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 Seal and Insulate DIY Guide (PDF, 12.8 MB).

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