Frequently Asked Questions

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Top Frequently Asked Questions:

  1. Why is mold a problem, and how should I get rid of it?
  2. What is toxic black mold?
  3. What is relative humidity, and how does it affect mold growth?
  4. How can site grading and drainage outside my home affect indoor air quality?
  5. Should I be concerned about mold and moisture in my crawlspace?
  6. What is negative building pressure?
  7. What about my heating, ventilation, and air conditioning (HVAC) system?

Why is mold a problem, and how should I get rid of it?

Mold growth is one of the leading causes of indoor air quality problems. Mold spores are potential allergens, meaning that they can trigger allergic responses much like animal dander or pollen. In addition, some types of mold produce toxins that can be harmful to humans. All visible mold growth should be removed from the indoor environment as quickly as possible. Mold sampling should only be performed to determine a specific hypothesis. If mold growth is already visible, sampling can be a waste of time and money, as all mold growth is cleaned in the same manner.

Light mold growth may be cleaned by the homeowner or building manager (while using the proper personal protective equipment, including eye protection, gloves, and dust masks). Cleaning methods should include an antifungal detergent and light abrasion to remove all fungal growth. If the area of mold growth is greater than ten square feet, or if the growth has invaded porous building materials such as drywall, a professional mold remediation contractor should be consulted.

What is toxic black mold?

The term “toxic black mold” actually refers to some species of the genus Stachybotrys. This particular genus of mold can produce harmful mycotoxins that may be toxic to humans. While no definitive link between Stachybotrys and specific human health effects exists, anecdotal evidence suggests that this type of mold can cause a wide variety of symptoms. Other types of mold have also been shown to produce mycotoxins that may be harmful to humans, including Aspergillus and Penicillium.

“Toxic black mold” is actually a misnomer, as many varieties of mold can appear black. In fact, Stachybotrys can appear in a variety of different colors, including pink. All black mold growth should not be assumed to be “toxic black mold.” However, any mold growth in thehome needs to be removed as soon as possible to prevent allergic reactions or other adverse health effects.

What is relative humidity, and how does it affect mold growth?

The amount of water that can dissolve in a given volume of air is dependent on the temperature. Absolutely dry air has a relative humidity of 0%; completely saturated air has a relative humidity of 100%. When a constant volume of air and dissolved water is cooled, the relative humidity goes up. When the temperature has dropped to a point where the relative humidity is 100%, water will condense to a liquid in the air itself or onto surfaces with a temperature below the dew point (defined as the temperature at which a given volume of air reaches 100% relative humidity). Ideally, the relative humidity indoors should be between 35 and 55%. If the relative humidity is too low, your skin will dry out and static electricity will increase. If the relative humidity is too high, condensation will occur on cool surfaces (such as windows, air conditioning vents, and on floor joists and beams in crawlspaces). This condensation provides the moisture necessary for mold growth.

How can site grading and drainage outside my home affect indoor air quality?

Site grading and drainage plays a huge role in indoor air quality. If surface water is directed towards or trapped against the foundation wall, moisture may enter the basement or crawlspace. This moisture will raise the relative humidity of the home, encouraging mold growth. To keep surface water from entering the home, the homeowner or building manager should take a number of actions. These include the installation of gutters (provided they are well-maintained and that the downspouts discharge at least six feet from the foundation wall), re-grading the soil near the foundation to ensure the proper slope to direct water away, and eliminating swales or gullies that carry water towards the home. Accumulations of mulch or organic debris against the foundation walls or on the roof (pine needles, leaves, etc.) that might trap and hold moisture should also be eliminated.

Should I be concerned about mold and moisture in my crawlspace?

Crawlspaces are not intended to be part of the breathing environment; however, because of negative building pressure and unsealed penetrations between the crawlspace and the living area, mold, moisture, and even radon are being sucked from the crawlspace into the home above. For these reasons, it is important to ensure that crawlspace air cannot travel into the living area of the home or building. All plumbing insertions, ductwork, or other penetrations should be properly sealed to eliminate air movement between the crawlspace and the breathing environment.

