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Ductwork 101: leaks, sealing, and airflow

5 min readSunshine State Team

You can have the best AC in the world — if your ducts leak, you're paying to cool your attic.

The average home in our service area leaks 20–30% of its conditioned air into the attic through duct joints, plenum gaps, and boot connections. That's a quarter of your electric bill, gone.

How to spot duct problems from your couch

  • One room 5°F warmer than the rest
  • Dust that reappears the day after cleaning
  • AC runs constantly but never quite gets there
  • Attic feels unusually cool in summer (that's your cold air)

What sealing actually looks like

Real duct sealing means mastic (a paintable rubber compound) or Aeroseal, not foil tape. Foil tape dries out in a Florida attic within two summers. Mastic outlives the ductwork.

When to seal vs. replace

If ducts are metal or hard board and mostly intact, seal them. If they're flex duct that's older than 15 years, crushed, or disconnected at multiple boots, replacement usually pays for itself faster than repeated seal jobs.

Get it tested

A duct blaster test measures leakage in CFM at 25 pascals. Any HVAC contractor worth hiring will do one before quoting a new system — because installing new equipment on bad ducts is throwing money into the attic.

Ready when you are.

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