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What is the Difference Between Mold and Mildew?

September 6, 2021

Unmasking the Organic Intruders: Exploring the Enigmatic Stains of Mold and Mildew on Houses and Buildings

By Steve Lawrence; Klein, TX

Mold and mildew are both mold types that can grow in damp, dark areas. They can be found on siding, bricks, shingles and window sills. The difference between mold and mildew is that while both are a form of mold, mildew has a flat growth habit. Mildew also grows more quickly than mold because it needs less water to grow. Mold will grow in any moist area and does not need light to do so. Mildew prefers moisture from high humidity levels which occur on damp, shaded surfaces. If you notice signs of mold or mildew such as light or dark patches on your home's surface then contact us for how we can help eradicate them!
Mold on Siding
Pictured above showing house siding with moderate growth of mold.

Eradicating mold living on your home's exterior surfaces

Let's start with mold on the exterior surfaces of your house. While it is important to remove mold as quickly as possible, you need to exercise caution when using chemicals that could harm other plants and animals outside. Also, if the mold has started growing into or through certain types of paint or coatings (e.g., stucco) thereby rendering them failed surfaces, then removing and replacing these might be necessary for successful removal efforts. Of all the homes we have soft washed at Klein Pressure Washing, we have only seen this a few times.

Black Roof Streaks

Not to be confused with either mold or mildew, black streaks on your roof or a form of algae called gloeocapsa magma.

On the other hand, if mold has only just begun appearing on an interior surface such as siding for example; there are many ways to eradicate this - most notably from a professional applicator's perspective is by doing spot treatments using borate or ammonium sulfate with a measured addition of a quality surfactant (soap) formulated for this application. We use Southern Drawl, Apple Wash or Lemonator. These also work great when treating the black stains and streaks on a roof caused by a black algae called gloeocapsa-magma.

For severe infestations that are beyond cosmetic in nature, we would recommend a professional mold remediation and restoration professional.

Removing mildew living on your home's exterior surfaces

Mildew has a similar appearance as other molds but produces less damaging effects. Its removal from your home's surfaces is virtually the same process but employs overall uniform coverage vs. spot-treating.

Applying a uniform coat of sodium hypochlorite with surfactant to the entire surface and allowing it to dwell for approximately 5 to 15 minutes will yield the best results. Two or three treatments may be required for more stubborn stains / infestations.

Again, and as stated above, if mold is present, it can be treated in much the same way with one major difference; bleach cannot kill mold spores that may be hidden deep within your home's surfaces. This is why it would require an application of chemicals like borate or ammonium sulfate to eradicate the mold growing inside sealed cavities.

Mildew on Siding
House siding showing the growth of mildew and/or gloeocapsa magma algae.


Mildew living on exterior surfaces should also not pose a problem when dealing with its eradication unless there are layers upon layers of mildew growth over several years which will prove difficult if not impossible for sodium hypochlorite alone to eliminate all at once without leaving unsightly residue behind after treatment. Mold usually inhabits only one area while mildew tends to spread faster than mold. But if the infestation is severe, we would recommend a professional mold remediation and restoration professional.

For you next house wash, contact Klein Pressure Washing who has the equipment, experience, and know-how. We’re available to give you some helpful advice and provide expert guidance in any way we can! Give us a call or book a free quote today.

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