4 Or 6 Duct For Bathroom Fan? Which One Should You Choose?

Ductwork is a crucial part of your ventilation system. The ducts are responsible for moving air from one area to another and can be an important factor in making sure your home’s ventilation system works efficiently. 

Ductwork isn’t just limited to carrying air; it also carries water vapor or steam from radiators and clothes dryers. 

There are four main types of ductwork such as supply, return, branch and exhaust. Supply ducts take fresh air into the house from outside; return ducts go outside after warming up inside. 

Branch vents transfer warm moist air from rooms with radiators or clothes dryers to other house parts, and exhaust ducts carry stale indoor air outdoors through a roof vent.

4 or 6 Duct for Bathroom Fan

The size of a duct is determined by the bathroom fan capacity. It’s OK to have a duct that is as long or longer than the fan opening.

Always use a 6″ duct if the fan housing has a 6″ connection, even if the fan comes with a 4″ adapter. Install a 6″ duct regardless of the fan’s size if the length is more than 25 ft.

Bathroom Exhaust Fan Duct Size

Most bathroom fans are available in four-inch (4″) and six-inch (6″) diameters. Fans for larger rooms require stronger motors and bigger ducts.

Increasing duct diameter allows for longer ducting at a constant fan strength. Increasing fan strength lowers the permitted duct length at a constant diameter.

A bathroom fan duct is used to take the air out of the bathroom. The duct needs to be the right size so that there is enough pressure to get air from one room to the next. The length and width of the ducts can change the pressure system.

How Ducts Conduct Bathroom Air to Outside

When you install a bathroom exhaust fan and duct system, the space is actively exhausted.

Because of this, when you close the door to your bathroom, all of the air in the house is pushed toward one area. The negative pressure gradient causes air to move from the bathroom to the outdoors.

The fan’s rotation creates a high-pressure zone behind it, drawing air into the duct.

Because the air outside is not restricted to a specific volume, its pressure is lower than the air in the duct. Furthermore, as the distance from the fan grows, the duct pressure decreases.

Ambient air is sucked into the fan from the highest-pressure region, which causes relatively low pressure behind it. A series of increasingly lower pressures thus replace the high-pressure area, and pollutants are carried with it as they move through each progressively lower pressure zone toward the outside.

For fixed duct dimensions, the more powerful the fan, the greater the pressure produced and the faster the air travels along with it. This is why bathroom fans are rated in CFM.

How Does Duct Diameter Influence the Airflow?

The greater the duct’s width, the more efficient the ventilation system is in terms of duct diameter. This is because a larger number of air currents flowing through the duct will not contact the duct’s walls.

The narrower the duct, the greater is its resistance to airflow and airflow rate. Furthermore, utilizing a duct that is too small generates a significant amount of noise.

However, you must think about how much ducting space you have since there is no use in using a wide duct if you can’t fit it in the walls.

4″ or 6″ Duct for Bathroom Fan

The most common duct diameters for bathroom exhaust fans are 4″ and 6,” and there’s a reason for that.

The first thing to consider is that the size of the bathroom determines how large a fan you’ll need.

On the other hand, the fan’s capacity determines the proper duct diameter because it affects the pressure developed in the duct.

Most common bathroom fan capacity needs can be met with 4″ to 6″ ducts.

Furthermore, because the most common diameter for bathroom fan ducts is 4″ or 6″, most bath fans are constructed with 4″ or 6″ connection points.

It’s also relatively simple to determine the appropriate size duct; all you have to do is look at the connector.

Some fans come with an adaptor for a different size duct, but you should never go smaller than necessary. So, if your fan has a 6″ connection and a 4″ adaptor, go with a 6″ duct.

5″-diameter ducts are available, but they’re not necessary. This is because a bathroom fan that demands a duct diameter greater than 4″ will work better with a 6″ diameter duct than a 5″ one and 6″ diameter ducts are more readily accessible.

If you think your bathroom is larger than usual, use the calculator linked above because you’ll need a stronger fan and larger duct.

Bathroom Duct Sizing Table

If the exhaust is intermittent, the minimum mechanical exhaust capacity for bathrooms and toilet rooms is 50 cubic feet per minute (CFM). This is why we omitted CFMs less than this in our tables.

However, you should verify your state and local laws, as they may require a higher requirement.

For example, if a toilet is included in the bathroom, Oregon’s Code requires that it must be 80 CFM minimum. However, if it is simply a toilet and does not contain a bath facility, 50 CFM is enough.

Bathroom Fan Capacity (CFM)

The ideal duct diameter for your home is based on the type of heating system you have. Although these are the diameters we recommend, the manufacturer’s recommendations should always be followed as a first choice. If they do not specify the correct duct diameter, you can consult our table to find out.

How Far Can a Bathroom Fan Vent Run?

