In order to better understand the growth of mold in buildings it is necessary to understand what is required for mold growth. To start with we must have the fungus present. Since mold causing fungi are ubiquitous in most areas, their exclusion is not practical. A susceptible substrate is another requirement. Most buildings are made up to a large extent of wood or paper products and these are for most fungi, acceptable food sources. It is not feasible to do away with susceptible plant based building products (Figure 1). Certain fungi such as the Aspergillus group require a higher energy level from seeds or other energy rich sources. The Cladosporium group will often require energy sources such as dead insects or insect and animal feces. These food sources can be eliminated and consequently the Aspergillus and Cladosporium fungi can be avoided. All fungi require a favorable environment in which to grow. Water and warm temperatures make up this favorable environment. Water is the requirement that can be most easily eliminated and is thus the single most important parameter in the prevention of mold in houses.

Points to consider in the prevention of house mold deal primarily with the prevention of free moisture in the house. The most common moisture traces from bottom to top in the house must be investigated and the specific source eliminated before any control or remediation is attempted.

Lot elevation is the first parameter investigated (Figure 2). Most cities have building codes which require house floors to be constructed at least 9.5 inches above curb height. In most cases this will prevent flooding unless the whole area floods. Unfortunately many people ignore this requirement and suffer water damage even in non flood prone areas. The higher that we can build floor levels above curb height the better (Figure 3).

Roof leaks are thought by many to be the primary source of water entrance into a building. This may be true in high rainfall areas, but not in Central Texas. Our rainfall in Central and West Texas is so infrequent that water leaks through the roof usually have the opportunity to dry out before fungal growth can become a problem. Roof slope is an important consideration in construction. The less pitch or slope on a roof the more likely the roof will be to leak (Figure 4). Valleys are the primary leak locations. Obstructions in valleys such as leaves or chimneys block water escape and enhance leaks (Figure 5). Advance architectural planning and frequent valley cleaning can both help to avoid this problem. Most homeowners look up and see circles on the ceiling and think they are seeing mold when what they are seeing in most cases is simply dirt that has been moved by the water leak before it dried out (Figure 6).

Hot water heaters constitute one of the worst chronic water leaks in many houses. The question is not if a water heater will leak but when it will leak. They will all eventually leak and the leak may be so small it goes undetected for long periods of time. All water heaters should be placed in a pan which is plumbed to the sewer or have a drain built into the slab underneath them. This will allow most leaks to not cause a mold problem during the early stage. Placement directly on the floor (Figure 7) is just asking for a problem.

Leaks occur in all types of house plumbing (Figure 8). In new homes, this may be due to faulty plumbing that was not properly pressure tested or where a pipe was pierced with a nail. These leaks usually surface rather quickly. In older homes, the plumbing does not last forever. Depending on water quality, copper pipes usually survive for 30-40 years. Without proper grounding of copper pipes, leaks caused by electrolysis may occur much quicker. This will be much more common in a pier and beam house or even in a slab when some types of pvc or plastic is used though the soil and metal pipes are only used above ground. Tubs, showers, sinks, dishwashers, clothes washers and garbage disposals can all develop slow leaks. Leaks around drains are most common (Figure 9). Mixer heads on both tubs and showers wear out (Figure 10). These mixer heads should never be plumbed into a wall that does not have an access door. These access doors allow periodic inspection of the plumbing and detection of leaks before they become a problem. Due to the location of most plumbing at or near the floor, most mold problems are detected within 18 inches of the floor (Figure 11).

Air conditioning systems present another type of water problem. Condensation forms on cooling coils and drips into a pan before emptying into the drain. This results in algae growth which plugs the system at various points causing overflow of condensate. A clean properly maintained system does not experience nearly so many problems. Filters should be replaced at least monthly (Figure 12). A cup of household bleach flushed through the drain system 3-4 times per year will also help to prevent algae build-up in the drain (Figure 13). Air conditioning systems with copper coils (Figure 14) are much less susceptible to algae growth than are aluminum coils (Figures 15, 16, 17) . Copper is a natural algaecide/fungicide and sufficient copper erodes from the copper coils to significantly reduce algae growth. If a condenser unit could be manufactured with copper fins in addition to copper coils, an even cleaner systems would be the result. Copper condensate catching pans would also be helpful. Algae control results in a system less likely to clog and overflow. Without overflow there is no water and this point source for mold growth is eliminated.

Windows present several sources of water (Figure 18). In houses where the heating, ventilation and air conditioning (HVAC) system is oversized, windows are more likely to produce condensate which wet walls in winter. A system that is oversized runs for shorter intervals and is given less time to dehumidify the air and thus is more likely to produce window condensate. Double paned windows produce less condensate than single windows. Wood framed windows usually produce less condensate than aluminum framed windows. Aluminum frames filled with insulation materials will be helpful. Windows should be recessed into the building or under some type overhang which prevents moisture leaks from above.

Dead insects or feces may be present under heavy appliances that are seldom moved for cleaning (Figure 19). These are not a great health hazard until moisture becomes available in the area. When the evaporative pan overflows or the drain leaks, they provide the substrate on which several fungal species grow. The Cladosporium group is the most common. Animal feces in attics due to wild animal invasion can also provide a food source for additional fungi.

Houses that are excessively sealed with non permeable plastic moisture barriers are slower to dry following any type of moisture invasion. Plastic coated wall papers are nice until moisture invades the wall. They then provide an excellent mold atmosphere.

When house mold infestations are examined, they usually are made up of several distinct colonies of fungi that vary in growth type and color (Figure 20, 21, 22, 23, 24, 25, 26). Everyone seems interested in black mold. People apparently have been led to believe that if it is not black there is no need to worry. This is false reasoning. Some of the more toxic Aspergillus fungi are green. The Fusariums are usually pink. We see numerous white, brown, gray and blue fluorescent colonies in infected walls. Some of these colonies are more dangerous than others but any and all of them when detected in the home indicate an unhealthy environment and should be eliminated.

Elimination of house molds must start with identification and elimination of the water source. After that, removal of infested materials can be undertaken (Figure 27). Personal protection equipment should be worn by workers as they remove moldy boards and paper products. These removed products are not considered hazardous waste and can be burned or buried. It may be possible to clean structural material with soap and water followed by a bleach solution. After materials are completely dry, they should be painted with some type of paint sealer. As long as this cleaned area is kept dry, it will be no more susceptible to mold growth than any other materials in the house.

Construction type and material is very important in the prevention of house mold. Stucco houses are best suited to the desert areas where moisture is seldom a problem. When stucco is used in higher rainfall areas, it should be accompanied by steep roof slopes and good roof overhang to facilitate water escape (Figure 28). Selection of moisture impermeable sidings are very important where walls emerge through roof lines (Figure 29). This prevents wicking of water and consequential leaks. Flat roofs almost always leak and composition roofs are easier to seal than most metal roofs (Figure 30). The standing seam metal roofs are the most leak resistant of the metal roofs. A composition roof gets tighter each year and a metal roof is tightest the day it is installed. Adequate roof overhangs or guttering is necessary to keep water from running down walls and invading wall space (Figure 31).

This final photo depicts a house which exhibits good water management from the outside (Figure 32). Siding is of a type that repels water. Roof slope is steep enough that the water quickly runs off. Composition shingles are well sealed and there are no valleys. Windows are set under an adequate overhang. The porch is two inches lower than the floor and the sidewalk is five inches lower than the porch. The lot then slopes quickly to the street. These points address numerous water invasion points and all are inexpensive and esthetically pleasing (Figure 33).