What Kind of Duct to Use for Bathroom Fan


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Ventilation in a bathroom is vital, especially for bathrooms with no external windows, but the installation of bathroom fans is not just for odor control. There are laws regarding the airflow in confined spaces and moisture control in bathrooms, and the ducting used for the bathroom fans is similarly subject to building codes. Furthermore, of the allowable ducting, each has its own set of benefits and drawbacks. 

Flexible duct is easy to install and has a long lifespan, it is a common choice for bathroom fan ducting. A rigid duct will move air more efficiently and can be used over longer distances, but it is costly and hard to install. Insulated ducting (flexible or rigid) is always better than uninsulated.

There are two options for when it comes to bathroom fan ducts: flexible and rigid. We look at both options in this article. We also look at some other duct-related decisions you will have to make when installing your bathroom fans, including insulated or uninsulated, 4” or 6”, where to vent, and what fan to choose. 

Flexible Duct for Bathroom Fan

AC Infinity Flexible 6-Inch Aluminum Ducting, Heavy-Duty Four-Layer Protection, 25-Feet Long for Heating Cooling Ventilation and Exhaust

Flexible duct is perfectly acceptable for a bathroom fan. 

Every type of duct is going to have benefits and drawbacks (the pros and cons for flexible duct are discussed further down). In the case of flexible ducting, the benefits are believed to outweigh the drawbacks in most situations, which is why flexible duct is so commonly used. 

Now, it’s all well and good to say that flexible duct will do the job, but what does the building code say?

Flexible Duct for Bathroom Fan Is Approved by Building Codes

Chapter 15 of the International Residential Code (IRC) “is specific to exhaust systems related to clothes dryers, domestic cooking, toilet rooms, bathrooms and whole-house ventilation systems.”

Section M1504 discusses exhaust ducts and exhaust openings. While there is no line stating that you can use flexible duct, they include “flex duct” as a duct type in their table of allowable duct lengths. 

Flexible Duct Materials

The flexible ducts can be made from fiberglass, plastic, or metal. However, you should try to avoid cheap aluminum ducting because it tears very easily.

In fact, it’s very common for the thin aluminum duct to be damaged during installation. This is often discovered only when investigating the source of moisture damage in the attic years later.

Look for something that has a protective layer on top of the aluminum. PVC-covered aluminum flexible ducts are drastically more tear-resistant and are much easier to insulate and install since you don’t have to be so careful.

AC Infinity Flexible 10-Inch Aluminum Ducting, Heavy-Duty Four-Layer Protection, 25-Feet Long for Heating Cooling Ventilation and Exhaust

If you do wish to use aluminum ducting, then I recommend heavy-duty AC Infinity Flexible Aluminum Ducting (Amazon link). It is reinforced with a four-layer design and is thus much more “resistant to leaks, rips, and cuts”.

AC infinity makes excellent residential ventilation products. Couple this heavy-duty duct with a quality spring-loaded rubber-sealed backdraft damper (pictured below) and you have got yourself a long-lasting efficient ventilation solution.

Here is an helpful guide if you would like to learn more about ventilation dampers.

AC Infinity 6" Backdraft Damper, One-Way Airflow Ducting Insert with Spring-Loaded Folding Blades for 6” Ducting in Range Hoods and Bathrooms Fans
backdraft damper

Benefits of Flexible Duct for Bathroom Fan

The most obvious benefit of flexible ducting is its flexibility. How flexibility is beneficial for bathroom fan ducts:

  • It allows the ducts to be placed into more confined spaces with ease.
  • It will enable the duct to bend and follow angles that rigid ducting could not do without the use of joins and additional attachments. Bends are never desirable in ducting, but they are often unavoidable, and the curve of a flexible duct is better than the sharp corner of a rigid duct.

The most alluring aspect of flexible ducting for a bathroom fan, however, is probably the cost. Even the higher quality flexible ducting is significantly less expensive than rigid metal ducting. 

You save further on labor costs because it is easier and therefore cheaper to install, and you don’t have to buy the adaptors and joints required for rigid ducts. 

Drawbacks of Flexible Duct for Bathroom Fan

The purpose of exhaust ducting in a bathroom is to carry the warm and moist air out of the bathroom and release it outside. This prevents moisture from building up in the bathroom structures and surfaces and causing rot, mold, mildew, etc. 

There are two factors whereby flexible ducting is not the most effective type of ducting for this purpose. 

Increased Friction

The ridges found on the inner walls of flexible ducts create airflow turbulence. Turbulence increases the rate at which air particles collide with the walls of the ducts, thereby increasing friction. 

Friction is one of the key factors influencing airflow pressure through a duct. Duct pressure needs to be sufficient to keep the air and the moisture carried by the air moving through the duct until it is released outside. 

