Technical Updates

Posts Tagged ‘mold and moisture prevention’

Comparing the latent and sensible loads for several major cities in different geographic regions (Peart and Cook 1994) helps illustrate the new definition. A study was done showing the monthly average latent and sensible loads from outside air for Orlando, Florida; Atlanta, Georgia; and Columbus, Ohio. During the cooling season in Orlando, the latent load far exceeds the sensible load of outside air. The effect of these conditions, which occur for more than half a year, is that any outside air drawn into the building envelope or occupied space will likely cause moisture accumulation and microbial growth problems. Furthermore, because this outside air is used for ventilating the building’s occupied spaces, it presents a huge dehumidification challenge for the makeup air system. Clearly, under these conditions, Orlando is highly susceptible to moisture intrusion problems.

Atlanta was shown to be less susceptible to moisture intrusion problems than Orlando because, on average, the difference between sensible and latent load is small, particularly during the peak cooling months. Standard AC systems have a better chance of accounting for the latent load in Atlanta than in Orlando. Nevertheless, the latent load in Atlanta represents enough of a moisture accumulation risk that it belongs within the upper boundary of the humid zone. However, according to the ASHRAE-defined humid zone, Atlanta is outside the critical zone for humid conditions.

When looking at Columbus, the latent load from outside air is consistently less than the sensible load. The reversal of the load relationship explains why buildings in Columbus are not likely to develop moisture-related problems from outside air intrusion, because any outside air that infiltrates into buildings in Columbus will be adequately dehumidified before it is cooled.

The new definition also explains why, in certain areas of the country, building commissioning procedures are more critical than in others. For example, if the building exhaust systems are started before the AC and makeup air systems, as is typical, huge amounts of moisture may infiltrate the building, depending on the outdoor conditions.

In applying the new humid climate definition, however, two qualifications must be made:

• The definition is based on average climatological data. At certain times during the summer, the latent load of outside air can exceed the sensible load to a much greater extent than was reflected in the study. Such episodes of extreme high moisture entering the building can cause problems despite seemingly safe average conditions and must be considered in problem prevention.
• If the building envelope has an improperly located vapor retarder, moisture accumulation problems can occur, even if a favorable sensible/latent load relationship exists. Condensed moisture behind the vapor retarder will never reach the AC system for proper dehumidification but will accumulate in the wall system. Thus, architectural aspects of the building work in conjunction with outside conditions to create problems.

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…

The HVAC system is typically designed to control the temperature inside a building and, as a by-product, also may control relative humidity (RH). In addition to keeping most people comfortable, the HVAC system should also help control contaminants in three ways: by filtration (filtering contaminants out of the air before they reach the building occupants); by ventilation (diluting the contaminants in the air by adding fresh outside air); and by pressurization (maintaining the right pressure balances between building spaces to keep contaminants from moving into the wrong place). If the HVAC system fails to operate properly, IAQ problems usually occur.

Pathways involve both a route for contaminants to travel through a building and a mechanism like air pressure to push the contaminant along that route. Pathways are affected by the building design, the operation of the HVAC system, and the building use.

Building occupants who spend an extended period of time (an eight-hour work day, for example) in a building are likely to report symptoms when IAQ problems occur. They are a good barometer of the health of a building.

All four factors combine to create IAQ problems. A change in any one of them can cause a dramatic change in the types of problems and symptoms that occur.

A large office building in Los Angeles illustrates this interaction. Workers in one section of the building were exposed to chemicals, including paints and adhesives, from another section of the building that was being renovated. The fumes were migrating to the workers’ area through the HVAC system that served both areas. The workers sued the building owners and managers, as well as the contractors, product manufacturers, and installers, and won a large financial settlement. If the building owner or manager had been aware of the four IAQ factors and taken proactive measures, the problem could have been easily avoided. For example, the pathway or pressure that enabled the chemicals to reach the occupants could have been removed by setting up a temporary exhaust system in the renovation area and blocking the return vents to the building’s HVAC system. These simple steps would have prevented the chemical fumes from getting into the common HVAC system where they could travel to the occupied areas of the building.

To be continued….

Indoor air quality is influenced by a variety of factors, including outside air quality, weather, building operation, type of mechanical systems, existing contaminants, occupant types, and building. Buildings that are designed for one purpose often end up being used for something entirely different. The new use may be incompatible with the original building design, and if the building owners are unaware of the need to adjust the building or its operation to account for the new use, IAQ problems can result.

Most experts group all these interrelated influences into four primary factors that are common to every IAQ problem in every climate.

Contaminants that can result in IAQ problems are generally classified as:

• Combustion products (smoking and cooking)
• Volatile organic compounds (VOCs) from solvents and cleaning fluids
• Respiratory particulates (asbestos and dust)
• Respiratory by-products (carbon dioxide)
• Microbial organisms (fungi and bacteria)
• Radionuclides (radon)
• Odors (perfume, smoking, and mold)

These contaminants cause IAQ problems only when a specific set of conditions exists that promotes them or allows them to reach levels that cause reactions in susceptible building occupants. Sometimes these conditions can be changed easily and the problem quickly remedied. For example, simply increasing the volume or distribution of outside air may reduce elevated levels of VOCs within a building to acceptable levels. At other times, however, such as when microbial problems occur, the conditions can be complex, requiring modification of both the HVAC system and the building envelope along with careful removal of the microbially contaminated materials. In fact, in most cases, microbial problems only require a change in the environmental conditions for the problem to not return.

The major types of contaminants in a building depend on the building’s location and condition, the climate, and the building use. Moisture intrusion and mold are the number one problem in hot, humid, or rainy climates.

To be continued….

These blog posts are brief by design and are not intended to provide comprehensive engineering data available elsewhere, nor to substitute for professional expertise. Instead, they address key issues and  help owners, designers, and contractors work together to improve decision-making. This is especially needed as architects participate in more construction work (e.g., through design/build) and as builders provide more preconstruction services.

No single piece can address all the issues related to moisture intrusion and mold growth. However, we believe that this blog addresses a significant proportion of the problems that architects will encounter in the design and construction of a typical commercial building. Nevertheless, the design and construction team are advised to seek additional input from other documents as well, and there are a variety of readily available, technically sound resources.

While this blog is intended primarily for commercial construction, the fundamental issues are similar, if not identical, in residential construction and can be dealt with in a similar fashion. However, the scope of these posts remains targeted to commercial architecture.

Most of the information presented here relates to preventing moisture intrusion and mold problems in hot, humid, or rainy climates, which is a significant portion of the United States. Parts of this blog address other regions and climates (for example, cold climates), but not in significant detail. For technical information on issues related to design and construction in those climates, please refer to other publications.

The majority of our blog content relates to air- or vapor-borne moisture. Mold and moisture problems associated with plumbing leaks or groundwater intrusion are not included. In our experience, those problems are usually more easily identified and resolved. While rainwater and groundwater intrusion are briefly mentioned in this blog, readers would be advised to seek additional information in resources specializing in those issues.

Today, there is unquestionably more information about the causes of and solutions for moisture and mold problems. We hope we have converted this information into usable knowledge that can be applied to create problem-free buildings.

To be continued….