Is Metal Ductwork Better Than Flexible Ductwork

HVAC contractors have stopped using metal ductwork as their default duct material since the 1980s. The default ducts you’ll see in homes now is flexible ductwork, but that doesn’t mean that it’s a better for airflow. This post will compare metal ductwork with flexible ductwork and help you decide on which is better for your home. 

Metal Ductwork Types

​Metal ductwork comes in two types, rectangular sheet metal and round rigid ductwork. Rectangular sheet metal comes in 3-4’ sections and is held together with S cleats and drives. Ranch style homes that have registers above the bedroom doorways and a dropped 8’ ceiling in the hallway have this type of ductwork.

The rectangular sheet metal ducts are nailed into the framing of the house and impossible to take out without removing the ceiling as well. Homes built in the 1970s-80s have this ductwork type delivering the conditioned air from a central heating and cooling system but also from an evaporative cooler.

Rectangular sheet metal ductwork are great for airflow because the ducts are oversized to accommodate the large amount of air delivered from the swamp cooler.  Studies have shown that metal ductwork is not necessarily better than flexible ductwork but the advantage of metal ductwork is that is hard to install wrong.  Metal ductwork does not kink as easily as its flexible ductwork counterpart.  As I’ll discuss later, flexible ductwork is not only easy to install, it’s also easy to install wrong with kinks over trusses or electrical wires, too much length, or hard 90 degree bends.
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The second type of metal ductwork is round rigid metal ductwork. Round metal ductwork can either be spiral duct, which comes in one 10’ section of different sizes and is not insulated. This is a great look for restaurants, commercial buildings or even homes that want an industrial look. Insulating spiral ductwork more than doubles the price because the insulation is cut and glued between each of the spiral ribs. For this reason uninsulated spiral ductwork is mostly used in conditioned space. The second type of round rigid ductwork is KD pipe which is snapped together at a seem and comes in 3’, 4’ or 5’ sections depending on the size. This ductwork also comes uninsulated and can easily come apart if not mechanically fastened together and can be deformed and crushed if walked on (yes, I have seen cable guys, HVAC technicians, insulation installers, electricians all crushed this kind ductwork and left as a casuality of their work).  The airflow is slightly better in KD pipe than in compressed flexible ductwork, but if the flexible ductwork is pulled tightly the airflow will be the same. 

Why Metal Ductwork Is Bad

The downsides of metal ductwork is that it rusts and it leaks. With new homes the rust isn’t much of an issue but with homes that also had a swamp cooler installed with metal ductwork, there’s a high chance of rusting already occurring in the metal ductwork. I’ve seen rectangular metal ductwork rusted so bad that large gaps have been made and cold air pouring into the attic in the summertime.
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Because metal ductwork only comes in 3 foot sections (compared to a 25 foot bag of flex duct), there are plenty of joints and places where air can escape.  In addition, if the metal ductwork was never sealed before the drywall went up when the house was being built, the bottom half of the ductwork becomes inaccessible to seal by hand and requires a more expensive Aeroseal process to seal properly.

Flexible Ductwork

​The alternative to metal ductwork, and the most commonly used type of ductwork today is flexible ductwork.  Flexible ductwork is made from 3 different sections, a outer reflective sleeve, an insulation layer and a vinyl plastic inner liner with a metal spiral rib holding it’s shape.  Flexible ductwork was first used in Arizona homes in the 1980’s with a clear inner liner.  This type of ductwork has since been outlawed as it deteriorates and becomes so brittle it will crack and split open with the air pressure.  When changing your air filters, if you see a clear inner liner at the return duct, you know you have this type of ductwork.

Flexible ductwork must be installed properly to get the right amount of airflow to each room and a poor installation can severely affect airflow and comfort in your home.  This means no kinks, using 90 degree elbows at all ceiling cans and the ductwork is pulled tight with minimal slack to minimize friction resistance.

Can I Install Metal Ductwork On An Existing Home?

Yes it is possible to change out the flex ductwork in your home to all metal ductwork but it is costly. Metal duct installers used to have a name, tin-knockers, because of the skill required to measure, fabricate and install the ductwork was a skill. Unfortunately today, there is no name for those of us that install flexible ductwork. The fact is anyone can install flexible ductwork, so anyone and everyone does. With little to no training, installation best practices go out the window, which is one of the main reasons why I’m not worried about finding work for my lifetime but I digress.
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Installing metal ductwork in an existing home is expensive for two reasons. 

1. Material Cost Is High

I don’t know what prices for tin were in the 1970s but to purchase metal today is pricey. Consider that one bag of 16” x 25’, R8 insulated ductwork cost around $90. A 4’ section of KD pipe costs $23 or $138 for 6 sections and that still has to be put together and does not include wrapping the pipe in insulation. Spiral metal ductwork costs $51 for a 10’ stick, or $102 for 20’ and again it is uninsulated and difficult if not impossible to fit a 10’ section in an attic hatch and through the maze of trusses. Insulating the ductwork increases the spiral pipe to $200 for 20’. 

