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Should Range Hood Duct Be Insulated

When you are paying to install a range hood and its ducting, the cost very quickly climbs, particularly if you are not able to install it yourself and you have to pay for labor. So, when faced with the choice of cheaper, uninsulated ducting and its more costly insulated counterpart, it can be very tempting to choose the former. Perhaps it’s not even you who are making this choice, it was the house’s previous owners, and now you are left wondering if your house is up to code.

The international and local codes governing your area will probably cover range hood duct insulation. They could specify if and when it is mandatory, prohibited, or up to you. But at the very least, it should detail the standard of insulation to be met when it is installed. Here we look at the International Residential Code. While international, this code is not universal. However, if it does not apply to you, there are reasons why you should insulate your ducts, even if you don’t have to.

According to the IRC, range hood ducts are only required to be insulated if they run outside of the house. Insulation is prohibited where the ducts pass through fire-blocked walls and floors. While insulation is not mandatory, it is beneficial to minimize noise, corrosion, mold, dripping, and energy loss.

Range Hood Duct Insulation Code

When installing a range hood duct, you must make sure to look at the regulations pertaining to the insulation codes. The IRC does have rules in place that cover the installation of range hood duct insulation if you decide that you would like to install it. These can be found in both Section M1601.3 and Section M1601.4.6

Minimum Insulation Requirements

With the exception of one circumstance in which insulation is mandatory and one in which it is prohibited (I will explain these shortly), there is no specific rule requiring you to install the insulation, meaning that it is up to you to decide what you want to do.

While the easy and cheap route may be to omit the insulation, later in this article, you will see that there are quite a few reasons why you should rethink leaving the duct uninsulated.

Duct Insulation Installation

Section M1601.4.6 of the IRC covers the installation of duct insulation. There are three requirements listed, but the first applies to cooling supply ducts, which means that it is irrelevant to range hood ducts. Below are the remaining two requirements:

If the ducting extends outside, then this system must be protected against the elements, i.e., it must be insulated. This is the only time when range hood duct insulation would be a requirement.

Range hood ducting that passes through fire-blocked walls or floors cannot be insulated. The reason for this is that the walls or floors are designed to stop the spread of fire. If they are traversed by a flammable (albeit one with a low fire spread index), then the purpose of the wall or floor is negated. This is the only time when range hood duct insulation is prohibited.

375_Rooms-Kitchen_Duct Penetration of Fire Partition 2

Insulation Materials

Section M1601.3 of the IRC covers the requirements that duct insulation materials must follow. If the material is listed and labeled for use as insulation for range hood ducting, then you know it meets all of these requirements.

Flame Spread Index and Smoke-Developed Index

Insulation must be tested for how easily the material spreads fire (flame spread index) and how much smoke it produces as it burns (smoke-developed index).

The tests must be standardized, carried out according to the methods in ASTM E84 or UL 723 and ASTM E2231.

For the insulation to be code-compliant, it must have a flame spread index of 25 or less and a smoke-developed index of 50 or less.

Ducting runs through portions of the house. Should it catch alight for some reason, it is vital that it will not conduct these flames rapidly. If it did, the fire would quickly spread to many areas of the house, reducing the chance of the blaze being doused before it does extensive damage.

Similarly, if a lot of smoke is produced when the insulation burns, then this smoke can cause damage and loss of life far more rapidly than a material that produces less smoke when it burns.


You can use spray polyurethane foam on the outside of range hood ducts where they pass through attics and crawl spaces if the following conditions are met:

  • Flame spread index < 25 at required installation thickness.
  • Smoke-developed index < 450 at required installation thickness.
  • Ignition barrier requirements of IRC Sections R316.5.3 (foam plastic requirements in attics) and R316.5.4 (foam plastic requirements in crawl spaces) are met.
  • IRC Section R316 (foam plastics) requirements are followed.

Temperature Test

The insulation material must undergo the temperature test outlined in ASTM C411.

For the duration of the test, the temperature must match the temperature to which the insulation will be exposed during use. However, it cannot be less than 250℉ (121℃), so if the in-use temperatures are lower than this, 250℉ will be the test temperature.

To pass the test, the insulation material cannot catch alight or even glow, smolder, or smoke.

Correct Labeling of Insulation

External duct insulation must have the following clearly indicated at least every 36” (914 mm) along the insulation:

  • Type of insulation.
  • Name of the manufacturer.
  • R-value for the installed thickness (installed thickness must be determined in an approved manner as dictated in this section of the code).
  • Flame spread index.
  • Smoke-developed index.

While this may seem like a pointless regulation, it is important to ensure that all portions of the insulation are up to code, particularly in situations where insulation has had to be removed and replaced.