The following conditions determine the length of your bathroom exhaust fan duct:

  • The type of duct you’re using
  • The combination of fan airflow rating and duct diameter
  • The number of elbows used

The regulations that govern duct length are set to ensure that the natural drop in air pressure that occurs along the ducting does not exceed the permitted air pressure drop.

If you exceed the size, the air will not move through. And then ventilation quality will be bad.

You should have a duct that is the same size as the fan. This will help you to have a better experience with your fan. If you don’t, it will be frustratingly loud.

What Kind of Duct to Use for Bathroom Fan

The flexible duct is simple to install and has a long lifespan, making it a popular option for bathroom fan ducting.

A rigid duct will move air more effectively and over greater distances, but it is expensive and difficult to install. Insulated ducting (flexible or rigid) is always preferable to uninsulated.

There are two options when it comes to bathroom fan ducts. They include:

  • Flexible 
  • Rigid

Flexible Duct

It’s fine to use a flexible duct for a bathroom fan. However, each type of duct has its pros and cons. In most cases, the advantages of flexible ducting outweigh its drawbacks, which is why it’s so popular.

Now, it’s true that a flexible duct will suffice in most situations, but what are the building code requirements? The good news is that building codes approve the flexible duct for the bathroom.

Fiberglass, plastic, or metal are all potential materials for the ducts. However, you should seek to avoid low-cost aluminum ducting since it is fragile.

The thin aluminum duct is frequently damaged during installation. Unfortunately, this is typically discovered only after looking into the source of moisture damage in the attic many years later.

Look for a product that has a protective layer on top of the aluminum. For example, PVC-covered aluminum flexible ducts are much more tear-resistant and easier to insulate and install since there is no need to be careful.

Pros of Flexible Duct

  • Allows you to place the duct into more confined spaces with ease.
  • It enables the duct to bend and follow angles that rigid couldn’t do without joins and additional attachments. Bends are never desirable in ducting, but they are occasionally unavoidable, and a flexible duct’s curve is preferable to a rigid duct’s corner.
  • However, the most appealing feature of flexible ducting for a bathroom fan is, without a doubt, the price. Even higher-quality flexible ducting is significantly less expensive than metal ducting.
  • You also save on labor costs because it is simpler and therefore less expensive to install, and you don’t have to buy the adaptors and joints required for rigid ducts.

Cons of Flexible Duct

  • Increases friction – The ridges you find on the inner walls of flexible ducts create airflow turbulence, which increases the rate at which air particles collide with the walls of the ducts. This eventually increases friction.
  • Sagging duct – Flexible ducting can also sag if it’s not well supported. This sagging further impedes smooth airflow, increases friction, causes pressure losses, and collects water that should be taken outside.

Rigid Duct

The best option for a bathroom fan to vent air and moisture from the bathroom is rigid ducts.

But why would you ever use a flexible duct if rigid is the greatest? There are two primary disadvantages of rigid ducts that we’ll address in this section.

First of all, you need to note that rigid duct is also accepted for bathrooms by building codes. Additionally, if you’re using a range of hoods as your bathroom fan, then you’re only allowed to use rigid galvanized or stainless steel ducting. 

Benefits of Rigid Duct

  • Rigid ducting is made of metal and has smooth inner walls.
  • Rigid ducting is the best type of duct to use for a bathroom fan. It creates less turbulence than flexible ducting, so there is less pressure lost in rigid ducts.
  • In dry air, there is less space for moisture to collect. This is only a problem in places with joins and elbows.
  • Rigid ducts also last longer and don’t require any maintenance.

Disadvantages of Rigid Duct

  • Expensive – Rigid duct is expensive to buy, and if you’re having it installed professionally, you’ll incur more labor costs.
  • Hard to install – It’s quite difficult to install a rigid duct by yourself. You’ll need to know how to do it or hire an expert to do it for you.
  • Noisy – Unfortunately, rigid ducts tend to be too noisy. This can be frustrating for users.

Should I Use Insulated Duct for Bathroom Fan?

Bathroom exhaust ducts should be insulated. One of the most significant reasons to insulate your bathroom exhaust ducts is that it prevents condensation from forming in the duct.

One of the reasons for a dripping bathroom fan is that there isn’t enough insulation in the ductwork. Any duct can be insulated. 

Is It OK to Vent a Bathroom Fan through the Soffit?

While each house is different, it’s usually acceptable to vent a bathroom fan through the soffit. If your roof has already acquired numerous holes, the soffit may be your best alternative.

To enable venting your bathroom fan through the soffit, you must pick the appropriate duct size, duct material, duct slope, duct routing, and insulation.

Installing the soffit correctly is critical in preventing heated and moist air from returning into the attic through the soffit. Venting should only be done via the soffit, never into it.