When friction increases, it causes airflow pressure loss, and the efficiency of the duct is reduced.

Furthermore, the inner wall ridges can catch and hold moisture, preventing it from being taken outside, and the lower the airflow pressure, the easier it is for moisture to be deposited in the ducts.

While a flexible duct will increase friction, it’s generally nothing to be worried about. The performance difference compared to rigid ducts is negligible. Especially when using the AC infinity duct(it has a smoother inside compared to others)

Sagging Duct

Secondly, flexible ducting can sag if it is not adequately supported. The resultant sagging points further impede smooth airflow, increase friction, cause pressure losses, and can collect water that is supposed to be taken outside. 

The fact that moisture can build up in flexible ducts means that they require cleaning more often, making them relatively high maintenance. 

It is also worth mention here that range hoods are not allowed to be used with flexible ducting. 

This stipulation can be found in the IRC chapter 5 Exhaust Systems, section 505.3 Exhaust ducts:

“Domestic cooking exhaust equipment shall discharge to the outdoors through sheet metal ducts constructed of galvanized steel, stainless steel, aluminum, or copper. Such ducts shall have smooth inner walls […].”

Flexible ducts are not made from sheet metal, and they do not have smooth inner walls. 

Can You Use Insulated Flexible Duct for Bathroom Fan?

You can either buy flexible ducting and then insulate it yourself, or you can buy flexible ducting that has already been insulated. You definitely can use the latter for your bathroom fan.

However, pre-insulated ducting is often more difficult to cut, so you should consider the nature of your project carefully before making your purchase. Never leave the extra duct laying curled up in the attic, as this will drastically decrease the efficiency of the entire system.

I recommend using an uninsulated PVC-covered duct (like the one I recommended above) and then insulating it using a cotton duct wrap (amazon link). This will give a better end result.

Frost King CF55 Foil Backed "No Itch" Natural Cotton Duct Wrap, 12-Inch x 1-Inch x 15-Feet
Cotton wrap for duct insulation

No matter if you choose a pre-insulated duct, or you will insulate it yourself. Insulating the duct is very important. Water will condense inside a non-insulated duct and eventually it will start dripping down from the bathroom fan. Not only is the dripping annoying, but it will also cause brown stains around the fan.

Rigid Duct for Bathroom Fan

Rigid ducts are the best choice for a bathroom fan in terms of performing the purpose of venting air and moisture from the bathroom. Then why would you ever use a flexible duct if rigid is the best? Well, there are two main drawbacks of rigid ducts, which we will discuss shortly.

First, let’s make sure that rigid ducting is legal to use for bathroom fans. 

Rigid Duct for Bathroom Fan Is Approved by Building Codes

As with flexible duct, there is no line in Section M1504 stating that you can’t use rigid ducting for bathroom fans. However, they include “smooth-wall duct” as a duct type in their table of allowable duct lengths, and smooth-wall duct refers to rigid ducting.

Additionally, as mentioned previously, if you are using a range hood as your bathroom fan, then you are only allowed to use rigid galvanized or stainless steel ducting.

Benefits of Rigid Duct for Bathroom Fan

Rigid ducting is made from sheets of stainless steel metal, and so it has smooth inner walls. The smooth walls create much less turbulence than the ridged walls of flexible ducting, so there is less pressure lost in rigid ducts. This makes rigid ducting the most effective choice for a bathroom fan duct.

There are also fewer places where moisture from the air can collect and build up in the ducts. This is only a problem by joins and elbows.

Rigid ducts have a long lifespan and are low maintenance—they only require the occasional cleaning. 

Drawbacks of Rigid Duct for Bathroom Fan

Expensive

Rigid ducting is more expensive to purchase, and if you are having it installed professionally, then the labor is also more expensive because it requires more specialized skills. 

Difficult to Install

If you are going to try to install it yourself, you should be aware that it is quite challenging to do so. Additionally, once it is installed, moving it is not simple, so you have to make sure that it is correctly installed the first time.

Rigid bathroom fan ducts can’t just bend or be placed around obstacles. You will have to insert elbows and joins to move around obstacles and turn corners. The more elbows and joins, the more turbulence is created. 

Noise

A ventilation system exclusively made out of rigid ducts is going to be noisier when compared to flexible ducts. The noise from other rooms and even from outside is transferred by the continuous metal duct. This is why in most cases the last few feet of the duct run is made with a flexible duct.

The expense and the difficult installation are the two main drawbacks of rigid ducting, but if neither of these things deters you, then it is probably your best option. 

Should I Use Insulated Duct for Bathroom Fan?

Bathroom fan ducts should be insulated. Among the many reasons why you should insulate your bathroom exhaust ducts is that it prevents condensation from forming in the duct. This is one of the causes of a dripping bathroom fan. You can insulate any duct. We recommend using cotton duct wrap (amazon link).

