How to Insulate HVAC Ductwork

Does Ductwork Need to Be Insulated?

When talking about ‘ductwork’, we are referring to the system of pipes and ducts that circulate cooled or heated air throughout the house.

Unfortunately, the absolute majority of ducts are made out of thin material (fiberglass or sheet metal) and that’s why the air that is traveling through the system can easily get lost. 

Of course, you can always choose to not insulate the ductwork in your home. But adding insulation to the system has quite a few benefits that you should know about:

  • Reduced energy consumption ; lower electricity bills

Insulation helps ensure that the air that is traveling through the system stays at the desired temperature. Moreover, it prevents leakage which, in its turn, leads to energy loss.

In fact, without proper insulation, you can be losing up to 30% of the energy that is used to heat or cool your house.

Tip: to find out if your ductwork needs additional insulation, place your hand close to the supply register. The answer is ‘yes’, in case the air feels lukewarm.

  • No condensation

Whenever cool air passes through a very warm part of your house, it may cause condensation to appear in the ductwork. As a result, there will be moisture build-up that can lead to mildew and mold growth, and other problems.

High-quality insulation can prevent condensation from occurring in the system.

In a nutshell, ductwork insulation will help ensure that your home stays cozy and at an optimal temperature. And all that – without the cooling and heating systems having to work at full capacity all the time. 

But does all ductwork need to be insulated? Or are there certain areas where insulation is more necessary?


2. Gather Necessary Materials

Now, head to the hardware store or online to order all of your materials. You will need both foil-faced duct insulation (R-6 or better) and special high-temperature foil tape designed for duct applications.

Click here for foil duct tape from Amazon.

If air sealing is needed, use the same foil tape for the joints that you use for the fiberglass insulation. If the air leaks are particularly bad, you can couple the use of the tape with the use of specialized duct sealing mastic.

Click here for a utility knife from Amazon.

The easiest option to cut the fiberglass batts is to use a utility knife. If needed, grab one of these while shopping for the rest of the materials.

How much duct insulation do I need?

For fiberglass batts, calculate the total area of the outside of your ducts using geometry and the measurements you took during your inspection. Be careful when converting between feet, inches, and other units.

If your ducts are round, use the circumference of

If your ducts are round, use the circumference of a circle times the total length to get the total square footage needed. The letter ‘r’ in the following equation stands for the radius, which is the distance from the center of the circle to the edge. The circumference equation is as follows:

Circumference = 2πr

You can easily add up the perimeter for square or rectangular ducts with measurements taken with a ruler or tape measure. Then, multiply the perimeter by the total length of ducting to get the needed square footage.

Generally, buy at least 10-percent more of the material than you calculate for the square footage as corners and difficult areas often take more material than you hope. For the tape, calculate using your measurements and buy extra rolls which are returnable if left unopened.

How to Insulate Ductwork – Step-by-Step Instructions

General Recommendations

  • Check the system and your home for any problems

Some things need to be taken care of ahead of time. Before insulating the ductwork, make sure to fix any structural problems within the house and damaged ducts, and get rid of mold and asbestos.

Of course, you would have to take all the health and safety issues into consideration.

  • The surface has to be clean and dry

Make sure that the ductwork is not covered in dust and that there is no moisture. Otherwise, the sealants and insulation might not be as effective.

  • Find the leaks

Keep the system running and carefully examine it. Mark any holes and also the spots that have become discolored or have rust on them (they indicate a possible future leak).

  • Protect yourself

If you are insulating ductwork yourself, you have to know how to protect yourself. When working with fiberglass, for example, you should wear goggles, an approved dust mask, and protective clothing. Ideally, you would want to have gloves, but we do understand that it might be challenging to do the job while wearing them.

How to Seal Ductwork

Experts recommend sealing your ductwork before getting the insulation installed. This additional step will bring plenty of benefits in the future. If the ductwork has been sealed, your insulation is going to be more effective and will serve for longer.

Leaks are most commonly found at joints or connections and at vents and registers where they come into the room. Sealing all the leaks is incredibly important, but is far more critical in the unconditioned areas.

Once you have marked the leaks and made sure that the surfaces are clean and dry, you can start sealing using one or more sealants.

