How to Prevent Condensation on AC Ducts: 6 Steps

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Last updated: 
April 18, 2024

Preventing Condensation on AC Ducts: A Comprehensive Guide

Why Condensation on AC Ducts is a Problem

Condensation on air conditioning ductwork is a common issue during warm and humid summers. According to the principles of thermodynamics, when cool air inside the ducts comes into contact with warm, humid air outside, moisture condenses on the surface of the ducts, similar to how water droplets form on a glass of iced tea . This condensation is not only a symptom of underlying problems in your HVAC system or home but can also lead to secondary issues that can seriously impact your home and health.

Reasons to Address Condensation on AC Ducts

Condensation on AC ducts can indicate one or more of the following issues:

  • Lack of insulation on exposed ductwork
  • Gaps in ductwork allowing cold air to escape and promoting localized condensation
  • Dirty furnace or air handler filters slowing air flow
  • Ductwork running through excessively humid spaces
  • Blower motor not moving air efficiently

If left unaddressed, the moisture from condensation can:

  • Promote mold and mildew growth, causing odors and health risks
  • Stain and damage drywall
  • Cause wood framing to rot and attract wood-eating insects
  • Wet and compact insulation, reducing its effectiveness

Tools and Materials Needed

To fix the various issues causing condensation on AC ducts, you may need the following tools and materials:

  • For leaking ducts: Fiberglass mesh tape, mastic duct sealer, chip brush or cheap paintbrush
  • For non-insulated ducts: Duct wrap and cloth duct tape
  • For dirty filters: New furnace or air handler filters
  • For damp basements or crawlspaces: Vapor barrier, fiberglass or foam insulation, and a dehumidifier
  • For humid attics: Soffit, gable, ridge, and roof vents, circular or jig saw, drill, screws, flashing, and roofing tar
  • For inadequate airflow: New blower motor, if the current one cannot be adjusted or repaired

Steps to Prevent Condensation on AC Ducts

1. Find and Repair Loose Joints in Exposed Ductwork

Inspect seams visually, feel for air leaks, or listen for whistling air with the blower fan running. Apply mastic to loose seams and joints using a brush, then reinforce with fiberglass mesh tape pressed into the mastic. Cover the tape with a topcoat of mastic.

2. Insulate Exposed Ducts

Wrap the entire duct, when possible, or cover exposed surfaces. Seal the seams of the wrap with cloth-reinforced vinyl duct tape for long-term durability. The US Department of Energy recommends insulating ductwork to reduce energy loss and costs, even in the absence of condensation issues .

3. Clean or Replace Dirty Filters

Dirty filters slow air flow and allow cold air to linger in the ducts, promoting condensation. Replace or clean filters regularly.

4. Address Damp Basement or Crawlspace Issues

Cover dirt floors with a vapor barrier. Seal gaps around pipes and vents with fiberglass batt insulation or spray foam insulation. For persistent humidity problems, use a dehumidifier that can drain into a floor drain.

5. Vent the Attic

Proper attic ventilation is crucial to prevent moist air from causing mold, rot, and insulation damage, in addition to condensation on AC ducts. Installing vents can be challenging for most homeowners, so consider hiring a professional roofer or siding contractor.

  • Ensure existing soffit vents are not covered by insulation
  • Install soffit vents between rafter tails by cutting holes in the plywood soffit and securing the vents with screws
  • Install gable vents by cutting holes through the siding and sheathing from the outside, using the template included with most gable vents, and securing the vent with screws
  • For hip roofs, install roof vents every 20-30 linear feet near the ridge by cutting holes in the sheathing and roofing material, securing the vents, and using flashing and roof cement to prevent leaks
  • Install ridge vents by removing the ridge cap shingles, nailing down the ridge vent, and reinstalling the ridge cap

6. Adjust or Replace the Blower Motor

An HVAC technician can adjust multi-speed and variable-speed blower motors to meet the ductwork's airflow requirements or repair/replace the blower as necessary. Do not attempt to adjust the blower motor yourself due to risks of electric shock, injury from the high-speed fan, and damage to HVAC components.

Additional Tips to Prevent Duct Condensation

Until you complete the insulation project, two tips can help reduce condensation on AC ducts:

  1. Keep your thermostat set at 75°F (24°C) or higher when in air conditioning mode. The colder the air in the ductwork, the more likely condensation will form.
  2. If your HVAC system and thermostat have a fan-on mode, use it when humidity is high. Keeping the air flowing over the cold AC coil, even when the AC compressor isn't running, increases dehumidification, reduces condensation, and improves comfort.


Addressing condensation on AC ducts is essential for maintaining a healthy, comfortable, and energy-efficient home. By following the steps outlined in this guide, you can identify and fix the underlying issues causing condensation and prevent secondary problems like mold growth, structural damage, and reduced insulation effectiveness. If you have any experiences or success stories related to preventing condensation on AC ducts, please share them in the comments below to help other readers facing similar challenges.


  1. U.S. Department of Energy. (n.d.). Air Conditioning Ducts. Retrieved from
  2. U.S. Environmental Protection Agency. (n.d.). Mold and Health. Retrieved from

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