Occasionally (OK, more than occasionally) I receive phone calls from technicians, contractors, and students that I have worked with or taught in the past. You don’t teach a trade for 14 years without having some excellent graduates that keep your phone number in case of emergencies or questions. I welcome them because it makes me think and keeps me in touch with what is happening in the field.

Every year near the end of Air Conditioning, and the start of heating seasons I get a ton of phone calls and text messages about mold. Their customers or their maintenance technicians are “seeing mold” in the systems and/or ductwork. The questions always revolve around five questions:

1. Is it Mold?
2. What is causing the mold?
3. How serious is the mold problem?
4. How to resolve the mold issue?
5. How to prevent it from happening again?

Let’s see if I can provide reasonable guidance to each of these questions so other people can learn from them. I should point out that the thoughts in this post are my own opinions, supported by scientific facts and years of training. I am in no way representing my current employer with this post. Blame me, and only me if you do not agree.

Is it Mold?

OK, let’s start off with what mold is? Let’s keep it simple, and yes, people will argue with keeping it simple, but… Mold is basically a plant colony, microscopic in size. Mold is spread by spores. Some molds put off fumes that many people (including me) are very allergic to. For some people it has no affect. Some molds are very visible, some blend into their surroundings very well. Mold, mildew, and algae are three different things and sometimes are mistaken for each other.

Let’s start with a basic rule: There is no way any technician should or is able to diagnose mold by sight. Yes, you heard me right, it is impossible to accurately diagnose mold just by visually looking at it. I have seen technicians and their bosses be torn apart in court by a team of attorneys to the point they left the witness stand jobless and in tears because of their mistaken diagnosis of mold. Currently whenever someone says they have mold in their house my first question is, “may I please see the lab report?” No lab report I do not assume we are dealing with mold. The only way to accurately diagnose and identify mold is to do laboratory testing using surface and air samples comparing one area to another. For mold in a section of ductwork it takes a minimum of three samples being sent to the lab:

1) A surface tape sample of the area in the ductwork (swab sample works as well)
2) A surface sample of a clean area of ductwork for comparison.
3) A untouched slide (or swab) to prove that the material was not contaminated to start with.

Good Indoor Air Quality specialists will also take two air samples and send three canisters to the lab as well:
1) An air sample inside the affected area.
2) An air sample of the air outside the building for comparison
3) An empty, untouched sampling container to prove material not contaminated to start with.

Once the lab report comes back the specialist looks at the sampled areas vs the control areas. If the sampled areas (suspect areas) have more mold spores present than the controlled areas, they can say that mold is present. If the numbers are lower, or identical than mold is not growing in the area. Remember, mold is part of the natural environment. Every building in the wold has some mold, it is a matter of how much is present compared to the outside (control) environment. Inside should always be the same or slightly lower (within 10% if over). If not, need to find the source.

Any HVAC/R technicians that you know of that carry proper sampling materials around, or are even licensed or trained to identify mold? Do they carry a microscope to do the required counts? No? Than they should never be identifying mold, it is a tremendous liability and a source of many court cases.

What Causes Mold?

Mold itself is naturally occurring in the environment, it is everywhere. Yes. get over it. You cannot escape mold. But, what you can do is minimize it’s impact on your indoor environment. Mold needs certain things to grow: Moisture, warmth, and a source of food to grow on. Anything that is a porous plant based material is a great place for mold to grow. Mold also needs humidity (moisture), a humidity of over 50% allows mold to grow. Dryer environments are better for prevention, but do not go under 20% humidity as that will have a whole separate host of issues. Keep indoor humidity between 21 and 49%.

Water leaks, improper ventilation, improper dyer exhaust, improper cooling, incorrectly sized equipment, improper installation of HVAC/R equipment and ventilation equipment are primary causes of mold. It is important to have qualified contractors, and qualified technicians working on all cooling and heating equipment. It is important for home owners and property managers to locate and stop any leaks, fast! It is critical for HVAC/R technicians to do pressure measurements on ductwork in order to verify air flow. It is critical that installation crews read and follow the installation manuals each and every time. You have no idea how many times I find installation manuals crammed into equipment still sealed in their original shipping plastic bags.