  • Mastic is a paste that can be easily spread on holes and any gaps. Use gloved fingers or a stiffed paintbrush to apply the substance. The layer has to be about nickel-thick, in order for the sealant to work its magic.
  • You might want to use mesh in combination with mastic, in case the gaps are over an eighth of an inch wide.
  • Some choose to use butyl duct tape or foil duct tape. By the way, there are foil tapes are mastic tapes. Otherwise, you can use both mastic and any of the tapes mentioned. 

Warning! Old-fashioned duct tape is not a good fit for the job.

After you have sealed all the right places, make sure to double-check the whole system. Perform an air-flow test and a combustion safety test. Let the mastic dry, if you have used any, and only then continue with installing insulation.

How to Insulate Rectangular Ductwork

Rectangular ductwork is sometimes referred to as ‘sheet metal ducts’. It is, perhaps, the most popular type as it is relatively inexpensive to install. Unfortunately, such ducts are pretty ineffective – they are prone to leakage and condensation and allow excess noise in the house.

You should definitely install insulation around su

You should definitely install insulation around such ducts to make them much more effective.

Tip: use a measurement tape to understand how much material you are going to actually need. Don’t forget to allow some room for when insulating ducts in cramped places as you would usually need more material for such spots.

First things first, you have to check the speed of the blower motor. If the speed is too high, you can switch the wires to reduce it.

Use a simple utility knife to cut your insulation to the right size. After you have wrapped the material around the duct, make sure to pinch the seam closed. You can use short strips of foil tape to secure the seams or go for mechanical fasteners.

In case you are using tape, apply a long strip of it along the first short strip.

Hint: you can get hollow-core foam insulation – the thing is easy to install as it self-seams.

You may also choose preformed duct insulation.

All you would have to do to install it is disconnect the elbow to get the end of the duct exposed, then snap a cap onto the end and slip the insulation over the duct. Gently pull the material to get the whole length covered. 

How to Insulate Flexible Ductwork

The majority of manufacturers cover the wire coil and the bendable plastic that flexible ductwork is made out of with fiberglass insulation. So, in a lot of cases, you wouldn’t have to worry about insulating this part of your ductwork.

The great thing about flex ducts is that they redu

The great thing about flex ducts is that they reduce the number of joints and eliminate the need for offsets and elbows. 

Such ducts have to be sealed with duct mastic and held in place with tie wraps. This will help avoid thermal loss.

Tip: all supports have to be at least 1.5 inches wide. Make sure that they do not constrict the insulation as this may cause condensation.

If you ever decide to add another layer of insulation to your flex ductwork, follow the same instructions as for sheet metal ducts. It shouldn’t be too challenging, in case the flexible ductwork was installed correctly (without too many bends and kinks).

How to Insulate Ductwork Elbows

Insulating ductwork elbows can be tricky because of the angles that they bend at. You should definitely go for flexible insulation for this job.

  • The first thing that you want to do is clean the elbow junction. You can use a wire brush or sandpaper to rub it down.
  • Use an old rag to wipe the elbow dry and clean.
  • Measure the girth of the elbow and the length from top to bottom. After that, measure and cut the insulation
  • Make sure that the foil side of the material is facing outward. Use one hand to wrap the insulation around the elbows and the other to apply a short strip of tape along the seam

Hint: to make things a bit easier, you can cut the piece of insulation once again and apply one part on the top of the elbow and the second one – on the bottom.

  • Use longer strips of tape to secure all the seams

Warning! The insulation has to be tight but not compressed. If you press the material into the ductwork, you are going to reduce its effectiveness. 

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Is Insulating Ductwork Worth It?

By now, you should at least be considering the potential risks of uninsulated ductwork.

But is it even worth it to fix?

How much does the temperature drop?

When, if at all, will I deal with mold and mildew? 

If you plan on being in your house for the foreseeable future, it is absolutely worth it to insulate your ducts. Did you know that you typically lose 20 to 30 percent of heating or cooling energy?

Up to nearly a third of your money is going to waste! 

Not to mention the state of your furnace and air condition. Insulated ducts can save them from working as frequently or as hard, which may increase their longevity. This is what I call a win-win situation.

As for mold and mildew, you may never have to deal with this problem if your ductwork was correctly installed with the necessary precautions. But if you do have a mold problem, it’s more often than not that you won’t realize until it becomes a gigantic headache for you and hole in your wallet.

In Closing

In this post, we provided a step-by-step guide on insulating ductwork. The advice here is appropriate for ducts in all locations, including basements and attics. We also answer an important related question. Good luck!


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