Don’t forget to place insulation above the fan to prevent heat loss.

4” or 6” Duct for Bathroom Fan

Duct size depends on the bathroom fan capacity. Installing a duct as big or bigger than the fan port is OK. Avoid using adapters. If the fan housing has a 6” connection, always use a 6” duct even if the fan comes with a 4” adapter.

If the duct length is over 25 ft, install 6” regardless of the fan. 

How Far Can I Vent a Bathroom Fan?

The distance that you can vent a bathroom fan depends on the fan’s airflow rating, the duct diameter, and whether the duct is flexible or rigid.

You can check out my easy-to-use tables on duct lengths, which are based on the information provided by the IRC, to ensure that you never vent further than is allowable by the building codes.

Is It OK to Vent a Bathroom Fan Through The Soffit?

While the final decision depends on each individual house, it is typically OK to vent a bathroom fan through the soffit. If your roof already has a few holes through it, the soffit might be your best option. 

The main benefits to venting a bathroom fan through the soffit are:

  1. It is often the shortest route.
  2. You preserve the integrity of your roof.
  3. You eliminate the chance of back-drafting.
  4. It can help with room heat preservation.

In order to make venting your bathroom fan through the soffit a viable choice, you have to choose the correct duct size, duct material, duct slope, ducting route, and insulation.

The installation has to be done correctly to prevent the warm and moist air from flowing back into the attic through the soffit. Additionally, venting should only ever be done through the soffit and never into it. 

Is venting a bathroom fan through a soffit sometimes a bad idea?

You should never vent a bathroom fan within 10 ft of a soffit air intake point. This defeats the purpose of venting the warm and moist air outside because it can easily be drawn back into the attic.

If you are seriously thinking about venting your bathroom fan through the soffit, you should first read this article that explains in detail the factors you need to consider when venting through the soffit

Does a Bathroom Fan Need to Vent Outside?

This is a common question, with a simple answer: A bathroom fan must always vent to the outside.

The purpose of bathroom fans is to remove moisture and bad odors from inside the bathroom so that mold and mildew do not grow. However, mold and mildew are not just ugly to look at; they are also hazardous to your health. If you vent a bathroom fan into your attic, crawlspace, soffit, etc., then you are just moving the risk of mold and mildew from your bathroom to that area.

In fact, it is such an essential condition of venting that it is stipulated in most building codes, including the IRC. In chapter 15, section M1505.2 Recirculation of air, it says:

“Exhaust air from bathrooms and toilet rooms shall not be recirculated within a residence or circulated to another dwelling unit and shall be exhausted directly to the outdoors.”

Ductless bathroom fans are available on the market, but they will only create airflow in the room; they are incapable of removing moisture, which is the primary purpose of a bathroom fan. 

This article on Does A Bathroom Fan Need to Vent Outside provides more information and also lists a few functional alternatives for bathroom ventilation if you are not able to vent your fan outside. 

How to Choose a Bathroom Fan

There is no shortage of bathroom fan options on the market today, and trying to choose the correct one for your bathroom can become rather overwhelming. To guide you through making your decision easier, I have written an article on the 6 easy steps to find the right bathroom exhaust fan

Here is a summary of the 6 helpful steps:

  1. Find the optimal CFM for the bathroom fan. CFM refers to cubic feet per minute, and it represents the airflow rate through the fan. If you are not sure what CFM your fan should have, check out the calculator in the article linked above.
  2. Choose an optimal location. 
  3. Choose a type of bathroom fan (ceiling mounted, wall-mounted, or inline).
  4. Measure cutout dimensions. This refers to the actual hole you are going to have in the ceiling or wall for your fan.
  5. Find out the duct size.
  6. Consider the noise level. 

Then, once you have chosen your fan, you will need to decide how you would like it to be operated (unless you have a continuous ventilation bathroom fan). You could go with a physical on/off switch, a timer switch, or a humidity sensor switch.

Can a Bathroom Fan and Dryer Share the Same Outside Duct?

A bathroom fan and dryer cannot share the same outside duct. According to IRC’s chapter 15, section M1502.2 Independent exhaust systems:

“Dryer exhaust systems shall be independent of all other systems and shall convey the moisture to the outdoors.”

So, unfortunately, you will have to vent your dryer and your bathroom fan separately. To learn more about why this stipulation exists, check out my article on Can Bathroom Fan and Dryer Share a Vent.

Related article: Can Bathroom Fan Be Vented Using PVC Pipe

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Joonas

I like it when I'm able to fix everything that needs fixing around the house. In order to do that, I have to do a lot of research. This site will cover everything I learn and maybe help others do the same.

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