Article by A Accredited Mold Inspection Service, Inc. 

At A Accredited we do not believe that a Florida based mold inspection service should just check for water leaks and airborn spores. Often we find unusual humidity related causes of mold problems such as the entrainment of hot humid outdoor air into buildings, homes, and AC ductwork via the actions of a damaged HVAC system pulling in humid outdoor or humid attic air.

Below is a summary page prepared by our mold inspection service in which we identify a damaged roof mounted AC unit plenum as being a contributor to mold problems in a Miami townhome. 
A hole in the plenum was allowing the AC unit on the roof to pull hot humid air from outside into the interior. This air formed condensation when it hit cold surfaces inside and around the AC plenum. This resulted in mold problems in the plenum and wall under the air handler.  

Client: Florida Mold Inspection Service Client
Property Address: Miami Florida
Date: 9-11-06

INTERIOR AREAS INSPECTION

The client reported that mold odors from the ducts were excessive during rain, and an explanation for excessive mold odors in the ducts during rain was found. This explanation also explains the client's suspected humidity problems and the inspectors confirmation of  such humidity problems.  The explanation is as follows:

The supply plenum and return plenum connect to the air handler on the roof were both leaking air. The supply plenum leaks cold air out and this cold air hits the insulation around it, causing the surrounding air to form condensation on the insulatio. This inspector observed insulation to be very cold and wet from condensation water. This water has  to drip down to hidden plenum areas directly below due to gravity and this accounts for the mold odor coming from the clean blower fan.

The return plenum was also leaking air, but because it is negatively pressurized it pulls hot humid outdoor air into the AC and this accounts for the mold odors in the ducts as well as  the humidity problems indoors.  During rain,  the humidity n the AC system will be extremely high and may form condensation in the plenum.

Humidity in the AC system will not only result in mold and mold odors but also will cause dust mites due to excessive dust in the return plenum to multiply. Excessive dust was seen in the plenum when the AC filter was removed.

Moisture was not entering as rain because the damage in the plenum's was hidden under the air handler where rain would have a hard time getting in.

The inspector confirmed that the supply was leaking air out because he could feel the cold air with his hand. 

The inspector confirmed that the return was leaking air in with the use of a smoke pen. In the photos you can see the smoke being pulled in.  

The opening in the plenum is well hidden under the air handler. Removal of plenum's sheet metal cover, open plenum and replacement of  any sections of plenum ducts as well as insulation are recommended corrective measures.

Photos show water inside this plenum area in the past. This inspection shows how condensation forms and likely drips down inside or around plenum's under the AC unit.

Encapsulate any plenum or ducts in the house that cannot be removed, especially in the areas near and under the roof mounted AC unit.

ALLERGEN INSPECTION

OBSERVATIONS:

This is a mold inspection service but the following evidence of possible rodent infestation was found. The inspector was informed during the client interview that rodents were visible in the past at the present property. The inspector observed what appeared to be rodent droppings during the inspection.  Droppings were in the opening in the upstairs guest bathroom ceiling. Rodent droppings were also detected.

These droppings were likely very old and rodents may all be gone. Seal this opening so rodent allergins do not enter the living space.

HVAC INSPECTION

Because the AC was leaking air out and pulling air in it as not removing as much humidity as it should have. AC was pulling in 53 grains of moisture and putting out 50 grains of moisture thus only pulling out 3 grains of moisture. This is very poor.  10 grains removed is more common. On a rainy day,  the unit would likely not pull any moisture out but bring moisture / humidity in.

This is a mold inspection service only and any AC system evaluations done by a mold inspector were done as a very basic preliminary courtesy to the client only and should not be relied on to provide complete information regarding the proper operation of the air condition systems operation. It is recommended that a qualified licensed AC service person or AC contractor review your AC system in regards to proper operation. Proper operation of your AC is very important in controlling humidity and mold problems. 

If professional mold removal also known as mold remediation is needed, then information on mold removal will be in the remediation section of the report.  Other general conclusions and important information can be found in various other parts of this report.This is just a summary of the inspector's findings. It is very important that you read the entire document.