There are some things anyone can look for and most likely find the cause of mold:
1. Bathroom exhaust fans should vent outside, and not into the attic.
2. Dryer vents should vent outside and not into the garage, basement, or attic.
3. When you open your front or sliding glass doors the air should slightly push outwards, it should never feel like a breeze coming in.
4. All ductwork that runs through the attic or basement should have all seams and joints sealed. With the system fan running you should never be able to feel air anyplace except at a register. For any leak you find assume there are five or six that you cannot find.
5. Ventilation is important. A house that is too tight, with no outdoor air exchange, will buildup mold spores and humidity. Both will cause problems.
6. Humidity. Check the humidity. You do not want indoor humidity without ventilation to be over 50%. If you have windows open and are ventilating properly you really do not have to worry about this. Generations of people before you lived without air conditioning and with open windows in 90% humidity and did not have mold issues. Mold issues are greater in recent years with tight constructed buildings, and a lack of outdoor ventilation.
7. Air conditioning size. Around 400 sq ft per ton for average construction. so, a 1200 sq ft home, that is conditioned space, will have roughly a 3 ton system. A 2000 sq foot house will have a 5 ton system. The larger the system the faster it will shut off, and the less it will dehumidify. The smaller the system, the longer it will run, and the more it will dehumidify. But the smaller system can’t be too small or it will not cool properly and will leave you with higher power bills. Never trust the contractor that asks you if you want to install a larger system than the one installed when the house was built. In reality many homes can downsize their air conditioning because of the newer and more efficient systems.

How Serious is the Mold Problem?

Any mold that is significantly greater than the outdoor comparison sample is serious. While it may not be bothersome to many people it can put others in the hospital. There have also been studies done that suggest that mold may be partially responsible for some pretty serious memory related conditions. Look it up online, you can find the same studies. Mold will not go away on it’s own. Once a mold colony has formed, has a reliable source of nutrients, has a reliable source of moisture, it is there to stay. Also, even if the moisture dries up and goes away the mold spores can last for hundreds of years, just waiting for the correct moisture content to come back, out of it’s dormant state, to start growing and spreading again.

Treat every single verified mold problem as extremely serious.

How to resolve the mold issue?

The most important thing is to get immediate testing of the affected areas. Air and surface sampling, sent off to a reliable lab, with verifiable reports returned. Equally important is to remove sources of water and humidity immediately.

Mold growth on non-porous materials, such as metals, hard plastics, tile, Formica, and some other materials can be cleaned. Mold growth on any porous material such as sheet rock, some woods, leather, cloth, paper, furniture, carpet, and other such materials cannot be cleaned. The materials must be safely removed from the building and disposed of. Mold covered materials are considered hazardous waste.

There is a basic rule with regards to what requires professional attention. Porous materials over one square foot will need to be removed and disposed of by a professional. Any flooding of a building with non-potable (not chlorinated) water requires professional attention (remediation). The remediation should begin as soon as possible or the condition will just get worse. It is important to remember that it is not just what you can see, but what is happening behind the walls and in other non-visible areas. Removing porous materials, beginning ventilation and de-humidification as soon as possible is key to a successful cleanup.

How to prevent it from happening again?

Mold is not preventable. It is naturally occurring and is everywhere. I know have said that earlier and it deserves repeating. Mold infestations, colony growth inside a home or other building is preventable. There are several common sense approaches to the prevention of mold infestations:

  1. Remove all sources of water. This includes roof leaks, basement leaks, pipe leaks, and standing water.
  2. Make sure the building is properly ventilated, that includes buildings that are air conditioned. See my next point.
  3. Make sure the building is maintained at a positive pressure. The goal is to always maintain the air pressure inside the building at a slightly higher pressure than the air surrounding the building. We do this by bringing in more outdoor air than the building exhausts. This slightly greater pressure allows bathroom exhaust vents, kitchen exhaust vents, combustion appliances (water heaters, and furnaces) to work better. It also makes sure that the building is not “sucking air in” through uncontrolled and unfiltered cracks and crevices such as our outlets and window frames. This is easy and should be part of standard construction practices and should be checked yearly.
  4. Make sure that the heating and air conditioning systems are properly maintained and cleaned. Removing humidity is part of what an air conditioning system does, wouldn’t it make sense to maintain it so it can do it’s job?
  5. If you are living in a flood prone area make sure you have the ability to prevent water from entering the building. This may include something as drastic as raising the building off the ground using beams and concrete footings.


Mold is a serious issue, but it is important that it be properly diagnoses through lab reports and testing. It is not, nor should it ever be, a way to sell consumers additional equipment. Mold is all around you, but with proper ventilation and de humidification it does not have to affect you, nor do you have to worry about it.