How To Insulate Ductwork [Inc. In The Basement Or Attic]

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Our ductwork performs the critical task of bringing conditioned air to the various parts of our homes. Given this, you may be wondering how to insulate your ductwork to maximize energy efficiency. This post uses industry professional knowledge and up-to-date research to answer your question thoroughly.

No matter where your ducts are, wrapping your ductwork with fiberglass batts is usually the most efficient and easiest duct insulation option. However, there are some situations where loose-fill insulation or no insulation are more appropriate.

To decide if and how to insulate your ducts, follow these steps:

  1. Inspect your Ductwork for Size and Form
  2. Gather Necessary Materials
  3. Optional: Seal all Duct Seams
  4. Apply Appropriate Insulation
    1. Fiberglass Batt Insulation
    2. Loose-Fill Insulation
  5. Enjoy the Added Comfort and Savings

Keep reading the rest of this post for details on each of the above steps. We pay particular attention to the substeps required for wrapping ducts in fiberglass batts. To conclude, we provide an additional reading list to help expand your knowledge of in-home energy efficiency.

A pipes of HVAC system for heating ventilation and air conditioning, How To Insulate Ductwork [Inc. In The Basement Or Attic]

1. Inspect your Ductwork for Size and Form

The first step in insulating your ductwork is to inspect your entire system. Usually, there are ducts only in either the attic or basement/crawl. However, some homes have ducts in both locations. Often, accessing these ducts requires movement in dirty and cramped locations.

Click here for a protective suit from Amazon.

Therefore, it is wise to wear proper protective equipment when performing the inspection and the subsequent work and only enter safe spaces. Wear full-body suits, breathing protection, eye protection, and gloves (optional).

During the inspection, have the fan of your HVAC system running. This allows you to feel for areas that are leaking air out of your ducts. Overall, look for and note the following during your duct inspection:

  • Diameter/Perimeter
  • Length
  • Air leaks (and length of leaky duct seams)
  • Location

Diameter, length, and air leaks are information that allows you to plan for which and how many materials you need for your project. The location of your ducts enables you to prepare for how to accomplish the duct insulation and whether your ducts even need insulation.

As you perform your inspection, be sure to note the diameter/perimeter and length and the specific lengths of each different size of ducting. Usually, a single home will have two or more different-sized ducts throughout the system.

Remember, if the task appears too daunting, it is always possible to call weatherization professionals to provide a bid and do all of the duct insulating work for you.

Does exposed ductwork need to be insulated?

Exposed ductwork needs to be insulated if it is in unconditioned space. On the other hand, if the ductwork is in a conditioned attic or basement, there is no need for insulation.

Any heat or cold that escapes from ducts in conditioned spaces escapes to already heated or cooled locations. This means that jacket losses in conditioned spaces are not energy inefficient or costly.

What type of insulation is used for ductwork?

According to the North American Insulation Manufacturers Association, fiberglass insulation is the standard for duct insulation. For at-home remodel applications, you will want to use foil-faced fiberglass batts with an R-value over R-6 for all duct insulation.

Click here for foil-faced fiberglass batts from Amazon.

However, for ducts within the belly of mobile homes, surrounding the ducts with loose-fill insulation is standard. Further, for attics that have ducts very close to the ceiling, it is sometimes the most practical to blow loose-fill insulation over the top of the ducts and over the ceiling in one go.

Foil-faced fiberglass batts are the best option because they are easy to install and the foil acts as a moisture barrier that stops condensation on the duct surfaces.

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.

Contractor estimating the cost of a construction project with a calculator and a ruler over an architectural blueprin

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.

3. Optional: Seal all Duct Seams

If you noticed a good deal of leakage during your inspection, it is now time to seal all of the duct seams. In fact, and according to Energy.gov, duct sealing is one of the best ways to improve in-home duct efficiency.

Seal your ducts by applying foil tape over the seams. Take the time to press the tape into place to ensure a good connection.

You can either perform air sealing just on problem areas or on the entire system. For an added bonus, seal the boot connection with tape by removing the register and taping the end of the duct to your home.

Click here for duct mastic from Amazon.

If the tape does not provide enough of a seal, you can go over the tape with a product known as duct mastic. Using gloves and a paintbrush, apply the mastic to all of the tape seams, following the directions on the container. Be sure to let the mastic dry before performing more work.

4. Apply Appropriate Insulation

Now comes the real meat of this project, applying the insulation. Below, we cover the directions for fiberglass batt and loose-fill projects.

Fiberglass Batt Insulation

As stated above, most ducts are best to insulate with foil-faced fiberglass batts. While effective as insulation, the installation process is relatively labor-intensive.

First, cut the batt to perfectly wrap around the duct, with the two ends just touching. For best cutting results, compress the batt down with a straight hard edge against a flat hard cutting surface such as a piece of plywood. Then, draw the utility knife along the straight edge to get a flat, even cut.

Worker insulating an attic vent duct with aluminum foil tape

Now, wrap the batt around the duct, with the foil facing out, and tape the loose edges together. Continue this strategy for the next section of batting. Also, be sure to tape the joint where each piece of batt meets.

When you get to branches and corners and ends, you will have to take care and cut the batts to fit the odd angles and joints. Overall, the goal is to fully enclose the duct with batting so there are no gaps and the batt is entirely in contact with the duct.

For a video on how to seal and install fiberglass batt insulation, check out the following:

Loose-Fill Insulation

You may choose to apply loose-fill insulation around and over your ducts for some specific situations. While this strategy does not provide a moisture barrier, loose-fill is the most economical choice for manufactured homes and ductwork that is right on the attic floor.

Spraying blown fiberglass insulation for roof

Spray the loose-fill insulation into the attic or up into the manufactured home belly as usual. For manufactured homes, the goal is to fill the available space. For attics, you will want to fill at least 2-inches to 3-inches of loose-fill insulation overall ducts.

Usually, blowing insulation is a job performed by professionals. However, it is possible to rent specialized insulation blowers to perform the work yourself.

5. Enjoy the Added Comfort and Savings

Congrats! You are all done. The money saved on energy cost through duct insulation and duct air sealing is usually fully realized over the life of the measure. Duct insulation is considered an excellent investment.

How do I stop my ductwork from sweating?

As mentioned above, ducts sweat because of condensation that occurs on the surface of the ducts. This is especially prevalent on ducts primarily used for cooling. To stop this, wrap your duct in a water-impermeable membrane. Here, we suggest foil-faced fiberglass batts.

Additional Reading

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

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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