How to Insulate a Prefab Fireplace


Prefabricated fireplaces can turn plain living rooms into showplaces. Without proper fireplace insulation and sealing, however, those showplaces may become very drafty in the winter when air rushes in through small openings around gas lines, underneath cantilevers and at unsealed corners. Safe fireplace installation requires about 2 inches of empty space between the outside of the prefabricated fireplace and any combustible materials (such as batt insulation). While you cannot insulate a prefabricated fireplace directly, you can stop drafts by insulating the structure surrounding the fireplace.

Things You'll Need

  • High-temperature sealant
  • Tape measure
  • Utility knife
  • Foam board insulation
  • Hammer
  • Nails
  • Insulated tape
  • Construction stapler
  • House wrap
  • Apply high-temperature sealant to the space where the fireplace's gas line enters the house. Apply sealant around all vents or ducts that open to the outside to eliminate drafts between these fittings and the wall.

  • Remove the siding on a fireplace cantilever. Measure and cut rigid foam insulation to fit around the structure. Nail the rigid foam insulation to the sides, bottom and top, if possible. Cut holes for any ducts or vents and seal them with high-temperature sealant. Tape the foam joints with insulated tape to prevent drafts.

  • Staple house wrap over the foam insulation, preferably in one piece, being sure to wrap the underside of the cantilever. Overlap the house wrap by 2 inches, if more than one piece is used, and tape firmly. Cut holes for vents and ducts, firmly taping or nailing the house wrap around the cutouts. Replace the siding on the fireplace cantilever.

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