If your home has a furnace and also an attic, chances are the furnace exhaust pipe runs through the attic to the roof. This pipe, also known as a furnace flue or attic flue, expels excess air and exhaust from your furnace. However when installed there can often be a gap between the flue pipe and the attic floor. This gap means that air (both air conditioning and heat) will escape into your unfinished attic wasting energy and money. 


Fortunately when you are sealing an attic for insulation purposes you can also seal the gap around the flue and keep air from escaping into the attic.


First you will need to verify that there is at least one inch of clearance around all sides of the flue pipe. You also need to make sure that any insulation nearby is at least an inch away from the flue pipe on all sides to prevent a fire hazard.


Next you will need to measure the width between the two attic beams on either side of the floor around the flue. If the beams are covered by a finished floor simply measure a few inches out on either side of the flue. Cut a piece of metal flashing to this width. In one side of the flashing you should cut a small curved half circle. This allows the flashing to sit flush against the furnace flue pipe.


Place the metal flashing flat on the floor or between the beams so the curved edge of the flashing lays right up to the flue. Note if the floor gap is on both sides of the pipe you’ll need another piece of metal flashing with the same curve cutout to cover the other gap. 


Staple or nail the flashing into place onto the beams, drywall, or floor below. Do not nail or staple into the flue pipe. Use high-temperature caulk in your caulk gun to caulk around the metal flashing edges including where the flue pipe meets the flashing.


Next cut a larger piece of flashing that is long so it can wrap around the circumference of the flue pipe. Slice a few slits into the bottom edge of the flashing piece to make it simpler to bend it flat on the bottom. 

 How to Seal an Attic FlueCredit: Jamie Beverly 2007

Wrap this piece of metal flashing around the lower part of the flue pipe with the cut slit section touching the floor where the pipe meets the floor. Slide it down and use a rubber mallet to flatten out the bottom section of this flashing piece. The cut slits will help it bend and flatten this segment on top of the other flashing already in place. When in place some of this piece of flashing will lay flat on top of the existing flashing while the rest will wrap around the lower 6-12 inches of the flue pipe. 


Use more high temperature caulk around all the edges of the flashing. If needed use nails or staples to carefully hold the bent piece of flashing to the other flashing but do not pierce the flue pipe and be aware of any electrical wires or duct work. Affix more caulk around the wrapped up piece of metal flashing to complete the seal. 


Consider using heavy gloves, protective goggles, and a facial mask to avoid the irritants in the surrounding insulation. And remember do not place any stuffing, insulation, or paper around the flue pipe due to the heat being produced. 


This will help minimize the amount of air escaping through the attic flue gap and will improve your completed project of sealing an attic.