Sealing HVAC ducts can help eliminate air leakage through air ventilation duct seams, plus minimize or prevent ambient foreign particles and gases from entering the return air vents.

Understanding Duct Joints and Seams

A typical HVAC ductwork configuration consists of large rectangular trunk lines connected directly to your furnace: one forced air, and the other return air. Then round rigid galvanized metal piping branches off to different sections of the home. Note: Proper duct sizing calculations are configured by a professional and educated HVAC technician.

Trunk Lines (i) Each section of the main trunk will have folded over edges at each end, and then use what is called cleats, that slide up and pull the ducts together.

HVAC Duct Trunk with Cleat Connection
HVAC Duct Trunk with Cleat Connection

Round Galvanized Pipes (i) Each connection is done with a slip joint, and accomplished by one end of a pipe or fitting being crimped to make smaller. (ii) The first step on the round piping is to add 2-3 sheet metal screws around the perimeter to secure the connection. Proper screws like Malco; which can found at Home Depot; or equivalent sheet metal specific screws are to be used. The bottom half is the most important, as this is where the weight is, and the screws strengthen this area from sagging.

Round Duct and Floor Boot Connection
Round Duct and Floor Boot Connection

Sealing HVAC Ducts with Tapes & Mastic

  • Duct Tape – Duct tape may share the same name as duct work, but it’s not a suitable product. The sealing properties are not great, and the tape will dry out from the heat; eventually leading to deterioration. Note: The timeframe that this happens will vary on what kind of duct tape was used and how often the furnace is pushing hot air.
  • Foil Backed Tape There are various sizes, brands, and types available; and several can be found at your local hardware supply store. A much better alternative to plain duct tape, but still not superior because of the thin level of adhesive backing. Better used on foil covered insulated flex duct.
  • Duct Mastic – Comes in either a tube, just like caulking, or a plastic container much like joint compound, and looks similar as well. Apply with a brush, putty knife, or wear some type of rubber based gloves to spread it on by hand. The mastic will dry and seal any gaps between the ductwork seams, while remaining pliable. Readily available at most local hardware supply stores in the HVAC section, or amazon.
  • Foilmastic Tape – The Polyken 367-17 by Berry Plastics is a 2mm thick butyl rubber adhesive tape with a foil backing. This is our preferred choice, since it’s easier to get around joints in pre-installed ductwork, and it’s less mess than the duct mastic, while doing a great job at sealing. Plus it’s 3″ wide compared to more basic foil tapes that typically come in at 2.5″ or narrower.
  • If there is existing duct tape, and it’s not coming off too easily, you may consider sealing over it.
  • When considering the horizontal seam where the round galvanized ductwork locks together; you’ll have to decide if you want to go through the extra time and expense to seal that area as well.