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When the home's duct system isn't properly sealed, carbon monoxide and other toxic fumes that are byproducts of natural gas appliances can build up in the home and create an unhealthy and potentially lethal living environment. Another problem occurs when noxious fumes from common household items, such as cleaners, solvents, fertilizers and paints, are drawn into the home's return system through poorly sealed joints. Once these smelly and potentially dangerous fumes are drawn into the duct system, they can be circulated and recirculated through the home.

Here are some key steps the HVAC contractor or remodeler should follow:

  • Make sure the joint area has been wiped clean to remove particles and dust before applying mastic. If the joint isn't clean, the mastic won't create a tight seal.
  • Apply enough mastic to form a continuous coating on the surface of the duct. Use a brush, trowel, gloved hand or caulking gun to apply the mastic. A cheap paintbrush can work well, but many people find it faster and easier to simply use their hands to scoop out mastic. If the trade contractor prefers this method, make sure that they're wearing heavy rubber gloves that won't tear if pressed against a sharp edge of sheet metal duct.
  • Work the mastic into the joint or crack and press lightly to get an even coating. There should be at least a 1/2-inch overlap around any joint, crack, or hole in the duct.
  • Use a mesh tape to reinforce the mastic coating on areas where there's a gap of 1/4 inch or more. This is especially important if the area being sealed is a joint that will be under stress. It's always important to make sure the tape used for sealing the joints is designed specifically for mastic being applied.