Locating Air Leaks 5 - Furnace Flues Sealing Techniques

The opening around a furnace or water heater flue or chimney is a major source of warm air into the attic. Because the pipe gets hot, building codes require 1 in. of clearance from metal flues (2 in. from masonry chimneys) to any combustible material, including insulation. Photos 5 and 6 show how to seal this gap with lightweight aluminum flashing and special high-temperature silicone caulk. Before you push the insulation back into place, build a metal dam (photo 7) to keep it away from the pipe. Use this same technique for masonry chimneys.

5. Cut Aluminum Flashing

Cut aluminum flashing to fit around the flue. For round flues, cut half circles out of two pieces so they overlap about 3 in. in the middle. Press the flashing metal into a bead of latex caulk and staple or nail it into place. If there's no wood, staple or nail it directly to the drywall, but be sure not to go through the drywall.

6. Seal with Silicone Caulk

Seal the gap between the flue and metal flashing with special high-temperture silcone caulk. Don't use spray foam.

7. Form an Insulation Dam

Form an insulation dam to prevent insulation from contacting the flue pipe. Cut enough aluminum from the coil to wrap around the flue plus 6 in. Cut slots 1 in. deep and a few inches apart along the top and bend the tabs in. Cut slots about 2 in. deep along the bottom and bend out the tabs.Wrap the dam around the flue and secure the bottom by stapling through the tabs. Now put insulation back right up against the dam.

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