Vapor barriers should be installed in the crawlspace to minimize the introduction of soil moisture into the crawlspace air. Many vapor barriers are constructed of 6-mil polyethylene, which must be replaced every three to four years. Ideally, a sturdy 20-mil vapor barrier should be used. All soil should be covered and all joints should be taped or otherwise sealed. By extending the vapor barriers up the foundation walls and over the crawlspace vents, soil moisture is allowed to escape and the crawlspace itself is kept dry. Encapsulating a crawlspace in this manner also can reduce energy bills by up to 30%.

Crawlspace vents have long been installed with the intent of allowing moisture to escape. However, outside air (especially during the summer) is often much more humid than air in the crawlspace. As outside air moves through the crawlspace and cools, it loses some of its moisture. Over time, enough moisture can accumulate to encourage mold growth and water damage. This phenomenon is especially true in homes with un-insulated pipes or ductwork in the crawlspace. These exposed metal surfaces are often well below the dew point for outside air and are therefore prime locations for condensation. Floors should also be insulated (even in warmer climates, where the floor insulation actually prevents condensation on floor surfaces cooled by HVAC systems).

We recommend that vented crawlspaces be fully encapsulated to prevent this moisture problem. Crawlspaces should be inspected regularly for signs of moisture or other problems.

What is negative building pressure?

Negative building pressure results when an HVAC system inside a building draws in more air from the return ducts than it expels through the supply ducts. A building can be wholly under negative pressure, or exhibit zones of negative pressure in certain areas (e.g. a room contains a return air grille and no supply grille). Given the current configuration of most HVAC systems, most buildings operate under some degree of negative pressure.

When buildings operate under negative pressure, some air must be introduced from outside to balance the pressure differential. Ideally, this air would enter the HVAC unit through a valve or duct and be dehumidified before being introduced to the indoor environment. Unfortunately, however, this is rarely the case. HVAC units often have no allowance for outside air, meaning the air must be brought in from different areas. This often results in air being sucked in around doors and windows, through cracks in the foundation, or through other penetrations in the building skin. When this occurs, the introduced air is not dehumidified before entering the indoor environment. Over time, the indoor relative humidity will increase, potentially causing mold growth on cool interior surfaces or inside wall cavities. In addition, negative building pressure can introduce soil gases into the home, including radon.

To eliminate negative building pressure, the HVAC system must be reconfigured so that any outside air that comes into the home passes over the condenser coils. This process requires careful calculation and precision, so it should only be attempted by a professional.

What about my heating, ventilation, and air conditioning (HVAC) system?

Proper operation of the HVAC system is the backbone of any indoor air quality plan. The HVAC system should be regularly inspected by qualified personnel to ensure proper operation. In addition, regular examination of the ductwork is necessary to identify and repair all leaks, damage, or other problems.

The thermostat inside the home should be set to the proper room temperature (~72°F) to ensure both regular system cycling (which provides dehumidification) and also to prevent abnormally cool surfaces that might encourage condensation. In addition, the fan should always be set to the “AUTO” position. Setting the fan to “ON” will cause the fan to continue running even when the thermostat set point temperature has been satisfied. Areas of negative building pressure will develop, causing humid outdoor air to enter the home at an increased rate. This air will be circulated throughout the home by the HVAC system (which no longer dehumidifies the air when the compressor is not operating). Over time, the indoor relative humidity will increase and mold growth will begin.

Microbial growth (including bacteria, mold, and algae) often occurs in the condensate drain pan, since this area receives a constant supply of moisture during the cooling season. This microbial growth can clog the drain line, resulting in an overflow that can damage underlying building materials. The drain pan should be regularly inspected to prevent this microbial growth. The regular addition of chlorine bleach or an algicide tablet will discourage microbial growth and prevent clogging. The discharge point of the condensate drain line should be located outside the home or building, at a minimum of six feet away from the foundation wall.

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