Condensation on AC Ducts: How to Prevent and Fix It

Sweating air conditioning ductwork is a significant issue when summers are warm and humid. The cause is simple: The air inside the ducts is cool, and moisture from humid air outside it condenses on AC ductwork like moisture on a glass of iced tea. Condensation on AC ducts is both the symptom of solvable problems and the cause of secondary trouble that can have serious consequences for your home and your health.

This guide gives clear steps to solving problems with condensation on AC ducts so that it does not return.

Why You Might Need to Prevent Condensation on AC Ducts

Condensation on AC ducts demonstrates there is one or more repair issues in your HVAC system and/or your home. Solutions are available below, so you can fix those that apply to your situation:

Exposed ductwork isn’t insulated;
There are gaps in ductwork that allow cold air to escape and promote condensation in spots;
Dirty furnace or air handler filters are slowing the flow of air;
The ductwork is running in space that is too humid;
The blower motor isn’t moving air as rapidly as it should.

Here are secondary reasons to prevent condensation on AC ducts. The moisture:

Promotes mold and mildew that causes odors and is a health risk to people and pets;
Stains and damages drywall;
Wets wood framing causing rot and attracting insects that feed on it;
Wets insulation, compacting it and lowering its effectiveness.

Tools & Materials Needed to Prevent Sweating Ducts

That’s a troubling list! The good news is that you can fix the issues. The tools and materials are grouped by the problem they solve:

Leaking ducts: Fiberglass mesh tape, mastic duct sealer, chip brush or cheap paintbrush;
Non-insulated ducts: Duct wrap and cloth duct tape;
Dirty filters: New furnace or air handler filters;
Damp basement or crawlspace: Vapor barrier, fiberglass or foam insulation and a dehumidifier;
Humid attic: Soffit, gable, ridge and roof vents, circular or jig saw, drill, screws, flashing and roofing tar;
Inadequate airflow: New blower motor, if your current motor can’t be adjusted or repaired.

How to Prevent Condensation on AC Ducts in 6 Steps

Rather than sequential steps, these steps start with basics and go to specifics.

1. Find and Repair Loose Joints in Exposed Ductwork

Visually inspect seams or feel for air leaks and listen for air whistling through the seams with the blower fan running. Spread mastic on loose seams and joints using the cheap brush. Then, reinforce it 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 the exposed surfaces. Cover the seams of the wrap with cloth-reinforced vinyl duct tape to ensure it remaining in place long-term.

The US Department of Energy recommends insulating ductwork to reduce energy loss and energy costs, even if condensation isn’t an issue.

3. Clean or Replace Dirty Filters

Dirty filters slow the flow of air and keep cold air in the ducts longer than it should be. Meanwhile, it is promoting condensation on AC ducts.

4. Address Damp Basement or Crawlspace Issues

Cover a dirt floor with a vapor barrier. Seal gaps around pipes and vents with fiberglass batt insulation or spray foam insulation.

Persistent humidity problems below grade require a dehumidifier, preferably one that can drain into a floor drain.

5. Vent the Attic

Attics need to breathe. Otherwise, moist air will cause mold and rot and ruin insulation in addition to causing condensation on AC ducts. Installing vents is a challenging DIY project for most homeowners. Roofers and siding contractors are your best options for pro installation.

Make sure 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 the hole through the siding and sheathing from the outside. Use the template included with most gable vents, and securing the vent with screws;
If your home has a hip roof instead of gables, install roof vents every 20-30 linear feet near the ridge. You can do this by cutting holes in the sheathing and roofing material, securing the vents and using flashing and roof cement to prevent leaks;
Install ridge vent 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. The technician can also repair or replace the blower as necessary.

Caution Notes

If your attic has exposed rafters, walk on them or lay a board across them to walk on. If you step between rafters, you’re likely to damage the drywall below you or possibly fall through the ceiling.
Unless you have excellent experience with working on ladders and roofs, installing gable vents, ridge vents and roof vents are jobs better left to professionals.
Finally, do not attempt to adjust your blower motor. Possible problems include electric shock, injury caused by the fan rotating at more than 1,000 rpm and damage to the blower motor, fan or other furnace parts.

More Ways to Prevent Duct Condensation

If your ducts are insulated, condensation won’t form on them. Until you complete that project, two tips will help reduce condensation on AC ducts.

First, keep your thermostat set at 75F or higher when in air conditioning mode. The colder the air is in the ductwork, the more likely condensation will form. So, go easy on the AC.
Secondly, 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. The reduces condensation and keeps your home more comfortable too.