Why Does Mold Grow On Walls

Mold growth is a common occurrence in many homes. If we observe mold on our walls, it can be concerning and raise questions about why it is happening. In this article, we will explore the key reasons mold grows on walls and how it can be prevented.

What Is Mold?

Before diving into why mold grows on walls specifically, it helps to understand what mold is. Mold is a type of fungus that produces microscopic cells called spores that spread through the air. There are over 100,000 species of mold. When mold spores land on a damp spot, they can grow into mold colonies when conditions allow.

Mold serves an important role in nature, helping to decompose organic matter. Indoors, however, mold growth indicates excess moisture and can lead to damage or health issues if left unchecked. On walls, mold colonies often appear fuzzy or have coloration ranging from black to green to white.

Key Factors That Allow Mold To Grow

For mold to grow, there are three key conditions that must be met:


Mold needs moisture to grow. Mold can thrive with a relative humidity level consistently above 55% in a home. Damp spots with moisture available can provide an environment mold needs.


Organic materials that mold uses as nutrients must also be available. Drywall, wood, wallpaper, dust and dirt that accumulate can supply nutrients mold needs to flourish if moisture is also present.

Warm Temperatures

While mold can grow at cold temperatures, warmer indoor temperatures between 50-100°F allow mold to thrive with accelerated growth. Stagnant air without circulation also creates an inviting environment for mold.

Where Does The Moisture Come From?

If controlling moisture is critical to preventing mold growth, where does troublesome moisture come from? There are a few common ways walls become exposed to sufficient moisture for mold:

Plumbing Leaks

Leaks from appliances, pipes, sinks or toilets can allow water penetration into walls. Small leaks over time often go unnoticed, slowly soaking walls and surrounding materials. Even repaired leaks can leave moisture residue.

Rain Leaks

Penetration of rain, snowmelt or runoff due to degraded or missing exterior protection also provides a moisture source. Poor drainage and grade sloping toward the home help funnel moisture to the structure.

High Humidity

Without proper climate controls like air conditioning, daily activities like cooking, showering, cleaning and even breathing raise humidity. If walls lack vapor barriers or adequate ventilation, moisture condenses on cooler wall surfaces.


Severe weather or plumbing disasters can also cause flooding. Water from floods quickly saturates wall cavities. Even after water extraction, residual moisture gets trapped within wall materials if not completely dried out.

Why Do Walls Provide Good Conditions For Mold?

There are a few specific reasons why damp walls often foster mold growth:

Organic Materials Present

Interior walls typically have abundant organic nutrients present. Drywall paper, wood studs, or layers of old wallpaper provide food for mold. Dust and debris in cavities or gaps give mold a boost.

Difficult Drying Conditions

Wall air pockets combined with porous materials trap moisture easily. Narrow cavities lack airflow, slowing evaporation. Trapped water takes much longer to fully dry out.

Unseen Growth

Wall mold is hidden from view in most cases. Out of sight, homeowners may be oblivious to mold threats taking hold inside until major damage becomes evident. Unchecked moisture and humidity sustain mold colonies behind walls over time.

Signs Of Mold Growth On Walls

Hidden mold still manifests outward signs with clues related to excess moisture. Homeowners should watch for:

  • New or spreading stain marks on surfaces
  • Bubbling, cracking or soft drywall
  • Peeling wallpaper
  • Dark spots on exterior surfaces
  • An earthy, musty odor
  • Increased humidity and colder surfaces on inner walls

Catching mold early requires being on alert for subtle signals of extra moisture. Don’t ignore those first tiny stains or cracking in painted drywall before mold infestation escalates.

Health Concerns With Indoor Wall Mold

Why be concerned about mold hidden in walls? Beyond directly damaging materials, mold introduces health risks including:

Allergic Reactions

Inhaling airborne mold spores can trigger allergy symptoms like eye irritation or skin rashes. Severe mold allergies require avoiding exposure.

Respiratory Issues

Microscopic mold fragments easily enter lungs. Exposure can worsen chronic lung illnesses such as asthma. Those with other comprised immune conditions also face heightened vulnerability.

Toxic Mold Dangers

Some types of mold generate mycotoxins that cause toxic mold syndrome with flu-like, long-term health consequences in those sensitive to exposure. Stachybotrys chartarum or “black mold” is most notably associated with mycotoxin production.

Preventing Mold On Interior Walls

With an understanding of why mold grows on walls, prevention comes down to limiting moisture. Useful proactive measures include:

  • Maintaining relative humidity at or under 55%
  • Ensuring bathroom exhaust fans ventilate properly
  • Avoiding overflow from leaks, floods and heavy condensation
  • Repairing building envelope issues early
  • Allowing airflow circulation on inner wall surfaces
  • Catching leaks right away and fully drying affected materials

With diligence to keep walls dry, the threat of hidden mold remains minimal. Indoor humidity control combined with fixing moisture intrusions promptly restricts the conditions that allow sneaky mold growth behind walls.


Mold takes hold on interior walls by exploiting hidden cavities where persistent moisture and nutrients exist. Taking action at the first sign of water intrusion reduces the likelihood spores take over unseen. Keeping humidity in check while identifying and addressing leaks quickly helps ward off ideal mold growth conditions. Remaining alert to subtle signs of excess moisture helps catch expansion threats within walls early. With vigilance, proactively keeping walls dry prevents Surreptitious mold from taking hold. Addressing root causes restricts troublesome health hazards, property damage, and inconvenient remediation down the road.

Dealing with indoor mold on your walls? Contact Southeast Water Restoration today!