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Furnace Flues Require Special Sealing Techniques

The opening around a furnace or water heater flue or chimney can be a major source of warm air moving in the attic. Because the pipe gets hot, building codes usually require 1 inch of clearance from metal flues (2 inches from masonry chimneys) to any combustible material, including insulation.

These gaps should be sealed with lightweight aluminum flashing and special high-temperature (heat-resistant) caulk. Before you push the insulation back into place, build a metal dam to keep it away from the pipe. Use the same technique for masonry chimneys.

Caution: Furnace flues (the pipe that removes your furnace exhaust) can be very hot.

Identifying Attic Pipes

Flues/Vents/Pipes Made Out Of Seal Around With
Furnace/Water Heater Galvanized Metal Aluminum flashing and high-temperature silicon caulk
Chimney Masonry/Metal Aluminum flashing and high-temperature silicon caulk
Plumbing Cast Iron or PVC Expanding foam or caulk, depending on the size of the gap

For additional details and images, download the DIY Guide PDF (2MB).

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