How to Reseal a Flexible Duct

What You'll Need
Duct Mastic
Metal-backed Foil Tape
Metal Scraper
Tie Wrap or Plastic Collar Tie
Crimping Tool
Duct Connector
Sheet Metal Screws

A flexible duct is prone to air leaks, especially in the areas where it is attached to another flexible duct (or rigid duct) or to a rigid duct elbow. When the HVAC connected to the ductwork fails to work efficiently, check the ductwork for any signs of damage or leaks and reseal it as soon as possible.

Step 1 – Assess the Air Tightness of the Ductwork

This can be done by visual inspection. Check the entire length of the ductwork to check for any poorly sealed ducts. Check the seams of duct elbows to see if the mastic has become loose. Inspect duct to duct connections as well and see if they have been sealed with duct tape. Ordinary duct tape will not seal the ductwork efficiently. It is recommended you use metal-backed foil tape for best results.

For maximum assessment, contact a professional to inspect the duct work completely or do it yourself. Rent a blower test door, read the instructions on the manual and perform the test. For a low fee, contractors can do the job as well.

Step 2 – Resealing flexible Duct to Rigid Duct or Elbow

Scrape away the loose mastic on the seams using a metal scraper to reseal the connection between the flexible duct and a rigid duct or elbow. Remove the debris using a rag. Apply duct mastic on the seams using a paintbrush.

Remove the plastic collar tie securing the flexible duct to the elbow. Peel back the insulation and vapor barrier about 8 inches away from the connection but do not cut it off or remove it. Remove the duct tape around the ductwork.

Re-attach the flexible duct liner to the elbow or rigid duct. Reseal the connection by wrapping a metal-backed foil tape a couple of times until the connection is secure. Pull back the insulation and vapor barrier to cover the connection. Wrap a plastic collar tie over the insulation and tighten it with a crimping tool to secure the connection.

Step 3 – Resealing Duct to Duct Connections

Locate all connections joined together using a duct connector. To reseal leaky connections, remove the tie wrap over the connection. Peel back the insulation and vapor barrier on both ducts about eight inches from the connection. Remove the duct tape and any metal screws mounting the duct to the metal connector. Replace the metal connector with a new one.

Attach one duct liner to the metal connector. Secure the duct to the metal connector by drilling in the sheet metal screws. Reseal the duct liner to the metal connector by wrapping around metal-backed-foil tape. Slip the other duct’s liner into the metal connector until it meets the other duct. Secure it to the metal connector by drilling the sheet metal screws. Wrap metal-backed foil tape over the connection and pull back both insulations and vapor barriers. Wrap the insulation with a tie wrap.