If you need to replace the ductwork in your home or you’re building a new home, you should think carefully about what type of ductwork you want to use. There are four main types of ductwork commonly used in HVAC systems: sheet metal ducts, flexible ducts, fiberboard ducts and fiberglass ductboard. The type of ductwork you choose will affect how much the project costs and can also have an impact on the effectiveness of your HVAC system and the air quality in your home. For these reasons, it’s important that you understand the pros and cons of each option to help you determine which one is the best choice.
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Sheet metal ducts are the most common type. In the majority of homes and businesses in the US, most or all of the air ducts are built out of aluminum or galvanized steel sheet metal. Sheet metal ductwork is constructed by cutting and bending individual pieces of metal to create each duct branch. All of the branches are then installed and joined together, and then each joint is fully sealed with foil tape or mastic to ensure the entire system is airtight and doesn’t leak.
Sheet metal is one of the most expensive ductwork options. All of the ducts are usually built in a sheet metal shop and then delivered to the site for the technicians to install. This means that sheet metal ductwork takes more time to build and install since the other options can typically be built on-site.
The only other real drawback to sheet metal is that it is a poor insulator. If you have sheet metal ductwork in an unconditioned area like an attic or crawl space, the outside of all of the ducts needs to be wrapped with insulation. If not, the sheet metal will absorb lots of heat in the summer and cold in the winter, which will make your HVAC system much less effective and lead to it using much more energy.
Despite the higher cost, sheet metal has numerous advantages over the other types of ductwork that typically make it the best option. Sheet metal ducts are extremely durable, as the metal used to build the ducts is fairly thick. This makes sheet metal ducts less prone to damage and leaks compared to the other options. The smooth metal surface also allows air to easily flow through the ductwork, which helps to improve the effectiveness and energy efficiency of the heating and air conditioning system.
Sheet metal is also non-porous, which makes it less prone to mold growth than other ductwork options. Any type of ductwork will collect dirt and other debris over time and should be professionally cleaned every few years. However, sheet metal ducts tend to collect less dust and are much easier to clean than the other options.
Flexible or flex ducts are made of a coiled metal wire that is coated in a thick sheet of flexible plastic. It is extremely rare for all of the ductwork in a home to be made from flex duct, as at least the supply and return plenums that connect to the air handler and furnace need to be made from some type of rigid duct material.
Flexible ductwork is most commonly used for things like exhaust vents and attic fans. It can also be used for normal air ducts, but this is typically only ever done in situations where there isn’t sufficient room to install rigid ducts. If you’re installing something like a whole-home air filter or air purification system onto your HVAC system, flex ducts will typically also be used to connect it to your existing duct system.
Flexible ducts are the cheapest option and the easiest to install. However, they are also prone to sagging and kinking, which can interfere with airflow and make the HVAC system much less effective and less energy efficient. It is also extremely easy to puncture the plastic coating on flex ducts, which means they are also more prone to leaking than rigid ducts.
Fiberboard ducts are made of stands of fiberglass that are embedded within a special resin. The outside of the ducts is also covered with foil that acts as a moisture and air barrier and helps to keep the duct better insulated. The inside of the ducts also has a special coating that fully seals the fiberboard to prevent any of the fiberglass strands from circulating in the air.
Fiberboard ducts are quickly becoming one of the most popular options and have a number of advantages over sheet metal. One major advantage is that fiberboard ducts are much better insulated to prevent heat loss and heat gain as the air flows through the ductwork. The insulation also has acoustic properties, which means it limits the amount of noise you hear when the HVAC system is running. This noise reduction is why fiberboard is so popular for commercial and industrial buildings.
Fiberboard is also less expensive and easier to install compared to sheet metal. Fiberboard ducts can easily be fabricated and the sheets cut to fit smaller areas or places where it would be difficult to install other types of ductwork.
The biggest issue with fiberboard is that the inside surfaces of the ducts are somewhat rough. This means that fiberboard ducts will trap much more dust and debris, which can negatively impact indoor air quality. Fiberboard ducts are also much more difficult to clean compared to sheet metal. The rough surface also creates more airflow resistance, which can make the HVAC system somewhat less effective. Fiberboard ducts are also much more prone to mold and mildew growth, which means they’re generally not recommended for use in humid climates.
Fiberglass ductboard consists of standard sheet metal ducts that have a fiberglass lining that serves as an extra layer of insulation and also helps to dampen noise. The lining also helps to prevent condensation from forming on the inside and outside of the ducts. Fiberglass ductboard tends to be the most expensive option. It is most commonly used in cold climates, as it is better at preventing heat loss compared to the other options.
