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).