Ductwork Insulation: Everything You Need to Know

Perhaps, the main thing that you should know about ductwork insulation is that insulating the ducts is never excessive. Properly installed AC ductwork insulation will help you reduce energy consumption (which results in lower electricity bills) and will also help avoid condensation-related problems.

What types of insulation should you use for the ductwork? What R-value should you go for and will you manage to insulate the ductwork in your house all on your own?

We present to you the “ductwork insulation: everything you need to know” guide. Here, you will find the answers to all of your questions (and even more).

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.

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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?

Where Does Ductwork Need to Be Insulated?

It is incredibly important to install insulation on the parts of the system that travel through unheated space. However, insulation will be just as beneficial in all the other areas of your house.

So, if you find yourself asking ‘does exposed ductwork need to be insulated?’, ‘does ductwork need to be insulated in conditioned space?’, or ‘do HVAC return ducts need to be insulated?’, the answer is positive.

Some experts say that ductwork located inside the building thermal envelope does not necessarily need any insulation, but you can still install it to prevent condensations (especially, if it will be delivering air conditioning).

You can follow these guidelines when choosing the right type of insulation for your house.

LocationInsulations R-value
Supply and return ducts in attics, if they are more than 3 inches in diameterAt least R-8.0
In attics, smaller than 3 inches in diameterR-6.0
Supply and return ducts in other unconditioned spaces (basement and crawlspace outside the building thermal envelope, a garage), if they are more than 3 inches in diameterR-6.0
In other unconditioned spaces, smaller than 3 inches in diameterR-4.2

What Is the R-Value?

R-value or ‘thermal resistance’ is a number that shows an insulating material’s resistance to conductive heat flow. 

In a nutshell, the higher the R-value, the better the insulating effectiveness.  

The thermal resistance depends on the type of insulation, its density, thickness, and on how and where it has been installed. Typically, the increase in the insulation’s thickness will proportionally increase the R-value.

Ductwork Insulation – Details

Types of Ductwork Insulation

Fiberglass is the most common material used for ductwork insulation. It comes in a flexible or rigid format and has R-values that range from R-4.0 to R-11.

Image source: https://www.certainteed.com/insulation-insulation/improve-indoor-air-quality-fiber-glass-duct-insulation/

Flexible fiberglass is wrapped around the air ducts. After that, the outer backing is backed by foil. On the other hand, rigid fiberglass is great for rectangular ducts. To keep the rigid board against the ductwork, experts use clamps and clasps.

Sometimes, more than one type of fiberglass can be used in a single system.

Such insulation prevents condensation, helps conserve energy, and provides temperature and acoustic control.

The second widespread type of insulation is made out of polyethylene bubbles that are located between radiant barriers (they look similar to simple foil). It is a cheaper option that is relatively easy to install. 

However, do bear in mind that in order to take advantage of the barrier, you would have to leave 2 inches of air space between the duct and the foil. And that is not always a simple thing to do. 

Foil-backed self-adhesive foam duct insulation is easy to install and can be wrapped around irregular ductwork. The material is relatively thin, but it dampens sound very well and, if you ever need to, can be used with other types of insulation.

Image source: https://www.thermalinsulationfoam.com/sale-11239456-self-adhesive-hvac-insulation-foam-13-25mm-thickness-long-service-life.html

Warning! Make sure to use only the foam that is specially designed for insulating as a lot of foam products can become toxic when burning and are highly flammable.

Ductwork Insulation Cost

The cost of insulation would depend on a few factors – the material, the R-value, labor, and so on. Usually, duct insulation cost falls in the range between $0.95 and $2 per square foot installed.

Let’s have a closer look at what you are paying for:

  • The length of the ductwork

Of course, the longer the actual ductwork, the more expensive the insulation is going to be. The cost will also depend on the insulation ratings (R-3.5, for example, is cheaper than R-8.0).

  • The thickness of the ductwork

The thinner the ductwork, the more insulation material you would have to add to achieve the desired thickness. This is done in order to meet the recommended insulation levels.

  • Supplementary materials

Unfortunately, you can’t simply stick the material to the duct, you are going to need quite a few supplementary materials. Expect to pay $25-$50 for the supplies for every 1.000 square feet of insulation.

