If you cannot do the side layout, consider running the duct through the roof.

Cover it by some elbow or bend it to prevent rainwater from entering the duct. 

Always insulate your duct after installation. 

Duct size

The 4-inch or 6-inch ducts are the common duct sizes for bathroom fans. However, it depends on the fan capacity. 

If the fan housing has a 6-inch connection, use a 6-inch duct, even if the fan contains a 4-inch adapter. 

If the duct length is over 25 feet, install a 6-inch duct, no matter the fan size. 

Can the bathroom fan and dryer use the same outside duct?

Avoid using the same duct for the bathroom fan and the dryer. 

According to the IRC, Chapter 15, Section M1502.2 Independent Exhaust Systems, the dryer exhausts and ducts must be separate from other systems. 

So, you need to install a separate duct for your dryer. 

Some best ducts for the bathroom fan

Final thoughts

Two main types of ducts for bathroom fans are rigid and flexible. 

Flexible ducting is preferred over rigid ducting due to easy installation and low cost. 

But you can’t use flexible ducts for longer distances.

Some people prefer rigid ducting because their inner walls are smooth, which does not retain moisture and is good for longer distances. 

But installation is very difficult and costly.

For better results, you can use both ducts together. 

Another good option is PVC ducts instead of rigid ducts. 

Insulate your ducts after installation. 

You can also get pre-insulated ducts, but insulation after installation would be better. 


How far should you vent the bathroom?

It depends on the fan’s airflow rating and the duct’s diameter and type.

Can I vent a bathroom fan through the soffit?

There is generally no problem venting the fan through the soffit, especially if your roof has a few holes. 

But it is completely up to the house owner. 

Most people prefer it due to the short route, preserved roof integrity, no back-drafting, and room heat retention. 

How to choose the bathroom fan?

To choose the bathroom fan, find the CFM of the fan, locate a good area, and find a suitable fan type. 

Measure the cutout dimensions, find the right duct size, and consider the noise level. 

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Arthur Smith

Howdy! I am Arthur Smith, an electrical engineer who is extremely passionate about electronics. I have lived in different parts of the US and currently reside in Wisconsin. I am one of those rare children who knew what he wanted to study and do in life right from the start. I was a curious child who wanted to know how switches work and how the AC works, and I would always observe my dad whenever he would handle the wires and fix things around the house. I currently work as an electrical engineer at a reputed company and write for this blog. And I read loads of books or play video games in my free time.

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