How to Cover Round Air Ducts With Insulation

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?


Rely on Trusted Professionals

Although it might be tempting to try and save some money by insulating the ducts in your home by yourself, this is actually a job that is best left to professionals.

For one, climbing into an attic and getting to the ducts that are located in ceilings, basements, crawlspaces and other out-of-the-way areas in your home takes a lot of physical effort and can be a dirty process. It’s also important to avoid dislodging ductwork or otherwise causing holes or tears in the process.

Professionals also know that it’s a good idea to insulate your basement walls at the same time you insulate the ductwork in the basement, since insulated ducts will give off less waste heat that formerly was helping to keep pipes from bursting due to excessive cold during the winter.

For better peace of mind, you can hire trusted HVAC professionals to take care of the job and do it right the first time, on time and for price that makes sense for your budget.

Additional Reading

To learn more about ensuring energy-efficient homes, read these great articles from HVAC Seer:

Step by Step Instructions

While it isn’t insulating your ductwork, taping joints in ducts should be done first.

Below are the necessary steps needed to properly install the insulation around your ductwork in your attic, basement, and/or crawl space.

Step 1: Choose what kind of insulation you would like to use for the job.  The most cost-effective type is foil-backed batts.  You get more for your money, but there are other options that were discussed earlier in this article. As long as it is heat resistant and has an R-factor greater than 3.5, it is a decent option for the insulation of your ductwork.

Step 2:  Clean ductwork by wiping it down.

Step 2:  Clean ductwork by wiping it down.

Step 3: Measure the area of ductwork you will be insulating, and then cut your heat-resistant insulation to the proper size. “Measure twice, and cut once,” as the saying goes.  You don’t want to have to use more pieces than necessary.

Step 4: Wrap the ductwork with the properly sized

Step 4: Wrap the ductwork with the properly sized insulation. Make sure that it fits tightly around the ductwork, but do not compress or dent the ductwork, because compressed fiberglass insulation doesn’t have as much air within the material – and air pockets help hold in heat. Also, make sure that the foil is on the outside with the fiberglass insulation on the ductwork.

Step 5: Use duct tape to secure the insulation around the ductwork.  Do this by simply wrapping the duct tape around the insulation and the ductwork.

Step 6: Using your foil tape, completely seal the

Step 6: Using your foil tape, completely seal the seams of the insulation by using one long piece of foil tape along the entire length of the seam.

Step 7: If there are any areas that you cannot fit

Step 7: If there are any areas that you cannot fit and seal the insulation around the ductwork, you can use mastic to seal the area via a caulking gun and a tube of mastic. Use a tool on the mastic to force it against the seam to get the best seal.

Step 8: Repeat the steps above on every section of

Step 8: Repeat the steps above on every section of ductwork.

Step 9: Double check all your ductwork to ensure that everything is properly covered, insulated, and there are not any exposed areas where heated or cooled air can escape via uninsulated ductwork.

These steps described how to use flexible, foil backed insulation to wrap your ductwork in your basement, crawlspace, or attic.  You can use the same basic steps when installing rigid insulation, too.  Rather than wrapping the insulation around the ductwork, you will cut the rigid insulation to fit around the ductwork and then use the foil backed duct tape to seal it all together.

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.

Steps for Insulating HVAC Ductwork

  1. Check the speed of the blower motor on the furnace. If necessary, switch the wires to reduce the blower to its lowest speed.
  2. Press a continuous strip of foil tape to all longitudinal seams along straight runs of duct.
  3. Use a paintbrush to apply duct mastic to the joints where an elbow connects to a duct.
  4. Use a utility knife to cut foil-faced fiberglass insulation to the proper size.
  5. Wrap the insulation around the duct, and then pinch the seam closed. Secure the insulation with short strips of foil tape.
  6. Apply a long strip of foil tape along the seam in the insulation. Repeat to insulate the remaining ducts.
  7. To install preformed duct insulation, start by disconnecting an elbow to expose the end of the duct.
  8. Snap a plastic cap onto the duct end, then slip the preformed insulation over the duct.
  9. Gently pull the insulation over the entire length of duct.

So, which is it? Insulate or replace?

It’s up to you, but it’s generally more cost effective to replace the ducts. By the time you pay for materials and labor for sealing and insulating existing ducts, it will usually shake out to a price that’s close to what it costs to get new ones.

If your contractor does things the way we do, replacement also results in a better duct system that keeps you more comfortable and maximizes efficiency.

With us, you get a brand new duct design that is accurately sized for your home and your HVAC system. Precious few HVAC contractors will do this. Most are “box swappers” who just replace what you have with the same exact thing. In other words, the system will still underperform – even after the ducts are insulated!

In the event you’re already replacing your HVAC system, that’s a great time to also replace your old ducts. The new duct system will minimize energy loss and, assuming it’s installed by a knowledgeable company, will offer better quality and comfort compared to whatever you’ve got now. It will also pair nicely with your new HVAC equipment. Your existing ducts, on the other hand, might not.

Why Should You Insulate Your Ductwork?

We mentioned energy efficiency by reducing energy loss through uninsulated ductwork when heating and air conditioning your home. Ecofriendliness shouldn’t be overlooked. The more energy that is conserved or saved, the better it is for the environment.

Plus, the fact your equipment won’t work as hard to keep your home comfortable.

It’s also true that if your HVAC system is properly sized but you’re losing a lot of the treated air with uninsulated and possibly leaky ductwork, your furnace, heat pump or AC might not be able to keep up, and parts of your home won’t be adequately heated or cooled.

OK, we’re all on the same page now. Let’s talk about how to insulate ductwork in the basement or attic.

Ducts that Run Below Joists

If your round air ducts run below the joists, lay one insulation batt on top of the duct. Direct your helper to hold a second batt up to the underside of the duct. Wrap the twine snugly around the duct to hold the top and bottom insulation batts in place. Direct your helper to hold up the bottom batt just ahead of where you are wrapping the twine around the duct. Tie knots to secure the ends of the twine.

Insulating your ducts

Adding insulation to your ducts isn’t as simple as getting some duct wrap and taping it together. Ok, some people and contractors do insulate their ducts this way. And technically, the ducts are insulated afterward.

But if you go this route, you’re forgetting something super important: duct leaks.

Duct leaks are the scourge of energy efficiency and indoor air quality. Gaps, cracks, holes, rusted out corners… When air leaks through your ducts, it brings contaminants into your indoor air and reduces the efficiency of your HVAC system. By not addressing duct leaks, you’re literally polluting your clean, filtered, conditioned air with dirty, unfiltered, unconditioned air!

Remember, a lot of your ductwork is either in your crawlspace or your attic. Do you really want to be breathing the air from those spaces 24/7?

Anyway, the point of all this is to highlight the fact that you’ve got to seal the leaks before you insulate the ducts.

Insulation blocks the movement of heat, but it can’t stop air leaks. Dirty air will still enter your ductwork through the leaks, even after you add insulation. If you decide to insulate your old ducts, just make sure you or your contractor follows this order of operations:

  1. Seal the leaks with mastic or mastic tape (we follow a highly precise “seal and test” protocol that identifies all leaky areas and results in 4% duct leakage or less).
  2. Wrap fiberglass duct insulation around the ducts and tape the seams together.

Of course, you might also consider…

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