Green Building Library
Weatherization Tips

How to Baffle Flues and Chimneys

Attic insulation must not touch chimneys or metal flues, because it could start a fire. Before insulating attics, you must install a permanent means of holding the insulation away.

Clearances To Insulation

  • Maintain a clearance of 3 to 4 inches between the chimney or metal flue and attic insulation material.  (Some flues, such as metalbestos and type B gas vent, may be listed for less clearance.)
  • Baffles should extend at least 4 inches above the final level of insulation.

Baffles

Baffles must be made from a solid, flame-resistant material, such as metal flashing or gypsum board. They must be firmly attached to the chimney or something in the ceiling structure, such as the ceiling joists.  Treated cardboard, sometimes used to baffle attic vents, is not acceptable for baffling chimneys and flues.  Wide aluminum flashing works well, because you can cut it with a utility knife, bend it easily by hand, and fasten it with staples.

Metal Flues

chimney flue

Solid metal baffles are needed around single-wall metal flues. It’s usually easiest to build the baffle in two or three sections.  Cut off a manageable length of flashing–2 to 4 ft.  Staple the flashing to nearby framing.  If necessary, cut another length of flashing and secure it with staples.  Shape flashing to maintain 3 inch clearance. 
If the baffle needs to straddle a ceiling joist, cut a slot for the joist with a utility knife. Slip the flashing over the joist.  Extend the flashing to the next joist and make another slot.  Work around the flue in the same manner.  Once a section is in place, staple through the tabs into the joist.

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