If you walk past a supply vent or a round metal duct in a St. Louis basement on a muggy July afternoon and see drops of water or a dark ring on the metal, you are probably looking at condensation. It forms when warm, moist air touches a surface cold enough to cool it below its dew point. Ductwork that runs through an unconditioned attic, crawl space, or garage is especially prone to this in summer, and St. Louis summers bring plenty of humid days. Sweating ducts are usually a sign of moist air and worn insulation rather than a broken pipe, and you can check for both yourself.
What Sweating Looks Like
Condensation on ducts tends to appear as a fine film of water, sometimes gathering into beads that run down the metal. It often shows up on the underside of a duct, near seams, or where a branch joins a main run. Over time it can rust bare steel, stain the insulation wrap, or drip onto the floor below.
Sort Out the Source First
Before blaming humidity alone, rule out water that comes from somewhere else.
- Note the time of day. Condensation tends to peak when outdoor air is warm and damp, and it often eases during cooler, drier stretches.
- Check whether the sweating happens on the cold supply side of the system. Return ducts can sweat too, but a drip beside the air handler or evaporator coil is a different problem, often tied to the condensate drain line.
- Look for water pooling under a joint. A steady trickle, or a stain that grows after rain, may point to a leak, a window, or a foundation issue.
- Touch the duct with a dry hand. Cold, wet metal in still, humid air is the classic pattern for condensation.
Check the Insulation
Many ducts in older St. Louis houses have insulation wrap that has sagged, torn, or lost its vapor barrier. Bare sections, crushed wrap, or gaps at joints let warm air reach cold metal. Walk the duct run in the attic, crawl space, or basement and note where the wrap is missing or damaged. Wrap that is still in place but soaked, stained, or discolored should be replaced rather than simply wiped down.
Look at the Air in the Space
Sweating ducts often mean the surrounding air is humid. A hygrometer placed near the duct, away from direct sun and away from the supply register, will show whether the space is holding a lot of moisture. Watch it through several days of different weather. If the space stays damp while the living areas feel comfortable, the crawl space or attic may be pulling in moist outside air.
Ventilation and Air Movement
Crawl spaces and attics in many St. Louis homes depend on vents, ground cover, and steady air movement. Blocked vents, missing plastic over bare soil, or vents closed for the cold months can trap damp air against cold surfaces. Confirm that vents are open and not buried under debris or insulation, and that ground cover is still in place where it was installed. These fixes are often simple to make, but the right step depends on what you find.
When to Call for Help
Some findings go beyond a weekend check. Bring in a qualified heating and cooling professional if the duct runs sweat heavily, if insulation is widely damaged, if the air handler or coil drips inside the house, or if the framing shows staining. A trained technician can measure airflow, inspect the coil, and explain the repairs. Ask for a written summary of what they found and what would correct it.
A Short Checklist
- Note when the sweating happens and roughly how much water collects.
- Identify the exact location, including whether it is on the supply or return side.
- Inspect the wrap for tears, gaps, and water damage.
- Check vents and ground cover in the crawl space or attic.
- Place a hygrometer near the duct and track it for several days.
- Dry any wet insulation, then confirm that the space vents and drains as it was designed to.