2. Installing Is A Laborious Process

Because the attic can only be access from a 2’x3’ access in the attic (not to mention all the roof trusses one has to navigate) a 10’ section of ductwork can be impossible to move to where it needs to go. Easier attics to get around have higher pitches and plenty of space to walk around but even then, it’s likely a two man task to just set the ductwork in place. Once set, the ducts needs to be secured, sealed and then wrapped in insulation. The right elbows and bends need to be in place to terminate the ducts into the ceiling cans and registers and all that labor can add up quickly. It can be done but but you have to ask yourself if a small improvement in airflow with metal ductwork is worth double the cost of installation verses decently install flexible ductwork.  

What’s The Difference Between Metal Ductwork And Flexible Ductwork?

Converting round rigid metal ductwork to flexible ductwork all depends on how the flexible ductwork is installed.  A 4% compression is a generally accepted practice.  To get  less than 1% compression would risk the duct being pulled so tight it would disconnect from one end.  At a 4% compression, you loose about 1”-2” of duct diameter going from flexible ductwork to round rigid ductwork.  That is, a 6” fleixble duct will deliver the same amount of airflow as a 5” KD pipe duct.

Can I Install Metal Ductwork On A New Home?

What about installing metal ductwork on a newly constructed home? Metal ductwork can better for airflow and the installers don’t have to deal with the space restrictions that come with existing homes, so it would make sense that the cost decreases also.  Well, not necessarily because insulated metal ductwork can be twice or more the cost of flexible ductwork.  If you are willing to pay double the costs of flexible ductwork and have the piece of mind that all the airflow coming out of your unit is going in your home, than metal ductwork is the way to go on a newly constructed home.

What’s Wrong With Flexible Ductwork?

Nothing is wrong with flexible ductwork when installed correctly.  It is on par with sheet metal ductwork for airflow.  What’s wrong with flexible ductwork is that it’s so easy to install wrong.  What could go wrong with flexible ductwork installation?  Here is a short list.

1. Compression with additional lengths
2. Hard turns
3. Not sealed, leaky ductwork
4. Hacked fittings
5. Double backing from poor planning
6. Wrong sizes forced to fit

These items can sabotage your airflow and HVAC efficiency.  With all the ways to install flex duct wrong, you would think that installing it correctly is pretty simple right?  You just pull the ducts tight and hook them up.  It’s actually harder to install flexible ductwork correctly than you would think.  On one hand we want a tight pull on the ductwork, but too tight a pull will disconnect the duct from one end or rip the duct if it’s mechanically fastened by a zip tie.  Flex duct that’s pulled too tightly creates tight bends at the terminals aka ceiling cans and registers in the ceiling.  A 90 degree elbow is required at the ceiling cans for optimal airflow going into the house.  That way the ductwork can be pulled tight and still make a 90 degree bend going into the house.  Many HVAC contractors like to strap the ductwork up along the roof trusses to help ease the drop down into the ceiling terminals, however this leaves the ductwork exposed to the hot attic air.  We like to drop the ductwork when possible, install 90 degree elbows  at the terminals and bury the duct in insulation. 

What’s Good About Flexible Ductwork?

There are good attributes to flexible ducts that in my opinion outweigh the bad.  We make our case below.

1. Flexible ductwork is cheaper than metal ductwork.  By the time you add insulation onto the cost of metal ductwork, you are almost double the cost of flexible ductwork that comes preinsulated (buy the R8 insulated ductwork verses the R6).
2. Flexible ductwork is faster to install than metal ductwork.  Even with the additional time installing 90 degree elbows, trimming the ducts to the correct length, properly sealing and running the ductwork most efficienctly, it is less man-hours and costly than securing 3-10 foot lengths of sheet metal ductwork, securing them together, sealing the metal joints and finally wrapping everything in insulation. 

If flexible ductwork is installed correctly, the airflow will be the same or better than a metal ductwork.  It’s all in the installation.  The advantage of metal ductwork is that you are taking less of a chance of the ductwork being installed wrong than with flexible ductwork.  It’s harder to install metal ductwork wrong than it is flexible ductwork.  If you can find an installer than follows the best flexible ductwork installation (like Green ID), then there’s no difference between airflow from flexible ductwork compared to metal ductwork.  The cost will be significantly less with flexible ductwork compared to sheet metal ductwork.

Loss In Flexible Ductwork Studies https://www.osti.gov/servlets/purl/836654

​https://www.mmmfg.com/wp-content/themes/va/pdf/060601_CC-KW_DuctTechPaper.pdf