Mold Found Inside an HVAC Supply Plenum in a Dallas Home

The Situation

An HVAC service professional reported suspected mold growth inside the home's attic HVAC supply plenum. The homeowner requested an independent assessment to document the condition, evaluate accessible HVAC components, and collect samples for laboratory analysis.

The limited assessment included accessible living areas, the attic air handler and supply plenum, visible ceiling supply grills, ambient measurements, and two surface swabs.

What Was Inspected

• The supply plenum was viewed and sampled through the existing opening at the air-handler cabinet. No plenum or duct material was cut, dismantled, or removed.
• Building materials were not removed, and wall, ceiling, and floor assemblies were not opened. Concealed duct runs and building cavities were not evaluated.

What We Observed

The accessible HVAC supply-plenum interior showed widespread surface colonization rather than one isolated spot. Separately, the living-room and bedroom ceiling supply grills had dark discoloration. Only the plenum and living-room grill were sampled.

In accessible living areas, no visible mold growth was observed on finish surfaces apart from the supply grills described in the report. No active water intrusion, standing water, or water-stained finishes were observed on accessible finish surfaces. Concealed areas were not opened.

What Was Found Inside the HVAC Supply Plenum?

The accessible plenum interior consisted of fiberglass duct-board material. Extensive white and gray colonization appeared on multiple accessible surfaces: the plenum floor, vertical liner panels adjacent to the evaporator coil, and liner around flexible-duct takeoff collars.

Parts of the facing were torn, delaminated, and hanging loose. Colonization was visible on exposed liner facing and underlying duct-board material. Dark discoloration and debris were also present on the plenum floor and near the base of the coil. These observations describe accessible surfaces, not concealed duct runs.

Mold Was Also Confirmed on a Downstream Supply Grill

Dark discoloration followed the louvers and face of the living-room ceiling supply grill, concentrated along its air-discharge path. A surface swab from this downstream grill returned findings consistent with fungal growth on the sampled surface.

The sampled grill and accessible supply plenum both showed fungal growth; these findings were consistent with one another. The assessment did not establish that the plenum caused the grill condition.

What About the Bedroom Supply Grill?

The bedroom ceiling supply grill had visually similar dark discoloration, but it was not sampled. Its condition remains undetermined; the living-room lab result cannot be applied to the bedroom grill.

Moisture / Building Conditions

At the attic air handler, the measured temperature was 78.1°F, relative humidity was 66.6%, and dew point was approximately 66.1°F. In an interior dining area, temperature was 75.3°F, relative humidity was 48.4%, and dew point was 54.5°F. These measurements describe conditions at one point in time, not typical or sustained conditions.

The report noted that sustained relative humidity above 60% is generally recognized as supportive of fungal growth. Cooled HVAC surfaces may also fall below the surrounding air's dew point and accumulate condensation. Neither observation established the specific moisture mechanism responsible here.

Sampling Performed

• Surface swab — Accessible interior surface of the attic HVAC supply plenum: One of two surface swabs; no air samples were collected.
• Surface swab — Living-room ceiling supply grill: The bedroom grill was not sampled.

Laboratory Findings

Understanding the Laboratory Rating

For these surface samples, the laboratory's categories were: No Mold Detected, fewer than one fungal structure; Minor, one or fewer structures per field of view; Moderate, greater than one to three per field of view; and Heavy, greater than three per field of view.

Heavy was the laboratory's highest rating and may indicate a sample was taken from a source of mold. These are laboratory surface-sample categories, not health-risk levels or measures of airborne exposure.

No Air Samples Were Collected

Both samples were surface swabs. Airborne fungal concentrations inside the residence were not measured. These results cannot establish airborne spore levels or quantify occupant exposure.

What the Findings Suggested

The report concluded that the combination of widespread visible colonization and Heavy Aspergillus/Penicillium-like structures, Cladosporium, and hyphal fragments was consistent with fungal growth on accessible supply-plenum interior surfaces.

The living-room supply-grill sample was separately interpreted as consistent with fungal growth on the sampled grill surface rather than merely incidental settled spores. The findings do not establish conditions on unsampled vents, concealed duct surfaces, or in the air.

What Caused the Moisture?

The limited mold assessment did not determine one specific moisture mechanism. It recommended that a qualified HVAC professional evaluate condensate collection and drainage, evaporator-coil condition, air-handler cabinet condition, insulation integrity, and air sealing at the plenum and duct connections.

The attic humidity reading and the possibility of condensation on cooled surfaces are relevant context, not proof that either was the sole cause of the plenum condition.

What Homeowners Can Learn From This Case

Discoloration on an AC vent does not automatically tell a homeowner what is happening elsewhere in an HVAC system. In this case, a limited investigation documented fungal growth on accessible plenum surfaces and on a sampled downstream living-room grill. The bedroom grill remained undetermined.

Depending on the question, a scoped investigation may include inspection of accessible system components and plenum materials, supply-grill observations, selected surface sampling, temperature and humidity measurements, and HVAC-professional evaluation of the moisture mechanism. Findings at one home should not be assumed to describe another.

Questions About This Case

Related Services

• Dallas mold inspection
• Dallas mold testing
• Moisture & humidity inspection
• Surface mold sampling