Why insulate and seal ductwork of HVAC?

It is important to note why ducts are insulated and sealed. Ducts can be insulated and sealed for sound control (acoustics), but they are also insulated and sealed for thermal performance, including controlling heat gain or loss, maintaining uniform heating or cooling throughout the air-handling system, and preventing condensation.

Duct insulation and sealing, especially insulated supply ducts delivering conditioned air within a building, save energy. Duct insulation and sealing are to keep mechanically warmed or cooled air as close to a constant, desired temperature as possible and prevent the conditioned air from escaping the duct system while it is being moved to spaces where it is needed.

DIY: How to Insulate and Seal Your Ductwork


Insulating and sealing ductwork increases the efficiency of your home by better maintaining its temperature. In winter, good ductwork helps to prevent heat from leaving the home. In summer, it stops heat from coming in. Overall, sealing and insulating your ductwork can lower energy bills, potentially saving hundreds of dollars a year. Like any home improvement project, there is  time and money involved, but it’s nearly always a worthwhile investment.


Find the Ductwork in Your Home


Attics, crawlspaces, and garages are common places to install ductwork. Builders may also place them in basements or ceilings, which are typically temperature-controlled, negating the need for much insulation. Ductwork looks like the stereotypical boxy, gray metal tubes seen in movies. Granted, you’ll be hard pressed to find ductwork large enough to crawl through to reach a destination unnoticed.


If your home was built before 1990, stay away from flaky, gray pea-sized insulation because it may be hazardous to your health. Contact a local health department to ensure the insulation is asbestos-free before disturbing it.


Test and Seal Your Ductwork


Insulation aside, it’s necessary to seal your ductwork before proceeding. Ductwork can be punctured, slashed, and otherwise rendered ineffective. Experts recommend performing a blower test to detect pinpricks and other hard to find leaks. Unfortunately, these tests cost anywhere from $100 to $250. You can skip the test if it’s unaffordable but keep it in mind for the future. If you’re upgrading the ductwork in a home so much that it requires a permit, you may be obligated to pay for a blower test anyway.


To seal ductwork, use mastic sealant or metal tape to cover all areas where the ductwork meets a wall, ceiling, or floor. Pay attention to the joints and where the builder originally sealed the ductwork. Again, don’t use regular duct tape because it will fail much faster than tape meant to be used for sealing ductwork.


Old ductwork may need replacing outright, especially if it is older than 15 years and showing obvious signs of wear. The longevity of ductwork depends on the quality of materials used, in addition to whether the ductwork was exposed to the elements, animals, or other unusual circumstances. 


Choose a Type of Duct Insulation


Duct insulation comes in two categories: blankets and sleeves. Sleeves require disassembling the ductwork, sliding the sleeve onto the duct, and then resembling it. Blankets are quicker to install because they wrap around the ductwork without need of dismantling. Some people purchase sleeves, slice them open, and then secure them to the duct. If you plan to do this, first consider purchasing blanket-style insulation, unless the sleeves are less expensive.


Experts generally recommend an insulation with an R-6 or higher value. R-values measure a material’s thermal resistance. These values range anywhere from R-0.2 (bricks) to R-50 (vacuum insulated panels). Fortunately, R-6 meets or exceeds most building codes, and it’s inexpensive to boot.


Regardless of the insulation’s R-value, it must be covered in foil to ensure maximum efficiency. Finding foil insulation is easy since it’s standard for ductwork; however, some homeowners make the mistake of purchasing paper-backed insulation. Paper-backed insulation can catch fire in hot attics, because it isn’t meant for HVAC use.


Install the Duct Insulation


Now that you’ve chosen an insulation and presumably hauled it to the attic or garage, it’s time to remove the old insulation. Dispose of it rather than leaving it in the attic.


Make sure the new insulation is wide enough by multiplying the diameter of the duct by 3.14 and then adding 2 inches of room for overlap. When cutting new insulation, ensure the fit is snug but not so tight that it compresses the fiberglass of your ductwork. Secure the insulation with foil-faced duct tape, if applicable, working carefully not to leave gaps. Some insulation comes with adhesive, in which case you need only peel the paper off the tape when you’re ready to secure it.


Sealing and Insulating Ductwork: Warnings

·         Remove all other clutter before getting started. Sealing and insulating your ductwork isn’t hard, but it can be time consuming, and trying to work around boxes of storage or old junk will be maddening in no time. 

·         If you’re completing the job in the attic during warm months, take frequent breaks, keep hydrated, and work with a partner. An attic an easily exceed temperatures of 100 degrees Fahrenheit. 

·         Fiberglass insulation is a severe irritant to your skin and respiratory system. Use gloves, eye protection, and a dust mask when handling it. If you don’t have a full protective suit, at least wear long sleeves and pants with close-toed shoes.

·         Most people will find themselves improving their current setup, but in rare cases, homeowners may need to start from scratch. For example, in homes with crawlspaces, small animals can find themselves under the home and making comfortable nests in the insulation. This is especially common in manufactured (mobile) homes without brick skirt or underpinning. Metal or plastic underpinning bends or breaks, leaving the insulation vulnerable to cats, possums, squirrels, and rats. After years of abuse, the home will essentially be without insulation. In all cases, you need to ensure that your ductwork and insulation is safe from destructive animals.


Insulating ductwork is straightforward work with proven benefits. Most people can complete this home improvement by themselves, but contact a professional if you’re feeling unsure.

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