How to Seal Around Exterior Furnace Vents

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High efficiency furnace vents are typically vented through the exterior wall of the home. These vents are a common place where outside air and moisture can get into the home. Outside moisture can cause mold and mildew problems within the wall structure itself, and infiltrating air can cause increased energy costs. To prevent these problems, seal around exterior furnace vents. With the proper sealant, you will keep unwanted moisture and drafts out of the home.

Things You'll Need

  • Soap and water
  • Expanding foam
  • Utility knife
  • Caulk
  • Caulk gun
  • Look where your furnace vents penetrate the exterior wall. Determine what kind of exterior wall structure you have---whether it is wood, siding, masonry, or something else to determine what kind of sealant to use.

  • Fill in large gaps around the exterior furnace vents with expanding foam, which you can find at your local home improvement store and which is sold in a spray can.

  • Wait until the foam cures, and cut the excess off flush with the exterior of the home with a utility knife.

  • Prepare your caulk tube by cutting the tip off at a 45-degree angle and poking the foil seal inside the nozzle tip. The caulk gun should have a cutter for cutting the caulk tube tip and a metal wire for poking through the seal.

  • Pull the plunger all the way back on the caulk gun and load the caulk tube into the caulk gun. Press the trigger of the caulk gun until the plunger is tight against the caulk tube.

  • Caulk around the furnace vents. Press the trigger on the caulk gun to dispense the caulk. Lay down a 1/4-inch bead around the entire circumference of the furnace vent pipes. The bead should adhere to both the furnace vent pipes and the exterior wall.

  • Smooth the caulk with your fingers to make a nice seal from the furnace vent to the exterior of the building.

Tips & Warnings

  • Before purchasing caulk, read the caulk tube label to make sure it bonds with your exterior wall surface. Silicone caulk will adhere well to all surfaces. Latex-based caulk works on wood and brick. Butyl rubber caulk works well on unpainted metal and masonry surfaces.
  • Don't apply caulk when rain is forecasted within 12 hours. The caulk will need time to cure, and a rainfall just after caulking can wash it away before it has a chance to cure.
  • Clean the area around the furnace vent where you want to caulk before you caulk it. Use a soap and water mixture to remove any dirt and grime to assist caulk adhesion.
  • For larger gaps---especially ones that you needed to seal with foam first, the caulk bead may end up wider than 1/4-inch. In these circumstances, spread the bead a little wider than the gap. It should make contact with both the exterior wall and the furnace vent pipe to provide a proper seal.

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