Fix it Fred: HVAC Duct Sealing

According to a recent Consumer Reports magazine, between 10% and 40% of HVAC airflow in a house can be lost through poorly sealed ductwork. That seems like a big number, but I have seen similar numbers reported in other sources as well.

After seeing this, I decided it was time to take on the project of sealing the ductwork that was accessible in our basement ceiling. I was not going to worry about the portion inside the finished ceilings, but anything in an area with no ceiling, or a drop ceiling, was on my list.

mastic and tape
the mastic and tape I used

I used two main sealing products for my project. The first is mastic paste; it normally comes in either a can or caulking tube. The other thing I used was foil tape (duct joint tape), not duct tape. Ironically, duct tape is a not an ideal choice for sealing ductwork. Duct tape will, over time, become brittle and lose its adhesive ability; no longer sealing the duct. After it has deteriorated, it is difficult to remove and replace.

old duct tape joints
old duct tape joints
more old tape
more old tape

Before I began the sealing process, I took inventory of all the gaps and cracks in our ductwork so I could estimate how much material it would take to complete the project. One Item to note: there are often more air leaks in ductwork than just the seams between pieces. I found patches covering locations where old dampers had been, and even a couple of registers hidden in the ductwork. These areas needed sealing as well.

this needed further investigation
this needed further investigation, it looked like suspicious

In addition, the plenum and transition pieces around a furnace and its filter can have lots of seams, gaps, and holes. These duct pieces may be large, and it can be easy to assume they are part of the furnace and should not leak.  This may not always be the case.  Another easily forgotten area of leaks is the return air ductwork. While these leaks are less significant in maximizing energy efficiency than other areas of your HVAC system, they are still worth sealing. Drawing return air from the correct locations, not only where the leaks let it in, is crucial for proper ventilation.

don't forget all these joints
don’t forget all these joints
return air duct can leak too
return air duct can leak too

My first priority was to seal as many of the seams and gaps as possible with mastic, since it dries to the touch, but remains pliable and can flex as the duct shifts. For easier application during my project, I chose the variety that comes in a caulking tube. After I applied a bead of the paste to the seam, I used a small piece of stiff cardboard to spread it evenly over the desired area.

smoothing the caulk with cardboard
smoothing the mastic with cardboard

For the spots I was not able to seal with mastic, I used the foil tape. In some areas, the duct was too close to the floorboards or joists to work around it with mastic and other joints were in nearly inaccessible locations. In these spots, I was able to thread the foil tape into position and then remove the backing. It may not have created a perfect seal, but it is far better than it before I began. After the mastic dried for a couple days, I went back and felt around the areas I had sealed to see if there were still any air leaks. If there were, I re-sealed those spots.

joint sealed with mastic
a joint sealed with mastic
a joint previously sealed with tape
a joint previously sealed with tape

Overall, the joints in the ductwork in our house were in good shape and relatively tight. However, a few joints had finger-sized gaps in them, mostly due to dents and improper fit-up. I can only imagine how much air leaked through holes like that. Some of the larger holes took many applications of mastic to effectively seal. My hands got cold while filling the gaps from all the air blowing on them in some of the worst areas. Note to self: fill gaps in the winter when the furnace is running with warm air next time.

large gap after multiple mastic applications
a large (1/2″) gap after multiple mastic applications

While I was only able access and seal some of our ductwork, every little bit will improve the airflow to the registers and allow us to better control the heating and cooling in our house. There are still more projects to do in our home so the HVAC system runs more efficiently, but this is a good first step. We still have windows and doors to update with more energy efficient models, and some other ductwork issues to address.  This sealing project is just a simple beginning, though, and a relatively inexpensive start to the process.


I enjoy fixing (most) things and extending the useful life of stuff as long as possible. I find the challenge of diagnosing and remedying problems mentally stimulating and gratifying. My family is more than willing to oblige me this pleasure. In fact, I am often greeted with a pile of broken items when I walk in the door after work. They have even created a designated spot on a shelf in the kitchen to store their damaged goods. Some may call me strange for enjoying what others call mundane; not my family, though. They call me Fix-It Fred.