How to Prevent Mold Growth in Your Home

Mold can be a frustrating thing to discover in your home and a frustrating task to clean and prevent from coming back. Once we understand how mold forms and what it needs to continue growing and spreading, we can better understand how to prevent it in our homes.

Conditions that Allow Mold to Grow:

High humidity

A relative humidity level of greater than 50% can lead to mold problems. Mold cannot grow with 60% moisture content, but it will grow at 61% moisture content. It is important to keep a home interior at 30% – 50% relative humidity reading. Some causes of excessive humidity: too many house plants requiring constant watering, poor ventilation, unresolved water damages/intrusion due to flooding, leaky pipes, exterior leaks in the foundation, attic, walls, windows, and the use of humidifiers without relative humidity controls. Damp basements or crawl spaces provide perfect conditions for mold colonization. Make sure you have a black 6-mil polyethylene vapor barrier covering all the soil in the crawl space to prevent moisture vapor from passing through the levels of the structure; this can cause a stack effect which may result in mold growth inside the attic as well as the crawl space. A vapor barrier will also help prevent mold from growing on the soil of the crawl space.

Leaky roof

Like other kinds of water leaks, water intrusion through the roof is difficult to find until it is too late. Check the attic regularly for signs of water damage or mold growth.

The exterior of the structure

Check for puddles of standing water or cracks around the foundation; this could mean you have water in your crawl space. Check to make sure windows and doors are sealed properly; if there is swelling or signs of mold you could have a problem in the interior wall as well. Make sure the ground around the foundation is graded away from the structure so water will run in the opposite direction. The water from downspouts should be directed away from the structure or have deep drain fields installed using gravel and rock.

Interior of the structure

Discoloration (water stains) of walls, soft spots, cracked or peeling paint, and loosening or bubbling of drywall tape, all are signs that moisture has infiltrated the wall. Do not set furniture flush against walls; leave space for ventilation to prevent condensation and surface mold growth. Avoid letting clutter of contents be stored against walls, especially in cabinets under kitchen and bathroom sinks; these areas are commonly affected by leaks that can initiate mold growth if not detected. Keep surface mold cleaned from windows and sills with a water/soap solution.

Mildew or musty odors

Odors can often be the first or only sign of a potential mold problem. In some cases, moldy smells will only be evident when the air conditioning or heat is turned on, indicating growth within the HVAC system.

Black mold in bathroom tiles

Tile grout is porous and will allow water penetration and mold growth behind the tiles themselves. In addition, dirt, skin cells, and grime (favorite foods for mold) are easily trapped in the grout making it an ideal environment for mold to thrive. Loose tiles will allow water into the inside wall which will result in mold contamination if not immediately taken care of.

Blocked gutters

Gutters that are blocked can cause water to seep into walls through the roof and can cause water to collect at the base of the foundation.

Warped wood

If wooden materials in your home have been infiltrated by enough water to actually warp, then sufficient levels of moisture are probably present to accelerate mold growth.

Clothes dryer and other appliances

If the steam from the appliances is vented inside, then this significant amount of additional moisture creates a great indoor environment for mold to flourish. Make sure all vents running to the attic are extended to the outside and above the roof line. The opening through the roof should be well-sealed to prevent leaking.

Poor ventilation

If the air pressure in your home is “negative”, meaning the air pressure outside is greater than it is inside, then it will force moisture and contaminates back into the home. If the air pressure is well into the “positive” side, then it can cause moisture to be forced into walls. The air pressure in your home should be slightly positive or at least balanced. A sign of inadequate ventilation is moisture condensation on windows and black surface mold.

How Does Mold Spread?

Mold releases spores into the air as a means of reproduction. In addition, cellular debris, such as cell wall material, can be released into the building environment from amplification sites.
Spores and cellular debris are micro-particulates and, as such, can remain airborne for considerable periods of time traveling freely through tiny cracks, crevices, and holes in building walls, and roofs. Spores, and cellular debris from mold, are allergens and result in reports of symptoms, i.e. itchy eyes, runny noses, headaches, and fatigue.
The responses to these allergens differ from person to person so neighboring building occupants may report very different reactions to mold amplification. Cellular debris from bacteria contains endotoxins which can produce allergic symptoms.
At high levels, this material is an irritant and can produce flu-like symptoms. Spores and cellular debris can remain an indoor air quality problem even after the active growth of an amplification site is corrected.

Rot is the result of a fungal infection of wood and wood products. Wood consists of cellulose and a complex hydrocarbon called lignin. The rot fungus can attack either the cellulose or lignin.
If it attacks the cellulose the brown lignin is left, and brown rot is the result. If the lignin is attacked, the white cellulose is left producing white rot. In addition, there is a rot process produced by the fungus Poria. This fungus can start in moist soil and send rhizomes great distances through wood structures.

What Does Mold Grow On?

Mold and bacteria live on hydrocarbon-based, organic materials. In buildings, there are abundant organic materials such as wood, wood products, paper, adhesives, and glues. Even in the absence of organic building materials, dust and dirt, which collect on surfaces in buildings, are primarily organic and can, in the presence of water, readily support mold growth.

Specific building materials that can support this growth include:

Paper products, i.e., the paper on gypsum panels used for drywall, facing for insulation, wall coverings, etc.
Wood and wood products including dimensional lumber, plywood, oriented strand board, and particle board.
Resins and adhesives. For example, the resin that binds glass fibers together to form fiberglass board insulation can provide food for mold growth unless treated with a biocide.
Glue such as wallpaper paste
Oily surfaces

How Much Water Does Mold Need?

Building materials may feel dry to the touch, but if they have absorbed enough moisture they can create a suitable habitat for mold. Mold can typically begin to grow when the moisture content reaches 17% or more.

Moisture constantly enters building envelope components such as liquid water, water vapor, and infiltrating moist air. In addition, the envelope may contain water that has been built in during construction.

If designed properly, the envelope will also continually dry due to moisture transport out of the envelope. If the rate of introduced moisture exceeds the rate of drying the water content of envelope components increases.

Many building materials have the capacity to safely store water. For example, wood is not considered to be wet until its moisture content exceeds 17%. If a building component takes on enough moisture to exceed its storage capacity, it becomes damp and microbiological growth and rotting can occur.

Additional Information

Mold occurs naturally and is a part of our environment; it is an integral part of nature’s digestive process. In the outdoors, mold feeds off and breaks down vast amounts of debris such as dead plants, leaves, or other organic materials. As the mold colony grows and matures there is a point when spores are released which then travel via air currents or water.

Mold does not need air, warmth, or light to propagate; mold grows best when there is little or no airflow. There must also be a food source in the mix; organic materials such as wood and paper products including drywall and insulation paper.

Mold grows best in moist, dark areas with limited airflow. Temperature can accelerate mold growth. Spores need a food source – organic material – to grow and colonize. Wood, paper products, cellulose, building materials, and natural fibers like cotton, leather, and soil are all examples of food sources. Mold can also grow on dust which contains dust mites and their feces.

Moisture is the main ingredient that promotes mold spores to grow. When moisture is introduced it supplies what is needed for growth to begin. There are mold types that can grow in very warm temperatures and types that grow even in freezing temperatures. As the mold colony matures and grows, it digests the available food source. When this occurs within a structure and the mold growth is unchecked, the structure will eventually lose its integrity and be destroyed.
If you notice signs of mold growth in your home, it may be time to call a mold inspector!