  • Labor costs

You can attempt doing the job yourself. It isn’t extremely challenging and won’t cost you a penny. However, if you are not used to working with such materials, you should expect to spend a wagon of time on the job.

When it comes to professionals, you will be asked to pay up to $0.80 per square foot for their services.

Tip: you can attempt completing the job on your own if it’s a small project. For larger tasks, experts recommend hiring professionals.

Even though the installation process might cost you a lot of money, in the end, it will definitely be worth it as high-quality duct insulation will help you save a small fortune on your electricity bills.

Ductwork Insulation Thickness

As a rule of thumb, the more extreme climate you live in, the more insulation you should add. People living in areas with freezing cold winters and boiling hot summers should consider going for the thickest insulation.

Moreover, it will make more sense to add a thicker layer, if you are planning on living in this particular house for decades. This will help you avoid a lot of insulation-related problems in the future.

You should plan on adding anywhere from an inch to 3 inches of insulation to your ductwork.

1-inch insulation gives you an R-value equivalent to 1.9.

1.5 inches – R 3.5

2.5 inches – R 6.0

3 inches – R 8.0

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 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 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. 

Ductwork Insulation in Different Locations

How to Insulate Ductwork in Attic

To be completely honest, you should try to avoid installing ducts in a vented unconditioned attic. In case ‘the damage had already been done’, move the ducts inside your home’s thermal envelope.

Your home has ducts in the attic if there are vents in your ceiling. And in case you are planning on leaving the ductwork there, then you would definitely need to insulate the system.

Installing insulation in unconditioned areas might be a bit tricky as you need to avoid ‘sweating’. Due to the drastic temperature changes, condensation might appear on the ductwork and that is something that must be avoided at all costs.

The first thing that you have to do is find and repair any leaks. After that, you can begin insulating using one of the methods that we have already mentioned above or you can try going for blown-in cellulose insulation: 

  • Ensure that the ducts are located on top of the rafters.
  • Use a quality hose and blowing machine.
  • Try to fully cover the ducts with cellulose.
  • If there are any uncovered areas left, you can use insulating blanket material.

Insulation Around Ductwork in Ceiling

There are quite a few disadvantages to having ducts in the ceiling. Heat rises and if it starts in the ceiling, it doesn’t really have anywhere to go. As a result, you are using your furnace inefficiently. 

However, if you have ducts in the ceiling and you know that the space is well-insulated, there might be no need in insulating the actual ductwork

Should I Insulate Ductwork in Basement?

The main thing that you should bear in mind is that insulating the ductwork in the basement will make the space much colder. If you have plenty of drains and water pipes running through the basement and you live in an area with super cold winters, then the pipes might freeze and burst in the winter.

To avoid that, you can use an electric heating tape wrap and apply it to the pipes. Or you can consider insulating both the basement walls and the ducts.

Insulating Ductwork in Crawl Space

Just like your attic, the crawl space is an unconditioned area with great temperature fluctuations. You should definitely install furnace ductwork insulation as this will help you save energy and money (including cash saved on furnace maintenance and repair). Insulation, if installed properly, also does a great job at preventing condensation.

You might want to consider hiring a professional to insulate your crawl space as it is challenging to work in such a tight space. 

The most common type of insulation used for the crawl space includes foil-faced blanket and spray foam insulation.

Outdoor Ductwork Insulation

You should consider installing exterior ductwork insulation or insulating any outside ducts with a minimum of R-8.0 insulation. In case you live in climate zones 5-8, a minimum of R-12 is required.

Image source: https://polyguardproducts.com/mechanical/weatherproof-jacketing/

A lot of outdoor duct systems come pre-insulated. If you want to add an additional layer, you would have to make sure that the material is durable, tear-resistant, and weatherproof

To Sum Up

This was everything that you need to know about ductwork insulation. 

In a nutshell, if you can install insulation in your ducts – do that as there will be plenty of benefits. The main advantage is, of course, energy savings.

In case you need to insulate a small area in your house, you might be able to do that without the assistance of a professional. No matter what space you have decided to tackle, the main guidelines would be pretty much the same:

  • Find and seal the leaks
  • Clean the surface and make sure that it is dry
  • Wear personal protective equipment
  • Carefully measure and cut the insulation material
  • Wrap it around the ductwork and secure the seams or simply apply your foam insulation

Insulating your ductwork is definitely worth it if done properly. So, make sure to take your time.