Technical Updates

Posts Tagged ‘gypsum wallboard’

According to ASHRAE, a humid climate can be defined as one in which one or both of the following conditions occur:

1) A 67 degrees Fahrenheit [20 degrees Celsius] or higher wet bulb temperature for 3,000 hours or more during the warmest six consecutive months of the year.

2) A 73 degrees Fahrenheit [23 degrees Celsius] or higher wet bulb temperature for 1,500 hours or more during the warmest six consecutive months of the year.

This definition is somewhat problematic. First, it is difficult to interpret and apply to problem solving. Second, high dew-point conditions can also indicate areas where moisture problems occur. Atlanta, Georgia, for example, does not qualify as a humid climate under the ASHRAE definition, but high dew points are experienced in this area and problem buildings are often found there.

Industry experience with building failures suggests the need for a new definition of humid climates that more clearly identifies the geography where problem buildings are more likely to be found, and better explains why these problems occur at all. This new definition is based on observations about latent and sensible load: A humid climate is defined as one where the average monthly latent load of outside air meets or exceeds the average monthly sensible load for any month during the cooling season. (Latent load is the moisture in outside air that is brought into the building and requires removal via dehumidification. Sensible load is the air temperature that is sensed and addressed by the HVAC system, either by heating or cooling the air, to reach the established set point.)

Infiltration of air with a high latent load will cause moisture to accumulate in building materials such as gypsum wallboard, with subsequent material degradation and mold growth. This infiltration may also exceed the ability of the HVAC system to remove moisture from the supply air. On any given day in many temperate areas, the latent load may be greater than the sensible load without causing problems; however, when these conditions persist for a longer period (a month, for example), the resulting moisture accumulation is sufficient to cause building failure.

The occurrence of a high latent load during the cooling season is a critical factor in building failure. Thus, defining hot, humid climates in terms of the relationship of sensible to latent load in ambient air expands the ASHRAE humid climate zone to include other parts of the  United States that are highly susceptible to moisture-related building failures.

To be continued…

This leaves water, in both liquid and vapor form, as the only element for fungal growth that we can easily control. Fungi need to break down nutrients before they can absorb them. They do this by secreting enzymes, which require the presence of water on the organic surface that is in contact with the fungal spores. Once the organic material has absorbed enough water, the fungal spores secrete the enzymes, absorb the dissolved nutrient, germinate, and begin to grow, sending out filaments called hyphae. These hyphae extend over the surface of the organic material, allowing the fungal growth to absorb more nutrients. As the hyphae thicken, they dig into the organic material forming a protective mat called a mycelium. This mat helps to hold in moisture, allowing the fungal growth to continue to feed even if the air is dry. Eventually the mass grows conidia, which generate new spores that are transported by air to other potential nutrient sources.

The amount of water in an organic material can be determined in a laboratory and given a quantitative measurement called water activity (aw). The required water activity varies by kind of fungi. Forensic investigators of building mold problems sometimes use this correlation to help determine the source of water that caused mold growth. Stachybotrys chartarum, which grows well on the paper surface of gypsum wallboard, needs a very high aw to grow. However, too much water can inhibit mold growth.

Liquid water from floods or rain can wet porous materials to the saturation point, often wicking up surfaces such as gypsum wallboard. Mold can grow after the gypsum wallboard dries to the aw needs of whatever fungal spores happen to be present. High indoor relative humidity can cause condensation on cooler surfaces. Condensation will absorb into porous materials, such as gypsum wallboard and ceiling tiles, and elevate the material’s aw enough to allow mold to grow. Therefore, control of moisture in buildings, including hidden spaces such as exterior wall cavities, in both a liquid state and a vapor state, is critical to controlling mold growth in buildings.

To be continued…

Mold is an important part of the earth’s ecosystem, breaking down dead organic matter. For fungal growth to occur, four elements must be present: fungal spores, nutrient sources, appropriate temperature, and water. Of these four elements, water is the easiest to control in occupied buildings.

It is estimated that more than 1.5 million species of fungi exist. Fungal spores are ubiquitous; they are found in indoor and outdoor air, and on, and imbedded in, the surfaces of building materials. Non-HEPA (high-efficiency particulate air) filters, which are found in many building HVAC systems, cannot remove fungal spores from the air because of the spores’ very small size. These spores tend to disperse through the air and settle on all building surfaces, where they can remain dormant for years. It is impossible, or at best impractical, to remove fungal spores from the indoor environment.

Nutrient sources are any organic materials. The paper surface of gypsum wallboard is a prime nutrient, because it easily absorbs moisture. Even on inorganic materials, such as the vinyl in furniture, nutrients exist in settled surface dust. Such nutrient sources cannot be easily eliminated from the indoor environment.

The temperatures that are best for humans are also ideal for fungi. Different species of fungi have different optimal temperature ranges. However, in general, fungi grow well between 40 degrees Fahrenheit (4 degrees Celsius) and 100 degrees Fahrenheit (38 degrees Celsius). (Some can survive at temperatures down to approximately -23 degrees Fahrenheit [-30 degrees Celsius] or up to approximately 140 degrees Fahrenheit [60 degrees Celsius].) Because the comfort range for people is well within the comfort range for fungi, modifying temperature is not an option for controlling mold growth. 

To be continued…