Duct Sealants and Tapes

What it is

Duct sealants close joints and seams so conditioned air stays in the duct. The two families are tapes (foil or film with adhesive) and mastics (thick paste brushed or troweled on, often with mesh in wide gaps). Codes require closures on duct board and flex duct to be listed to UL 181A or UL 181B. Sheet metal joints are usually sealed with mastic, listed tape or gaskets as the code and project specs call for.

How to identify

Listed tapes have the listing printed on the tape backing at regular intervals, such as "UL 181A-P" or "UL 181B-FX". Mastic containers show the listing on the label, such as "UL 181A-M" or "UL 181B-M". If the backing does not show a 181 marking, assume it is not listed.

Listing markings

Key points

• Match the marking to the duct type. A 181A tape is listed for duct board, not flex.
• Clean, dry and dust-free surfaces are needed for tape to stick. Cold surfaces are a common cause of failed tape.
• Heat-activated tape needs the right iron temperature and pressure to bond. Watch for the color change dot where used.
• Mastic is often the more durable choice for sheet metal and for joints that will be hard to reach later.
• Mesh tape embedded in mastic bridges wider gaps. Follow the maker's gap limits.
• Many energy codes call for duct leakage testing. Good sealing is what makes the test pass.

Common mistakes

• Using cloth duct tape on ducts. It dries out and lets go.
• Taping flex duct inner liner without a clamp, or clamping without sealing.
• Taping over dust or condensation.
• Using a mastic not rated for the duct's operating temperature or for outdoor exposure where needed.

Governing standards: UL 181A (closure systems for rigid air ducts); UL 181B (closure systems for flexible air ducts and connectors); International Mechanical Code and energy code duct sealing provisions; SMACNA seal classes for sheet metal.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.