Why Do I Have Mold?

Understand why you have mold in order to get rid of it

That musty smell, those unsightly splotches… could it be MOLD? Should you be concerned? And what should you do about it? If you, poor thing, are dealing with a mold problem, it will be a big help to you in the long run if you can understand why you have mold.

Mold is a serious issue, so yes, you should be concerned! Not only is mold very destructive to the materials in your house, it can have harmful health effects, even on previously healthy individuals. Those with more fragile immune systems are even more at risk.

Mold’s raison d’être

First of all, think about what mold’s purpose is in nature. It has a very important role as a decomposer! If mold wasn’t around, leaves on the forest floor wouldn’t break down, fallen trees wouldn’t decompose, and basically things would just pile up everywhere.  Mold is a recycler!

Unfortunately, it will also try to recycle (decompose) the materials in your home if the conditions are right. So what we want to do is keep mold growth outside where it can do it’s job where it’s supposed to, but not let it do that job inside.

How does mold get inside your home?

It is actually totally normal to have mold spores in your home. Mold spores are in our air, whether it’s inside or outside. In a healthy home we will find fewer mold spores in the inside air compared to the outside air. Why’s that? Because mold is supposed to be outside, doing it’s job of decomposer. It shouldn’t be decomposing things inside, so there shouldn’t be as many spores.

When you are outside, where do you see things decomposing? On the underside of a log, maybe. Or perhaps under a pile of leaf litter. If you have a compost bin, over time you will see that the organic matter you have added to it will break down, aided by molds, insects, and earthworms. What are the common conditions to these situations? Plant matter, darkness, and moisture. If you want to start a mold farm inside your home, by all means, keep it DARK, and HUMID.

While mold likes to grow on wallboard or wood joists in our homes (plant matter) it will also grow under carpet, on plastic showers, and in other places you would never expect it that are not plant matter. It will grow in dust on any type of material if you provide the right conditions: darkness and moisture.

mold in basement.png

Typically when people have mold problems in their homes, there are more spores inside than there are outside, and visible mold growth begins. It may not be visible to you, however, it may be hiding in your wall. Usually mold becomes a problem after events or changes in circumstance that introduce more water or humidity into the home than was there previously. These tend to be leaks, flooding, and high humidity.


A drop of water underneath your sink might fall every few seconds, hitting the fiberboard bottom of your bathroom cabinet. Eventually the moisture content will be high enough that the mold spores in the air will start to celebrate, call each other and set up camp in the nice moist fiberboard. These are prime conditions for mold to start doing its job – decomposing! Woohoo!


A flood is more water than a leak. If your water heater breaks down and starts emptying its contents onto your floor, we’d consider that a flood. If the water is allowed to sit for too long, the affected wallboards will develop mold growth on them. How long is too long? The rule of thumb is 24 – 36 hours. That means if you have a flood in your house, you need to get everything dried out within 24 hours, preferably.

High humidity

Let’s say that you live by yourself, take one shower a day, and turn on your bathroom vent while and after you shower. Then you meet the love of your life! He and his twelve year old son move in with you – into your lovely mid-century home that has only one bathroom. Your guy and his son never remember to use the bathroom vent when they shower. In fact, both of them close the bathroom door after they’re done. Where does all that extra moisture go that’s floating around in the air after the 3 showers a day? Well, it starts hanging out on the bathroom surfaces – the shower curtain, the walls, everything. If this goes on long enough without the bathroom being able to dry out… mold will find a way!

mold behind wallpaper.png

What’s next?

But what do you do next? If you can determine the cause of your problem – fix it! Fix the leak, rip out your moldy wallboard and replace it. If you are not sure of the cause of your problem, have a building biologist or other mold expert (one that does NOT perform mold remediation, as this is a conflict of interest) give you some help.

Just make sure you understand that if you have mold, it would be highly unlikely for the problem to go away on its own, so you really need to seize the mold by its stinky horns and tell it to go back outside!