What Size Flex Duct For Mobile Home?

The most popular ductwork for mobile home air-conditioning and heating systems is a flex duct. But what size should you go with to maintain adequate airflow? We’ve researched, and here’s what we’ve found: 

A 12” flex duct is ideal for mobile houses, but you can go for smaller or larger sizes, depending on CFM [Cubic Feet per Minute] that your room requires. 

Read further to learn how to calculate the proper duct size for your trailer and install a flex duct yourself!

Industrial Air-conditioning Ductwork and Pipework In Commercial Building Fitout, What Size Flex Duct For Mobile Home?

Why Choose Flex Ducts For Mobile Homes?

As the name implies, flex ducts are flexible. They bend around fixed objects and structures, making them easier to install in confined or awkward spaces such as mobile homes, compared to rigid or metal ducts. 

A flex duct might also serve as a crossover to join two trunks or be used to connect the heat pump to return the air grill if you are using a package heat pump. Lastly, flex ducts are cheap. 

ventilation system in a modern residential apartment during the construction phase

Why Do You Need The Right Flex Duct Size For Your Mobile Home?

Proper duct sizing sustains adequate airflow and prevents HVAC problems. An undersized or too small duct system leads to a higher pressure, causing the air to go back up and excessive noise. 

On the other hand, oversized ductwork results in weak airflow because the air won’t build up no matter how many times you adjust the temperature. Hence, you won’t be getting the maximum benefit of your HVAC system. 

How To Know What Size Flex Duct To Use?

The ideal flex duct size for mobile homes is 12”. Most people don’t go above it. But it’s still important to determine the proper duct size. Here’s how:   

Measure the square foot of the room that will be supplied with air. Just multiply the width and length. Then, determine the required CFM or air volume. Generally, one square foot requires one CFM. Once you’ve figured out that CFM, you can pick the duct size with such capacity. Refer to the table below: 

Flex Duct Size

Air Volume
5” 50 CFM
6” 75 CFM
7” 110 CFM
8” 160 CFM
9” 225 CFM
10” 300 CFM
12” 480 CFM
14” 700 CFM
16” 1,000 CFM
18” 1,300 CFM
20” 1,700 CFM

The information presented is provided by LearnMetrics.

How To Properly Install A Flex Duct In A Mobile Home

Air ducts in the attic

At this point, you should have determined the proper flex duct size for your home. If so, proceed with these simple steps:

1. Cut the flex duct

[Most flex ducts are 25” long, just cut accordingly with a duct knife.] Cut through the outer liner to the inner portion, then cut the wire flex with a tin snip. Roll and tuck the outer foil into [if necessary] to ensure that the insulation foil remains intact.

TIP: Make sure that the flex duct is not too long, otherwise, you will get kinks that can restrict air flow, reducing your BTUs. 

2. Connect the Flex duct to the Collar

For first-time flex duct installation, you have to drill a hole in the plenum where the duct the collar goes in. Use a sheet metal cutter to create a hole, then install the collar and flanges. 

Once the collar is set, connect the duct to it- make it as airtight as possible. Pull the inner liner, put it around the collar, then secure it with a zip tie. Make sure that all parts of the inner liner are tied. With a Panduit gun, tighten the tie, then cut it off. 

Double-check if the tie moves out of place. If secure, wrap a flex tip tape around it. Seal the joint with a duct sealant to avoid air leaks and dirt entry. Unroll the fiber insulation and outer foil to cover the inner liner. 

Wrap another zip tie on the outer liners and secure it with a Panduit gun. Finally, add another flex flix tape. 

3. Channel and support the flex ducts

ItIt’sime to plan out how to channel your flex duct, depending on whether it goes into the return or supply air system. Afterward, do the support- wewe’reeferring to the strapping. You may opt for metal, plastic, and ply web strapping for the straps.

Poly web strapping is recommended for beginners as they are easy to install with a stapler. But if you want a more secure connection, go for metal strapping. 

When it comes to flex duct support, make sure that the flex duct is off the ground to keep debris, insects, rodents, and other foreign material out of it and your home. HerHere’sw to properly suspend the flex duct from your mobile home floor. 

  • Place the straps 4 feet apart or closer so that no portion of the flex duct will be left unsupported, which might cause kinks or bends.  
  • Ensure that the supporting straps are ½” ”ider than the gap between the two spiral wires in a row to prevent them from slipping through the metal wires. This ensures that the ductwork remains straight and lowers the chances of it touching the ground.  
  • Place pads beneath straps for better support. 
  • Use sheet metal screws for metal straps. 
  • Select a quality duct saddle strapping system for improved efficiency and airflow. 

How Much Does It Cost To Install Flex Ducts?

Flex ducts cost around $40 to $55 per foot. A single unit ducting might take up to 60 ft of flex duct, and you can spend up to $3100. A double unit ducting will require approximately 120 ft of flex duct, and the price can reach up to $5000.

Securing the flex duct will cost more as the weight doubles. The total price may vary depending on how many flex ducts, straps, and collars your trailer needs. 

Check out this product on Amazon.

What Is The Common Problem With Flex Ducts?

The flex duct itself has no issue, but the installation might have. Due to their versatile nature, many people install them incorrectly, resulting in unexpected bends and sags. This increases the resistance of the system; hence, little to no air reaches the HVAC system.

They are often cut too short. Unless you are confident that you can install it properly yourself, don’t don’t hesitate to ask for the help of an expert.

air conditioner vent in the attic

How To Maintain Flex Duct

Regular cleaning [every five to seven years] is the best way to maintain ductwork. However, this process can easily damage the flex ducts. The reason is that the plastic layers inside get more fragile due to heat. That’s That’s often requires professional assistance.

But the most popular methods for cleaning flex ducts are vacuuming and brushing. The former involves blowing and sucking the air from one to another to get rid of the dust, while the latter uses a power brush system to remove and collect dirt. Failure to clean the ductwork periodically results in restricted airflow and early corrosion. 

Check out this item on Amazon.

How Long Do Flex Ducts Last?

Flexible aluminum dryer vent hose

Most manufacturers warranty a 10-15 year lifespan. Many homeowners question its reliability as you may have to deal with frequent duct replacement. However, some claim that a flex duct can last up to 25 years with proper maintenance!

Should You Insulate Your Flex Duct? 

Yes, you should insulate if your ductwork is placed outside to prevent air leaks and temperature drops. But some flex ducts already come with fiber insulation. Choose one with at least an R-4 to R-8 rating that will keep you comfortable through changing seasons.   

Flex Duct Vs. Rigid Duct: Which Is Better?

In ductwork, it all goes down to location. A flex duct is best for small spaces such as the attic and trailer floors, while rigid or metal ducts are best for areas where they can sit exposed, including basements and garages. 


Industrial Air-conditioning Ductwork and Pipework In Commercial Building Fitout

It’s ideal to have a flex duct for mobile homes, but you can install a smaller or larger size depending on the CFM that your home will require. Fortunately, it's fairly to determine this. Proper duct sizing is critical as undersized or oversized ductwork will lead to inadequate airflow. 

If you liked this post, you might also like: 

How To Install A Return Air Duct In Your Ceiling

Can You Use Flex Duct For Return Air?

Share this article

Leave a Reply

Your email address will not be published. Required fields are marked *