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The biggest problem is that this type of duct is that it tends to trap lots of dust, allergens and bacteria. This means it will need to be cleaned more often, and it is also much more difficult to clean than any other option. Another issue is that fiberglass ductboard doesn’t last as long. If not replaced soon enough, the fiberglass will start to deteriorate and release lots of particles into the air. This is a major health concern since exposure to fiberglass can irritate your eyes, skin, throat and lungs, and some studies have also linked fiberglass to an increased risk of lung cancer.
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In the U.S., over 76 million homes rely on central HVAC systems, making the duct system a critical part of the nation’s energy infrastructure. With an average home using 300 feet of air ducts, residential installations alone exceed 800,000 miles—enough to wrap around the Earth more than 32 times.
Given this scale, choosing the right type of ducting is essential. It directly affects airflow efficiency, insulation value, labor costs, and the comfort of building occupants. While sheet metal and flexible ducts remain common, fiberglass duct board is gaining traction—especially in residential and light commercial settings—due to its built-in insulation, ease of installation, and cost-effectiveness.
This guide will examine what fiberglass duct board is, how it compares to traditional ducting, and where it fits best in modern HVAC applications.
Duct board provides continuous thermal insulation across all duct surfaces, minimizing thermal bridging and leakage. Most formulations meet or exceed ASTM E84 and NFPA 90A/90B standards, ensuring compliance with U.S. building codes for flame spread and smoke development. The non-metallic structure inherently reduces vibration noise, while the sealed, foil-lined exterior offers moisture and condensation resistance, especially in humid environments. Additionally, the lightweight design eases handling and reduces the need for structural support in ceiling installations.
Compared to traditional ducting, duct board systems are quieter, lighter, easier to install, and more thermally efficient—making them a preferred solution in residential and light commercial duct systems, where energy codes, installation labor, and indoor air quality are closely scrutinized by both contractors and building occupants.
Duct boards are manufactured in several structural configurations to meet different installation and durability needs. Some panels are single-sided, with one laminated surface—typically used when the duct is mounted flush against structural elements or walls. Others are double-faced, where both sides are factory-laminated for full vapor barrier protection and enhanced mechanical strength, particularly important for suspended or exposed ductwork. High-performance duct boards may also feature multi-layer composite facings, combining aluminum foil, woven fabric, and polymer coatings to meet stringent requirements for hygiene, durability, or chemical resistance.
In terms of fabrication, most duct boards are supplied in flat panel form and cut on-site using the score-and-fold method, allowing installers to shape and seal ducts quickly with minimal tools. This approach offers excellent adaptability to on-site conditions and irregular spaces. Alternatively, some systems—particularly those using PIR or phenolic cores—are designed for modular prefabrication, where duct sections are fabricated in a controlled environment with pre-formed elbows, transitions, and joints. These pre-engineered systems are especially advantageous in large commercial projects or fast-track construction timelines.
Proper installation is essential to duct board system performance and code compliance. Most assemblies—particularly those using fiberglass duct board or phenolic foam—are fabricated using the score-and-fold method. Panels are cut with grooving tools or slot cutters to create V-grooves, allowing clean folding without damaging the facing. Joints are sealed using UL 181-approved foil tape, mastic, or mechanical fasteners, depending on system pressure and project requirements.
Different core materials require tailored handling. Phenolic boards need precise scoring to avoid cracking, while MgO-based boards require dust control and carbide tools due to their density. Fiberglass cores are the most installer-friendly but still demand proper sealing, especially in humid environments. All seams, corners, and take-offs must be sealed tightly to ensure airtightness and energy efficiency, with vapor-barrier reinforcement recommended for rooftop or high-humidity areas.
Duct board is best suited for low to medium pressure systems (up to 2″ WG). For long runs or higher pressure zones, supports like hangers or saddles must be added at regular intervals. Installation should follow SMACNA’s guidelines for fibrous glass ducts for joint construction, reinforcement, and layout best practices.
Compared to sheet metal, duct board reduces labor and eliminates external insulation. It is ideal for retrofits, confined plenums, and projects where acoustic performance and installation speed are key. Common mistakes—like under-sealed corners or over-bent joints—can lead to leakage or structural failure and should be verified through pressure or smoke testing.
When installed correctly and in accordance with ASHRAE 90.1, IECC, and local codes, duct board systems offer reliable thermal insulation, air control, and indoor air quality performance.
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