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How to Seal Leaky Ductwork

Professionals estimate that leaky ducts can reduce heating and air efficiency by about 20 percent in most households. Since many houses run either heating or cooling for most of the year, this can really add up in wasted energy and money. A few minutes of maintenance can help regain some efficiency and get the most out your air conditioner or furnace.

What you'll need:

  • UL 181 Tape. This type of tape create a better seal than standard duct tape.
  • Mastic. The sealant is applied with a brush and forms a permanent bond.
  • Paintbrush. Mastic is thick, so use a solid paintbrush to spread over the leaky areas.
  • Smoke pen. This optional tool will help you find the leaks in your venting system.

Step 1: Seal any large leaks with UL 181 tape.

Start with any obvious holes in your ductwork. Put a layer of tape over any area where your ductwork pieces don't meet. This will give the mastic you will later apply a solid base. This creates a solid base over which you will later apply the mastic.

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Step 2: Locate any duct leakages you want to permanently seal.

Run a smoke pen near any edges and seals in your ductwork. As most leaks happen where two pieces join, pay special attention to these areas. Leaks will pull in the smoke from your pen. You can either check your whole space and mark them, or take the smoke pen with you and repair as you go.

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Step 3: Apply UL 181 tape to any leaky areas.

Similar to step 1, the tape serves as a base for the mastic.

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Step 4: Apply mastic.

Load your paintbrush with mastic, and coat any areas you have placed tape. A good seal will cover the entire width of the tape plus a little extra on all sides of it. With any extra mastic, cover other joints and connections in your ductwork as a preventative step.

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Step 5: Wait for mastic to dry.

Mastic is a permanent sealant, so be sure to wait for it to dry before allowing it to touch anything else in your area. Keep boxes or other furniture away.

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Step 6: Apply tape to any non-permanent seals:

For areas that are losing air but you don't want to permanently close off, such as access panels, place UL 181 tape over the edges. This will prevent a good amount of leakage but still allow you to access the area.

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