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NC State Extension

How Does Mold Form in a Home?

Water, excessive moisture or dampness are the key reasons that mold gets formed anywhere in a home. Mold never forms in the absence of water or moisture. If these occur for some reason in a home but are fixed right away, mold will not be able to form. It is only when the water/moisture related issue is not fixed within 24-48 hrs of its occurrence that mold starts forming and becomes a problem.

Mold is actually a kind of “fungi” and reproduces by “spores.” These spores are akin to “seeds” of fruits or vegetables and hence essential to the growth of mold. Spores are not visible to the naked eye because of their minuscule size i.e.3 -40 microns. Just to get an idea how small this is, the size of human hair is 100-150 microns (Florida Solar Energy Center, n.d.).

Photo 1 depicts a magnified size of mold.


Photo 1: Enlarged mold and mold spores


Mold and mold spores are present naturally in the outdoor environments. The spores can come inside a home either by drifting through air (open doors, windows, vents etc.) or by attaching themselves to clothes, shoes, pets etc. (CDC, n.d.). When a spore lands on a wet surface inside the home, it gets the right environment to thrive and causes the mold to grow.

The common water related problems in a home that can lead to growth of mold (AIHA, 2013) are:

  1. Flooding of home (hurricanes, rain storms, bursting of a water pipe etc.)
  2. Dampness in basement or crawlspace (High water table or in inefficient rainwater drainage system)
  3. Back flow or overflow of sewer
  4. Leaking pipes in the structure of home
  5. Leakage from roof (damaged roof, ice dams or choked gutters)
  6. Leaky windows
  7. High humidity levels inside home

Mold growing under a leaky sink looks like the photo 2.

mold under a sink

Photo 2. Source:

When you can smell it but cannot see it….

There are many a times when one is unable to see the mold in a home but can smell it. If so, the situation should not be ignored as it can still lead to health problems for the residents of the home. When one is unable to see, and can only smell the mold, the first step to stop the growth of mold is to find the hidden (as one is not able to see it) problem area by narrowing down to those areas in the home which are damp or have water damage. Mold can grow on back sides of wood panels, dry wall, wall paper, insides leaky pipes, roofs etc. (EPA, 2010) and hence would not be visible readily. All these areas should be checked for signs of mold growth so that appropriate action can be taken to clean and stop any further growth of the mold.


1. AIHA. (2013). Facts about Mold. Retrieved January 13, 2015, from

2. EPA (2010). A Brief Guide to Mold, Moisture, and Your Home. Retrieved January 13, 2015, from 

3. Florida Solar Energy Center. (n.d.). Mold Growth. Retrieved February 02, 2015, from

4. CDC. (n.d). Facts about mold and dampness. Retrieved January 13, 2015