How to Cover Round Air Ducts With Insulation

Cut the duct wrap to fit, then install

Photo 1: Cut to size

Photo 1: Cut to size

Cut the ductwork insulation to match the circumference of the duct, plus 2 in. Then peel away the foil backing and cut off 2 in. of fiberglass.

Photo 2: Wrap the duct

Photo 2: Wrap the duct

Remove several duct hanger brackets and slide the insulation around the top of the duct. Overlap the 2-in. foil tab at the seam and seal with aluminum tape.

Photo 3: Secure sections with wire

Photo 3: Secure sections with wire

Relieve stress on the seam by wrapping wire around the insulation. Place two wires on each 4-ft. section.

Photo 4: Wrap round ducts also Repeat the cutting, tabbing and taping procedure for each round duct.

Uninsulated ducts that run through unconditioned spaces can lose more than 30 percent of their heating or cooling capacity. So you’ll save money by insulating them. The Residential Building Code calls for R-8 insulation for these ducts, but check with your building inspector for local code requirements. Before you insulate, however, plug any air leaks by sealing all the joints with caulk or tape.

Look for “duct wrap,” a fiberglass product with an outer foil vapor barrier. The foil barrier prevents condensation (and mold) from forming on the duct.

Unfortunately, R-8 duct wrap can be difficult to find. We checked three home centers and found only R-3 duct wrap. That didn’t meet our local codes, so we contacted a heating equipment supply house. That company knew exactly what we were looking for and had it in stock.

We bought a roll of 3-in. by 48-in.-wide by 50-ft. duct wrap for our installation. We also bought a few rolls of UL181 aluminum duct tape to seal the insulation seams.

You’ll be handling fiberglass, so wear long sleeves, goggles, mask and leather gloves. Measure the circumference of the duct, add 2 in. to the total, and cut the insulation to that length. Remove a 2-in. strip of fiberglass to create an overlapping flap for taping (Photo 1). To finish the job, butt the insulation edges together along the bottom of the duct, overlap the 2-in. strip of foil, and secure it with aluminum tape (Photo 2). Wrap wire around the insulation to relieve seam stress and prevent the seam from separating (Photo 3).

You’ll save lots of cutting and fitting time on rectangular ducts by removing the hanging brackets one section at a time. Remove the screw from the joist and rotate the bracket. Once the insulation is in place and taped, cut a small hole near each bracket, rotate it back up through the hole and reattach it to the joist. Seal around the bracket with tape.


Considering your insulation R-value

More important than the type of insulation (provided it is installed properly) is the R-value of the insulation you use. R-value is the measure of the ability of the insulation to prevent heat from either penetrating or escaping the object insulated. The higher the R-value (literally meaning resistance value), the better the insulation works. However, there is a ceiling on the effective R-value because, at a certain level, the cost of the material becomes greater than any additional savings.

Before selecting your duct insulation, determine the optimum R-value for your region. In general, the colder your climate, the higher the R-value you will need. Even then, different areas of the home may require greater or lesser insulating power. As a rule of thumb, expect to install a minimum of R-5 material. To be precise, consult the Department of Energy’s Duct Insulation R-value Chart.

When selecting your duct insulation, use the R-value you require to determine what product — or combination of products — you need. Use more than one layer if a single layer won’t give you the value you desire.

Evaluating your ducts

First, consider the duct location. The greater the temperature extreme between the air inside the duct and the air surrounding the ductwork, the greater your need for duct insulation.

Unfortunately, so many houses — especially newer homes — end up with ductwork running through the attic. Unheated basements, crawl spaces under the home and even garages are also unconditioned (neither heated nor cooled) runs for ductwork.

If, on the other hand, your ducts run across the ceiling of a heated basement or inside well-insulated walls and ceilings, your need for duct insulation is minimal. However, if the ducts have a lot of leaks, the air that makes it to your rooms will not be as warm or as cool as intended. So duct loss matters no matter what.

Then there’s the duct material itself. Most heating and cooling ducts are metal. These are the bulky, gray box-shaped ducts so common everywhere. Sometimes metal ducts are lined with duct liner, a 1-inch-thick fiberglass board that insulates the interior of the duct, rather than the exterior. Duct liner isn’t generally a DIY installation, and if your ducts already have liner, you don’t need to insulate the exterior.

Duct board, much like the duct liner, is also a fiberglass product. Made from 1- to 2-inch-thick sheets of rigid glass fiber and coated with an aluminum laminate for a moisture/air barrier, duct board comes in sections that fit together like metal ducts. Duct board’s advantage is that it is already insulated, eliminating the need for further work, as long as it is structurally sound.

Another product, known as flex duct, is a round framework of wires coated with fiberglass and encased in either foil or plastic to resist moisture and air leakage. Although flexible ducts don’t require further insulation, they are vulnerable to damage, especially punctures. Since they are generally used for short runs, when working on the remainder of your ductwork, check any flex ducts as well.

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.

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.

What to do Before you Seal and Insulate your Ductwork

As with most big home projects, there are safety issues to consider. Before you start sealing or insulating your ductwork, you should check your home and HVAC system for any problems that you must take care of ahead of time. Look for:

  • Structural problems within your home
  • Damage to or design problems with the ducts themselves
  • Mold, asbestos or other contaminants in your home or ductwork
  • Evidence that testing and sealing your ductwork would create health problems for individuals living in your home
  • The ability of the ductwork to be safely accessed and worked with
  • Any other health or safety issues, such as knob-and-tube wiring, animal or insect infestations, toxic materials, solvents, non-IC rated can light fixtures and so forth

Once you have either determined that there is no impediment to moving forward, or you have had a professional clear any problems you discovered, you can proceed with sealing your ducts. Remember, you must seal your ducts as thoroughly as possible before you insulate them. One possible exception is if you are using preformed insulation sleeves on round ductwork as discussed below.

Prepare to Wrap Your Ducts

Once you’ve ruled out a mechanical or other HVAC issues, consider insulating your metal ductwork. Uninsulated or poorly insulated metal ducts are often the culprit. Insulation prevents warm air outside the ducts from contacting cool, metal ductwork, thereby preventing condensation. Duct wrap, which is insulation made especially for insulating ducts, has an inner layer of foam or fiberglass and an outer layer of foil.

To insulate your ducts, you’ll need the following gear, along with your insulation:

  • Protective goggles
  • Metallic foil duct tape
  • Dust mask
  • Straight edge or carpenter’s square
  • Utility knife

Still Have Excess Moisture?

If after following the above, you still have moisture, you will need to take additional steps, such as installing a dehumidifier in either your attic or crawlspace. Visit our page on dehumidifiers to learn more.