Duct tape may have many useful purposes, but it simply doesn’t pass muster when it comes to sealing HVAC system ducts.

During a recent on-site visit of a Tideland served home, energy auditor Jim Rapin inspected ductwork located in the member’s attic. The ductwork looked to be in very good condition with no apparent disconnected ductwork. However, after 16 years of being located in the attic environment the duct tape used to seal the ducts at the connections was dry, brittle, and had lost adhesion.

Duct tapes just do not last, and as a result, we get air leakage at the connections and the duct insulation is compromised. Duct leakage on the supply side is conditioned air (heated or cooled) that escapes into the attic before it enters the living space. This leads to higher energy use. It can also cause the living space to go under negative pressure, increasing air leakage across the entire building envelope. Duct leakage on the return side allows unconditioned and unfiltered air from the attic to enter the HVAC system. That can cause issues with the system coil and is certainly not the healthiest air to bring into the living environment.

The HVAC system is meant to be a closed loop where supply air is brought back through the filtered return system. The preferred method of air sealing the ductwork is with mastic paste. This is preferred over all tape products, which break down and lose their adhesion over time.

NC residential building code now requires duct testing in all new homes. If the result of the testing is less than or equal to 5 CFM25/100SF for a total duct leakage test, or less than or equal to 4 CFM25/100SF for a “duct leakage to the outside” test, then the HVAC system air tightness is deemed acceptable. To achieve passing scores, more and more HVAC contractors now utilize mastic paste and consumers would be wise to insist on its use. In fact, we recommend stipulating the use of mastic paste in the HVAC quote to make sure the contractor understands you are serious about duct leakage.