How to Shop for Sealing & Insulating Ductwork

By Vernon Trollinger, October 30, 2017, Energy Efficiency, Home Improvement, Save Money

While most homeowners love their homes, not every home is perfect. There are one or two things the homeowner would like to change, such as an outdated bathtub for a new walk-in shower or updating the look of their kitchen. And then there’s occasional “uh-oh” of the unexpected repair. In our How to Shop for Home Improvement Projects series, we’re going to identify several projects and some of the ins and outs of what you can expect.

How to Shop for Sealing & Insulating Ductwork | Bounce Energy Blog

How to Shop for Sealing & Insulating Ductwork

Many newer homes have flex ductwork in their attic space and unless it’s been installed badly with kinks and poor connections, not much else will go wrong with it. But if your home has metal ductwork that was installed without being sealed or insulated, then you might have some work to do.

Generally speaking, metal ductwork has very tight seams —otherwise, it would come apart. But these seams and the swivel joints on elbows are not air-tight seals. Given temperature changes over time, the gaps in metal ductwork seams can expand and contract and pull wider apart. That’s one reason why homes have a 20 to 30 percent chance for conditioned air to leak out as it moves through the duct system. Sealing the duct improves both your HVAC’s performance and improves your home’s comfort — all while you save money on your utility bill.

Ductwork Sealing Materials: Mastic & Tape

The best stuff for sealing seams and joints is ductwork mastic. Cheap, gray, and about the consistency of peanut butter and tar, ductwork mastic is an acrylic compound that you can apply with a putty knife or paint brush on ductwork seams and work into crevices. Spread it along an entire seam and left to dry for 2 to 3 days and it produces a tough, air-sealing coat.

The second best thing for sealing ductwork is UL 181A or 181B aluminum HVAC tape. While its recommended use is for flex duct connections, it works fine for joining and sealing metal ductwork joints where there’s not enough roof to apply mastic.

Don’t use vinyl duct tape! Vinyl duct tape will hold ductwork together at the beginning, but lab experiments show vinyl tape’s adhesive dries out and breaks down over the first heating season. On average, vinyl tape falls apart in 1 to 5 years.

Things to keep in mind before you start sealing:

Most Texas homes have the majority of their ductwork in the attic. That means that you want to work up there when the weather is cool or cloudy. Attics are cramped and dusty because of insulation, so you also should wear an air filter mask and perhaps a pair of knee pads. Some of the common ductwork tools you also should take with you to the attic are:

  • a utility knife
  • putty knife
  • foam paint brushes
  • cordless drill with both slotted, Phillips, and small hex-driver heads
  • rags to wipe dust from ductwork seams
  • flashlight

For homeowners who want to seal and insulate ductwork that goes through a crawl space, remember that sealing and insulating your entire crawl space will reduce your home energy bills even further and keep out uninvited varmints.

How to Shop for Sealing & Insulating Ductwork | Bounce Energy Blog

Mastic Prep & Application

The ductwork must be free of dust and dry before the mastic can be applied. Wipe as much dust off of the ductwork as you can and clean off any signs of rodent urine or feces, as well. When the ductwork is clean and dry, apply the mastic to the seam with a putty knife, working the gray goop into the seams. Though manufactures say it’s ok to use a paint brush for spreading the mastic, I have found that using a cheap foam paint brush lets me work the mastic into all the crevices, seams, and crannies far easier without wasting mastic that gets trapped inside brush bristles.

Sealing the ductwork then is just a matter of spreading the mastic over all the seams on the ductwork. You’ll also want to fill in any small holes as well as all the swivel joints on elbows and boots. Depending on where a particular stretch of metal ductwork is, remember it might be easier to completely remove that section and take it outside to seal it. When it’s dry, you just reinstall it.

If you’re going to insulate your ductwork with sleeves, then you’ll want to leave some sections of ductwork dismantled while it dries.

Mastic needs to dry and cure, so it won’t be ready to handle the air pressure for between 48 and 72 hours (plan with the weather in mind). And with all the seams sealed, don’t be surprised if there’s more air pressure in your HVAC system.

Insulating Ductwork

Once the mastic is dry, it’s time to insulate. Ductwork doesn’t use very high R-value insulation (R-6 to R-8) because the conditioned air in the duct is moving. Once the heated or cooled air has finished moving through, it’s no longer a crucial job to prevent the remaining air from losing its temperature. Ductwork insulation comes in basically two forms, sleeves and wraps. And since the stuff is usually made from fiberglass, you’ll want to wear protective eyewear and gloves when working with it.

Insulating ductwork sleeves slide over the duct just like a shirt sleeve fits over an arm, so it’s very easy to put over the duct. Standard sizes (6” diameter or more) are usually available in 5 foot lengths at over $1/foot.

Duct wrapping insulation runs cheaper but you must cut it to size, wrap it around the duct, and tape it securely in place. Because ductwork layouts can be complicated with boards and wiring in the way, you’ll find that there will be instances where you’ll need to use both wrap and sleeves.

One more thing— Panned return ductwork uses spaces between joists and studs to provide airflow back to the furnace/air conditioner. It’s as easy as designating a particular joist or stud bay to be the return ductwork and covering it over with dry wall. While it’s no longer part of building code in many cities, it has been a common building practice in many states for over a century.

The reason behind the code change is that panned return ducts reduce system efficiency and effect air quality because they just aren’t sealed. Air gets pulled in through cracks and joints the entire length of the panned duct. Depending on the home and the layout of the HVAC system, that means air can be pulled into the system from places that you don’t want, such as a garage, crawl spaces, or even fireplaces. While installing sealed ductwork throughout these areas would be ideal, it’s also expensive. The best solution is to focus on sealing the areas of panned ductwork that pull in air from areas that impact your home’s air quality first. If you locate areas that pull in air from the outside, seal them. And if the job looks too complicated contact a professional HVAC company.

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About 

A native of Wyomissing Hills, PA, Vernon Trollinger studied writing and film at the University of Iowa, later earning his MA in writing there as well. Following a decade of digging in CRM archaeology, he now writes about green energy technology, home energy efficiency, DIY projects, the natural gas industry, and the electrical grid.

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