How to Repair Duct Work With Duct Mastic

Duct Mastic is used primarily on joints, fixed or flexible; but it can also be used for repair work.

Duct Mastic is a putty-like substance that is used to seal holes in ductwork, often in conjunction with mesh tape for added support. Its function is to seal the myriad holes that cannot be seen by the naked eye, not to provide any added tensile strength to the ductwork. If the damage is extensive enough, the ductwork may have to be replaced as there is a limit to the amount of surface Duct Mastic can cover without sagging and losing its sealant properties.

Things You'll Need

• Windex
• Paper towels
• Mesh tape
• Stainless steel trowel
• Wide paintbrush
• Safety goggles

Instructions

1. Turn your air conditioning unit off and look at the ductwork damage that you want to repair. If the gap is the size of your hand or smaller, it can be repaired with Duct Mastic. If the ductwork damage is greater than the size of your hand, replace that section of ductwork.

2. Clean the area around the damage and on the inside of the ductwork beside the hole. Use a clean paper towel and some Windex, making sure to not cut yourself on any jagged metal edges. Dry off the area with a separate towel as cleaning fluid can prevent the Duct Mastic from bonding with the ductwork.

3. Put on your safety goggles and use the trowel to scoop the Duct Mastic up and apply it to the inside of the ductwork damage. Apply it in a even layer around the edges of the hole.

4. Use the trowel to spread a light layer of Duct Mastic around the edges of the hole. Once the full circumference of the hole has been covered, lay down mesh tape in parallel lines across the hole. Use the paintbrush to apply a light layer of Duct Mastic across the mesh tape until you cannot see through the mesh tape.

5. Apply a perpendicular layer of mesh tape across the previous layer of tape. When the hole is covered with the second layer, use the paintbrush to apply another layer of Duct Mastic. Once mesh tape can no longer be seen, stop applying the Duct Mastic.

6. Allow the Duct Mastic to set for half an hour before turning the A/C on. Use a space heater or hair dryer to speed up the process.

Tips & Warnings

• Do not waste time trying to make the mastic repair look pretty--this will not be seen again once the repairs are done.

• Duct Mastic is not recommended for exterior applications--replace the damaged ductwork if it is on the outside.

References

• Good Common Sense: Duct Mastic
• Energy Designed Homes: Duct Repair
• Oikos: Applying Duct Mastic