Why Is There Mold On The Styrofoam In The Air Conditioner?

Mold growth typically happens where there is a lot of moisture and nutrients to feed it. Most mold strains are common to leaves and grass and more specifically the soil that is feeding the grass.

So how does mold get on the Styrofoam in the air conditioner?

1. The Styrofoam is dirty.
2. There is standing water inside the air conditioner.

Styrofoam is one of the most unlikely places for mold to grow. The reason is it has absolutely no ability to absorb moisture and it has zero nutrients.

But given the right circumstances even Styrofoam can have mold growing on it.



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1.It is dirty.

If there is mold growing on the Styrofoam inside of your window air conditioner, it is most likely because the Styrofoam is dirty and there is either rain or humidity droplets supplying moisture to it. All inside a dark humid space.

The outside of the window air conditioner is in fact outside in the elements.

Overtime dirt is going to get into the machine and get on the Styrofoam.
Combine the nutrients of the dirt with the humidity and darkness inside of the housing of the air conditioner and you have the ingredients of mold production.

2.  There is standing water inside of the air conditioner.

Most all household molds have one thing in common, they all thrive in standing contaminated water.

Condensation from the cooling coils inside of the air conditioner is meant to drip freely to the outside of the machine so as the inside of the machine stays dry.

If the inside of the machine is unnaturally filled with debris, the drip hole could be clogged to the point where there is standing water inside of the air conditioner.

This could explain why there is a continuing water supply for the Styrofoam.
Again creating the essentials for mold production which are nutrients, moisture and darkness.

Something to remember here is that black mold requires a constant supply of moisture over an extended period of time.

Standing water or drenched leaves inside of an air conditioner qualify for the requirements of black mold to grow.

If there is black mold on your Styrofoam then take the necessary precautions to clean it because certain molds that are black in color can be Stachybotry, the infamous mold that is toxic to breathe.

Don’t panic, just remember to wear gloves and a face mask. There are plenty of YouTube tutorials to show you how to do this without it being dangerous.

Another reason why there may be standing water is that the air conditioner is not level and condensation water is able to gather to the lower side of the unleveled unit.


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Where is the air conditioner located?

If the inside of the air conditioner has mold and you’ve already determined that the recipe for mold is nutrients, moisture, and darkness, then the question is , how is the source of nutrients getting inside of the machine?

Some of the possibilities include that it is located under a tree, or it is in an area like a wind tunnel where dirt is easily kicked up and can get into the machine.

There are many reasons why an air conditioner can be exposed to extra dirt and debris.

If that is the case then you should consider moving the air conditioner to an area where it is less likely to get dirty inside.

Also if the air conditioner has been recently brought in from storage, the likelihood of it being dusty inside is more than a little high.

Storage areas have a tendency to be humid, whether it be the Attic, the basement, or a storage building outside, they almost always seem to be moist.

Again you have all the components necessary for the production of mold in one spot. Humidity, dust, and no light.

Taking apart your air conditioner when you bring it out of storage is a great time to examine it and make sure that there is no debris inside the block the drainage hole and to clean off any dirt and dust on the styrofoam.

Another tip for keeping water from gathering inside the bottom of the air conditioner is to drill a couple of 3/8″ holes in the bottom of the square housing to give the moisture more area to escape through.

But regardless of whether you’re taking it out of storage or not, it is time to take apart the air conditioner and completely clean out any mold, debris, and dirt.

Another additional tip at this juncture is to add a mold inhibitor.

A mold inhibitor is a specific product that is manufactured to add to livestock feed. It keeps mold from being able to grow in their food.

It is also sold as a spray or solution to keep decks from mold growth.

While your air conditioner is taken apart for cleaning and maintenance, spraying a mold inhibitor on the styrofoam as well as the other components inside the machine before you reassemble it is a great way to help ensure that your air conditioner and more importantly your air is free of mold and mold spores.


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Even though Styrofoam is a material that facilitates mold easily. Any material given the right circumstances can develop mold growth.

If there is mold on the Styrofoam in your air conditioner, then more than likely the Styrofoam is dirty.

Mold needs nutrients, and without there being something like dirt on the Styrofoam, there is no way that mold could grow on Styrofoam.

Mold also needs moisture, if there is something blocking the exit of the moisture inside the air conditioner, like mud or leaves, then you have all the components of mould and the perfect environment for it to grow in.

In order to find out if you have these issues, you’re going to have to take apart your air conditioner to clean and maintain it.

Taking apart and cleaning an air conditioner is a very easy thing to do and there are plenty of tutorials and YouTube videos available to take you through the process.

But before you put the air conditioner back together and reinstall it, it is the perfect time to consider spraying a mold inhibitor on the Styrofoam and all the other components inside of the air conditioner.

Also adding another drip hole or two by drilling a 3/8 inch hole in the bottom of the air conditioner will not diminish its performance at all and will only help keep moisture from unnaturally gathering inside of the unit.

Air conditioners will last an amazingly long time when they are maintained and kept clean. Keeping mold from growing in your air conditioner comes down to exactly those two things.

Published by

Dennis Reed

Dennis Reed Owner and Author @ BreatheBetterAir.org