David Rowlett, the owner of Dracut, Massachusetts-based Ductworks, Inc. has sheet metal in his blood.

"My grandfather started his own sheet metal company in 1946 and I grew up in the sheet metal shop from a child," he says. "I was born to do this. My father was a union tin knocker and won all sorts of awards for layout in the late '60s, early '70s. I was born and bred to do this."

Rowlett, who has had his own business since 2003, knows that in order for HVAC units to work well, they need to be properly sealed. After all, leaky ductwork can lead to energy loss and issues with air quality. 

"The air will be leaking into areas you don't necessarily want it to leak into. It can leak into your basement or your crawl space," he says. "It can leak into the attic and then it can go right out to your roof and into the atmosphere. It's not actually helping the house either heat or cool."

In order to seal an air duct, Rowlett suggests using mastic. This water-based sealant is used for sealing the fabricated joint and seams of air ducts. While mastic can be messy, it's a far superior sealant than duct tape, which is quick and easy to use, but eventually loses adhesion and falls off. 

"It's almost like wet concrete," Rowlett says of mastic. "It's a little bit different consistency but it's easy to apply because it comes to a bucket and you can brush it on with a brush."

Some mastic is sold in a caulking gun, which is used more for a finer area that you may need to get to.

Before applying the mastic, Rowlett says to make sure you're starting with a clean, dry surface. Wipe off any dust or dirt from the joint between duct sections. 

"What I like to do is when I'm putting my duct work together, I take my paint brush and I put a dab in each corner where the ductwork goes together in the connections." 

From there, you'll brush it on. Make sure to apply enough mastic to form a continuous coating on the duct. You want to do your best to keep it neat, especially if you’re working on ductwork that is going to be exposed. 

Mastic takes about 24 hours to dry, Rowlett adds. "A lot of times when we are putting in a new system, we seal it and we leave the actual place where we seal it with the mastic open for it to dry overnight and then we go back the next day," he says. "A lot of times we do that for the inspector to see it has been sealed — to make sure it's up to code."