Every summer in Raleigh, your air conditioner works overtime. The indoor temperature drops to 72 or 74 degrees. Cold air flows through your ductwork. And somewhere in your crawl space or attic, the outside of those ducts starts dripping.
It's not a leak. It's condensation — the same physics as a glass of ice water sweating on a July afternoon. But unlike that glass, your ductwork sits hidden behind walls, above ceilings, and under floors. The moisture it generates feeds mold colonies you can't see, saturates insulation you can't inspect, and degrades the air quality in every room of your house.
This is one of the most underdiagnosed HVAC problems in central North Carolina, and it gets worse every year your system ages.
Why NC Humidity Makes This Inevitable
Raleigh regularly sees outdoor humidity levels above 80% from June through September. On many summer mornings, relative humidity approaches 90% to 95% before the sun burns it off. That humid air carries an enormous amount of moisture — roughly 18 to 22 grams of water per kilogram of air on a typical Raleigh summer day.
Your supply ductwork, carrying 55°F air from the air handler, has an exterior surface temperature well below the dew point of the surrounding air. Any time warm, humid air contacts that cold duct surface, water condenses. It's not a question of "if" — it's a question of how much and how fast.
In Five Points and other established Raleigh neighborhoods, many homes have ductwork running through unconditioned crawl spaces where humidity levels are even higher than outdoors. Crawl spaces without proper vapor barriers can register relative humidity above 90% for months at a time, creating a constant condensation factory around every supply duct.
What Condensation Looks Like (and What It Leads To)
The tricky part about duct sweating is that you rarely see the moisture directly. Instead, you see the consequences:
Water stains on ceilings — Brown or yellowish rings on drywall, especially where duct runs pass overhead. Many homeowners assume they have a roof leak and spend money on roofing repairs before anyone checks the attic ductwork.
Musty or earthy smell — Mold growing on damp duct insulation and surrounding building materials produces volatile organic compounds that create that unmistakable musty odor. You might notice it most when the system first kicks on after sitting idle.
Visible moisture or dripping — In severe cases, you can see water actively dripping from duct joints, register boots, or the underside of insulation wrap. Condensation pans under air handlers may overflow.
Mold on vent covers and registers — Dark spots or fuzzy growth on ceiling or wall registers is a downstream indicator that moisture has been present in the duct system.
Insulation degradation — Fiberglass duct insulation that gets wet loses its R-value and becomes a breeding ground for mold. Once saturated, it rarely dries out completely in a humid crawl space environment.
Pro Tip: If you can safely access your attic or crawl space on a humid summer day, feel the outside of your supply ducts. If they're damp or dripping, condensation is actively occurring. Check for any spots where insulation has pulled away from the duct surface — these bare metal sections condense the most moisture.
The Three Root Causes
1. Insulation Gaps and Failures
Every supply duct should be wrapped with insulation rated at R-6 or higher. The insulation serves a dual purpose: it keeps the cold air cold (improving efficiency) and it keeps humid air from contacting the cold duct surface (preventing condensation).
But insulation degrades over time. Adhesive backing fails. Seams open up. Sections sag or fall away from the duct. Animals in crawl spaces tear it. And every gap in the insulation becomes a condensation point.
In newer developments like Brier Creek, the ductwork insulation may still be in good shape, but improper installation — sections that weren't sealed at the seams, or areas where the vapor barrier on the insulation wasn't properly taped — can create the same condensation problems in a 10-year-old home as you'd see in a 40-year-old one.
2. Unsealed and Leaky Ducts
Duct joints that aren't properly sealed allow conditioned air to escape and humid air to enter. A leaky return duct in a crawl space actively pulls humid crawl space air into the system, increasing the moisture load on your air conditioner and raising humidity levels throughout the house.
The Department of Energy estimates that the average home loses 20% to 30% of conditioned air through duct leaks. In a high-humidity environment like Raleigh, those leaks don't just waste energy — they introduce moisture into the duct system and the spaces surrounding it.
3. Inadequate Vapor Barriers
Your crawl space's vapor barrier (the plastic sheeting on the ground) is the first line of defense against ground moisture entering the air. A missing, torn, or improperly installed vapor barrier allows soil moisture to evaporate into the crawl space, dramatically increasing humidity levels around the ductwork.
Many homes in Raleigh's Midtown and Cameron Village areas have crawl spaces with old, deteriorated vapor barriers — or no barrier at all. Encapsulating the crawl space with a proper vapor barrier and dehumidifier can cut crawl space humidity in half.
When to Call a Pro: If you're seeing water stains, smelling musty air, or noticing mold on your registers, it's time for a professional air quality assessment. A thorough evaluation includes checking duct insulation integrity, measuring humidity levels in unconditioned spaces, testing for mold, and inspecting the overall duct system for leaks and condensation damage. The longer moisture problems go unaddressed, the more extensive — and expensive — the remediation becomes.
What Proper Remediation Looks Like
Fixing duct condensation isn't a single repair — it's a system approach:
Re-insulate ductwork — All supply ducts in unconditioned spaces need continuous R-6 or higher insulation with a sealed vapor barrier. Every seam, joint, and penetration must be taped and sealed.
Seal duct leaks — Mastic sealant (not duct tape, which deteriorates quickly) should be applied to every joint and connection. This reduces both energy waste and moisture infiltration.
Address crawl space humidity — Install or replace the vapor barrier. In severe cases, full crawl space encapsulation with a dedicated dehumidifier is the most effective solution.
Check your HVAC sizing — An oversized air conditioner short-cycles, running in brief bursts that cool the air but don't run long enough to dehumidify it. This leaves indoor humidity high, which contributes to condensation throughout the system.
Consider a whole-home dehumidifier — In homes where the ductwork runs through particularly humid spaces, adding a whole-home dehumidifier to the HVAC system provides an additional layer of moisture control.
Frequently Asked Questions
How do I know if I have mold in my ducts versus just dust?
Mold on registers and inside ducts typically appears as dark spots with a fuzzy or slimy texture, often accompanied by a musty smell. Dust accumulation is generally gray and dry. If you're unsure, a professional air quality test can identify mold spores and determine the species and concentration levels.
Can I just spray mold killer inside my ducts?
Surface cleaning may remove visible mold, but it doesn't address the moisture source that caused it. Mold will return within weeks if the condensation problem isn't fixed. Additionally, some mold remediation chemicals can irritate respiratory systems when dispersed through the HVAC system. Professional remediation addresses both the mold and the root cause.
Is duct sweating a sign my AC is too powerful?
It can be related. An oversized system produces colder supply air temperatures, which increases the temperature differential between the duct surface and surrounding air, leading to more condensation. However, even properly sized systems will sweat if the insulation and vapor barriers aren't adequate for North Carolina's humidity levels.
How much does it cost to fix ductwork condensation?
Costs vary significantly depending on the scope. Re-insulating accessible ductwork in a crawl space might run $1,500 to $3,000. Full duct sealing adds $1,000 to $2,000. Crawl space encapsulation ranges from $5,000 to $15,000 depending on square footage. A whole-home dehumidifier installation typically costs $1,500 to $2,500.
Should I be worried about condensation on my return ducts too?
Return ducts are less prone to condensation because they carry warmer air (room temperature) rather than cold supply air. However, in extremely humid environments, return ducts with poor insulation in crawl spaces can still develop condensation. If your supply ducts are sweating, it's worth checking the returns as well.
