Sealing a Residential Oil-Fired Boiler Flue

Close off spacing around the attic flue passage with a sheet of aluminum flashing. Cut aluminum around the flue passage to cover the existing space between the flue and any insulation materials. For round flue passages, cut sheets into half circles that provide a 3-inch overlap.

Seal aluminum flashing sheets to the floor with caulk made specifically for hot-temperature surfaces. Nail or staple the flashing sheet to the flooring so that it stays in place. When nailing or stapling to drywall surfaces, make sure the nails or staples don’t go all the way through the drywall. Use caulk to seal the top portion of the aluminum flashing to the flue passage.

Build an insulation barrier to protect the surrounding insulation materials from the heat coming off the flue passage. Building code regulations require a minimum of 1-inch clearance between metal flue passages and insulation materials. Insulation barriers provide additional protection from flammable insulation materials. Wrap the flue passage with aluminum sheeting and allow for an extra 6 inches of sheeting.

Cut 1-inch slots along the top edge of the aluminum barrier sheet. Cut slots a few inches apart from one another. Bend the tabs of the sheet toward the flue passage. Cut 2-inch slots along the bottom edge of the aluminum sheet and bend the tabs outward, or away from the flue passage. Wrap the aluminum barrier around the flue and staple the bottom tabs to the floor. Position the insulation materials up against the aluminum barrier.