The Basics of Mold Growth in Fiberglass Duct Systems

Open attic ductwork showing disconnected flex duct and exposed interior near plenum, indicating potential for microbial contamination and air leakage.

Published: June 19, 2025
By: Full Spectrum Environmental – Palm Beach County’s Indoor Air Experts

A Hidden Source of Odor in Florida Homes

If your home has a musty, sour, or dirty sock-like smell—especially when the air conditioning kicks on—the culprit may not be your carpets or walls. It could be your ductwork.

In the high-humidity climate of Palm Beach County, microbial growth inside HVAC duct systems is surprisingly common. This growth—often a mixture of mold, bacteria, and biofilm—accumulates along fiberglass duct liner or internally insulated plenums, especially in older or poorly maintained systems. And once it’s there, it can push odor directly into your living space every time the fan runs.

What Causes Microbial Growth in Ducts?

Duct systems are supposed to deliver clean, conditioned air throughout your home. But when:

  • Ducts leak and pull in humid attic or crawlspace air,

  • Insulation inside ducts traps condensation, or

  • Filters are clogged and airflow is restricted,

you create an ideal environment for microbial activity. Interior duct surfaces, especially liner materials, can harbor mold and bacteria that feed on dust and moisture.

As this growth develops, it emits microbial volatile organic compounds (MVOCs)—gases that carry that all-too-familiar musty odor.

How Full Spectrum Diagnoses Duct-Related Odors

At Full Spectrum Environmental, we don’t guess—we inspect. When there’s probable cause to suspect duct contamination, we perform a targeted HVAC and air distribution assessment that includes:

  1. Visual Inspection of Interior Duct Surfaces
    We use scopes and cameras to photograph the inside of ducts, return plenums, and accessible branch lines. We document microbial growth, debris accumulation, and areas of liner degradation.

  2. Leakage Investigation
    Our team looks for signs of duct air leaks—like insulation darkening, sweating, or dust tracks—that can indicate system breach or negative pressure issues pulling in outside air.

  3. Odor Correlation
    We cross-reference musty smells with room locations, airflow patterns, and return/supply positioning to determine if odor pathways align with suspect duct zones.

  4. Moisture & Airflow Readings
    We measure temperature differentials, relative humidity, and sometimes carbon dioxide/VOC levels to assess whether the system is maintaining healthy air movement.

  5. Photo-Documented Report & Scope
    You’ll receive a clear, visual report with our findings—including annotated images of interior duct conditions—and a cleaning and restoration protocol you can take to any qualified HVAC contractor.

Borescope view showing suspected microbial growth on fiberglass duct liner during interior duct inspection with Milwaukee scope.
Onsite endoscopic inspection revealed visible dark microbial growth patterns on fiberglass duct liner material within a supply line.

 

What You Get: A Diagnosis, Not a Guess

Too often, homeowners are left with vague answers or unnecessary HVAC replacement quotes. With Full Spectrum’s approach, you get a professional diagnosis and a straightforward action plan:

  • Duct Cleaning Scope based on observed contamination

  • Material Notes (e.g., presence of fiberglass duct liner)

  • Recommended Cleaning Agents or Antimicrobials

  • Containment Guidance if microbial growth is active

  • Referral-Ready Documentation to help you collect bids or consult an HVAC contractor

Whether you move forward with Green Fox Air Quality or another certified duct cleaning provider, our goal is to empower you with data, documentation, and direction.