How to Install Duct 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.


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.

How to choose the thickness of ductwrap / duct board?

As we are a UK based online insulation store, we will be following the British Standard, BS5422:2009 to choose the thickness of our ductwrap and duct boards. Different codes and standards apply to different regions and countries, but you can be pretty confident British Standards are amongst the best and most rigorous in the world.

One caveat we will make is that the below advice is not meant to be a substitute for proper design calculations which strictly speaking should be made by a competent consultant or building control. 

The below advice is general in nature, whilst referring to BS5422 and can be used by the layman to help them choose the correct thickness, but all sorts of factors come into play when selecting the thickness of duct lagging, such as humidity, location of the ductwork, material the ductwork is made out of etc so professional advice should be sought or thorough research should be conducted to allow you to choose the correct thicknesses required.

For the exact thicknesses required, for England & Wales, Refer to TIMSA Guide Section 6.2.5, page 26 & BS 5422: 2009 Tables 12,13 & 14.

Below are snippets from Isovers own foil faced ductwrap data sheet which you can find on our website here.

The first snippet shows you how to choose the right thickness depending on the type of air that's being carried through the duct, to achieve the maximum permissible heat gain or loss. As you can see, 50mm is the specified thickness in most instances. If the ductwork is used for both warm and cold air, you should go with the central, dual purpose column which states 50mm.

However, if you're just looking to prevent condensation rather than preventing heat gain / loss then the below table details the thickness required.

Generally speaking, condensation prevention on ductwork can usually be achieved with just 25mm thickness, that's if the air within the duct doesn't drop to below 10 degrees which in most cases wouldn’t happen on conditioned air. 

However this is certainly possible with external air during frosty and snowy conditions, which is being sucked in via the fresh air ductwork, which would then require either 40mm or 50mm. 

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.

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…

Possible Risks Associated With Ductwork and Fiberglass

Nothing in the world of HVAC is risk-free. The good news is that you can have it safely installed by a professional. If you’re worried about what might happen when ductwork touches fiberglass, you’re in the right place.

Here’s a list of possible risks that might occur if ductwork touches fiberglass insulation:

  • If there are leaks along the ductwork, condensation can buildup. Condensation and other sources of moisture cause mold and mildew to grow on fiberglass insulation. It’s a health hazard that can also wreak havoc on the insulation and ductwork.
  • As mentioned above, loose fiberglass batts can cause breathing issues. Fiberglass batts are designed to stay compacted throughout the installation process, but that doesn’t prevent sloppy work from ruining them.
  • If the ductwork is porous or cracked, it could pull fiberglass fibers through. This problem would result in small shards of fiberglass spread throughout your home, which has obvious negative side effects. Although it’s incredibly rare, nothing beats a good old fashioned inspection.

So, above all else, your first step is ensuring that the ductwork is air-sealed.

As an FYI – According to the Engineering Toolbox, fiberglass has a heat threshold that’s well over 500°F or 260°C.

Whatare the benefits of having ducted hvac systems?

Properly installed and maintained ductwork has three main benefits: 

For starters, by supplying uniform temperature throughout the building, they maintain an even, comfortable temperature for its inhabitants, whilst eliminating hot and cold patches throughout.

Secondly, it filters out dust particles and other indoor pollution, making the indoor air cleaner and fresher. Special filters can even filter out germs and viruses, including coronaviruses.

Thirdly it is much more effective at heating or cooling a room quickly and evenly than say radiators or split air conditioners. Infact, unless there are split air conditioners in each room which is less efficient, then there is no better way to cool a building than using ductwork.

Ductwork can be hidden in ceilings and walls also, whereas split air conditioners and radiators cannot, meaning more living space and less ugly looking appliances on show.

The image below is of a huge office building's network of supply and extract insulated ductwork, before it was concealed by a hanging ceiling grid. 

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

  • Caulk gun
  • Cordless drill
  • Dust mask
  • Knee pads
  • Lineman’s pliers
  • Safety glasses
  • Tape measure
  • Utility knife
You’ll also need a trouble light and leather gloves