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Mold vs Mildew

Sam Smith Foundation Repair Consultant

Mold and mildew are necessary parts of the biological environment – their role in the ecosystem is to aid with the breakdown of leaves and other plant material, which helps enrich the soil. Their role is to take organic material and break it down. However, in a home environment, this task can cause major problems.

The first main problem with mold and mildew is to the health of your home. Because they both feed on organic matter, they can destroy the material they grow on, which may include the walls, furniture, or other materials in your home.

The second main problem is to your health. Inhaling mold and mildew spores can cause respiratory problems, especially to those who have pre-existing conditions.

Mold and mildew can also cause issues such as odor and can be extremely unsightly. This guide will walk you through the differences between mildew and mold to demonstrate the different treatments required for each, and the impacts both can have on your home. For more information on mildew and mold, see our guide on what is mildew and what is mold.


Both mold and mildew are fungi, meaning they are from their own taxonomic kingdom between animals and plants. Unlike plants, fungi don’t contain chlorophyll, and therefore can’t produce their own food from light. Instead, they need to absorb nutrients from their external environment. Fungi take nutrition from organic substances. 

Both mold and mildew reproduce either sexually or asexually by generating spores. The spores are released and, if they find a suitable environment, are able to create a new fungal body elsewhere.

Spores can travel by air or by water. Mold and mildew can start to grow within 24-48 hours, and release spores within 3-12 days.


Mold plays a critical biological function in helping organic elements to decay. Molds cause objects to decompose, which converts their nutrients into forms that other animals and plants can easily digest. They exist, therefore, to ‘recycle’ nutrients back into the food change. Without mold, nothing would decay.

As a result, there are hundreds of thousands of different types of mold in every different environment and niche. There is, according to the CDC, over 10,000 types of mold that can live indoors alone.

However, most molds you’ll find in your home are one of five different types:


Alternaria is a mold commonly found in the basement, on the walls in showers, on window sills and frames, and other places where moisture is present. Because it is so heavily connected with the presence of moisture, it is often found after an area has been flooded or damaged by water.

It can also be present in older buildings that may have suffered from water damage in the past.

Alternaria has a texture like cotton or down and can be black, grey, or dark brown. Alternaria spores are one of the most common causes of respiratory problems for those who have allergies or problems with asthma.


Aspergillus is the most common type of indoor mold. It is usually found on walls, paper products (including wallpaper), clothing, or insulation products. It has a wider range of colors than other types of mold – it can be gray, brown, yellow, green, white, or black.

Aspergillus is particularly harmful to those who have weak immune systems – the spores can cause allergic reactions as well as respiratory infections.


Cladosporium is unique in that it doesn’t require warmth in order to flourish. It prefers to grow on fabrics rather than on walls.

You’ll most likely first notice Cladosporium on carpets or curtains, or potentially on surfaces made of wood, such as floorboards. Cladosporium is a black or olive green color.


Penicillium is one of the worst offenders for producing odors. It’s usually found in places that are wet or have been in contact with water. It looks a typical ‘moldy’ blue or green color and can spread extremely easily.


Stachybotrys Chartarum is also known as black mold. The toxins it produces – called mycotoxins – can cause respiratory problems as well as problems like sinus infections, fatigue, and even depression. Other types of mold, and fungi in general, can produce mycotoxins as well.

Black mold requires constant dampness to grow, so it tends to congregate around leaking pipes, air conditioning ducts, or wet basements. You should remove black mold right away if you notice any in your home.


Mildew is, first and foremost, a plant disease that can cause major problems to crops in agricultural areas. If you notice a plant has splotches on its leaves, it may be infected with mildew. Although mildew comes from plants, it doesn’t need to be imported into your home via plants or plant products – it can travel in via spores on the wind.

There’s very little you can do to prevent spores from entering your home. The two most common types of mildew are powdery and downy mildew.

Powdery Mildew

Powdery mildew is the most common type of mildew you’ll notice in the wild. If you ever see patches of white or gray ‘splotches’ on a leaf or the petals of a flowering plant – that’s powdery mildew. Over time, these patches turn from light colors to a yellowish-brown or a black color. The mildew will eventually spread throughout the plant and cause it to decay.

The early signs of powdery mildew in your home are exactly the same – the splotches start as grey or white blotches, usually in a moist area of your home, before gradually becoming discolored as the mildew spreads. Although the light-colored splotches can be hard to detect, if you can catch them at this stage you can head off major infestations.

Downy Mildew

In nature, downy mildew is found less on flowers and plants and more on items like grapes or potatoes. They operate in the same way – spreading and decaying the ‘host’ – but have slightly different coloration. Downy mildew starts as yellow spots, but then these spots begin to turn brown. Again, it’s critical to catch the infestation at the early stage.


The first step in fighting mold or mildew is thinking about their optimal conditions. Both require all of the following to be present in order to survive:

  • Air
  • Water
  • Food
  • Temperature: 41-104 degrees F

Therefore, in order to remove the mold, you need to remove one of the four conditions.

Obviously, removing air is not feasible, and it can be tough to remove food (especially since this is usually the object you are trying to protect). Instead, you can use a dehumidifier to remove the moisture from the air or an air conditioning system to cool the area.


Knowing the difference between mold and mildew is important as it helps you more easily determine how best to treat it. The easiest way to tell the difference is based on appearance. The rule of thumb is that mold looks black or green, whereas mildew is gray or white. However, there are more subtle distinctions between the two.





Telling the difference between the two is only the first step; the crucial difference is what they can do to your home. Generally speaking, mildew is a preferable option for your home; only in extreme cases will it cause severe health problems. The spores can cause coughing, headaches, sore throats, and in rare cases, respiratory problems.

Mold can be much more damaging to health, particularly to those who already have health issues.

Some strains of mold cause allergic reactions, such as:

  • Irritation of eyes and throat
  • Sneezing
  • Skin irritation

Some strains can cause respiratory problems such as the following:

Some may also cause more serious health problems such as:


Aside from the differences in appearance, the easiest way to test whether you have mold or mildew is to do the ‘bleach test.’


  1. Place a few drops of bleach on an area of mold/mildew. Be careful as bleach can stain surfaces!
  2. Wait for five minutes
  3. After the five minutes have elapsed, look at the area again
  4. If it got lighter, then the infestation is mildew.
  5. If it has remained the same color, then it is mold.

Although there are commercial testing kits available, this is a fairly safe method of working out the specifics of your problem.

Ultimately, neither mold nor mildew is ideal to have in your home. However, knowing precisely what you are dealing with will make solving the problem far easier.

Understanding the difference between mold and mildew is the first step in that.

Mold and mildew are fundamentally important parts of our ecosystem, but neither are welcome additions to a home. Both have the capacity to damage furniture and cause health problems. While mold is definitely more damaging to both, mildew can also be troublesome. Getting rid of both before they spread is critical.

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Sam Smith Foundation Repair Consultant
SHORT BIO: Hey, I'm Sam Smith. I'm one of our service techs here at Regional Foundation Repair. I'm here to help you learn more about your home's foundation. I've been doing this for a while, so I have a few insider tips and tricks to share!

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