Local Codes May Differ

As always, while the IRC may not require range hood duct insulation, you also have to consider what the local codes of your area may say in case they have amended or added regulations regarding duct insulation.

Furthermore, the IRC does not apply to all countries. For example, in Canada, the IRC does not apply. Instead, they use the National Building Codes of Canada along with province-specific local codes, such as the Ontario Building Codes. As per these codes, range hood ducts must be insulated if they pass through unconditioned spaces.

For more information, check out Is a Range Hood Required by Code in Ontario.

Insulation Is Recommended

If it is not a requirement of your local building codes to insulate your range hood duct, should you?

Range hoods themselves are not always a requirement in kitchens. For example, in Ontario, you only need to ensure that there is some form of mechanical ventilation, not necessarily a range hood. Another example is North Carolina, where you don’t need a range hood if your stove has an integral exhaust system.

Despite this, having a range hood brings manifold benefits, making these appliances highly recommended by most industry professionals.

Similarly, while insulation may not be required to ensure building code compliance in homes that have ducted range hoods, it can be highly beneficial to homeowners.

Benefits of Insulating Range Hood Ducts

There are many benefits to insulation range hood ducts, including but not limited to:

  • Reducing noise
  • Reducing condensation
  • Saving energy (and therefore money)

Reducing Noise

Certain HVAC systems, ducted range hoods included, are known to make quite a bit of noise when in use. You may be able to tell yourself that the constant hissing, banging, thumping, rattling, clicking, and buzzing does not bother you, but we both know that is not the truth. 

All the extra noise can add to the pressure and stress surrounding the dinner preparation rush. It can also wake up your sleeping baby when you desperately need both hands to do the cooking. Why make yourself suffer through all these noises when there is an easy solution: duct insulation.

This insulation around range hood ducts can act as padding to reduce the amount of sound created by the ducts and as a dampener to absorb the soundwaves originating elsewhere but being transmitted by the ducts. Overall, this minimizes the sound output by the exhaust system.

If your range hood is making a noise when the weather is windy, regardless of whether the range hood is on, then duct insulation is one of the potential solutions available to you.

Reducing Condensation

Over time, condensation builds up in the range hood ducts due to the difference between the temperature of the gasses passing through the ducts and the air temperature outside the ducts.

Condensation may not seem like it would be a big deal; after all, it is just water. However, this is far from true when talking about ducts.

Firstly, these ducts are constructed of galvanized steel or aluminum rigid sheet metal. While these are designed to withstand extended periods of exposure to moisture before rusting, they are not rust-proof, especially at points of weakness, such as where nails or screws secure the duct.

Constant exposure to large quantities of condensation will hasten the corrosion process and significantly shorten the lifespan of your ductwork.

Another issue with water collecting in ducts is that it creates ideal environments to promote the growth of mildew and mold, which is detrimental to the health of the house’s occupants, both human and animal. According to the CDC, mold can cause fever, shortness of breath, and is extremely harmful to children. 

Once condensation develops within the ductwork, it can also run along the ducts and drip out of your range hood. This can result in water damage to walls, cabinets, stovetops, flooring, etc., which is costly to repair. The water will also contain grease and other cooking by-products, which are smelly and unhealthy.

Insulating your range hood ducts reduces the chance of condensation formation because it mitigates or eliminates the influence of the air outside of the ducts on the air inside of the ducts.

Saving Energy

For those of you debating whether to insulate your ducts based on the price of insulation, do not forget to calculate the energy savings.

According to Energy Star, in uninsulated ducts, anywhere from 20%-30% of the air that is meant to be moved through the system will be lost due to leaks, holes, and poor connections. Even when the air itself cannot escape from the ducts, heat can be transferred to the pipes and then to the surrounding air.

This air and heat loss is a relatively small amount, but it makes a big difference. It will result in an inability to keep your house comfortable no matter how your thermostat is set and a much higher electricity bill (without you even being able to enjoy the temperature you are paying for!).

Insulating your ducts will easily remedy this issue, allowing you to save money in the long run, and more importantly, enjoy the comfort of your home. 

Best Insulation Materials

When it comes to insulation, there are many materials to choose from, but what is the best material? There are a few good options to choose from, including:

  • Aluminum foil
  • Spray polyurethane foam
  • Fiberglass 

Both aluminum foil and spray polyurethane foam are great under specific circumstances. For example, spray polyurethane foam is best when used in places where vents go through unconditioned spaces since it will not allow for any water vapor to permeate the vent.

However, the best all-around material to use is foil faced fiberglass insulation. There is a wide variety of this insulation available on the market, and it is sold in many different thicknesses and R-values.

Please note that you can purchase this in both rigid and flexible ducting. However, only rigid ducting can be used for range hoods.  



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