How Did That Mold Get Into My Bedroom?

Does it seem like the news regarding mold problems is growing at an alarming rate every day? What has changed over the last 20 years to increase this phenomenon?

Several aspects need to be analyzed in order to thoroughly investigate the increasing alarm over mold. Let’s first take a look at how building practices have changed over the decades. The energy crunch in the 1970s resulted in the construction industry attempting to be creative, looking for ways to make a more energy-efficient home. The homes started to be constructed to decrease the rate of “air infiltration” and decrease the use of energy through greater and better insulation details. Additionally, this conservation led to the removal of our aesthetic air exchanger of the past (the wood burning fireplace). In its place, several different methods were used to provide air to the homes for heating.

The study conducted in Colorado by the City of Fort Collins, titled The Evaluation of New Home Energy Efficiency, revealed that ductwork in recently built homes was extremely leaky. Two-thirds of duct leaks existed in the “return ducts”. The “returns” carry house-temperature air to the furnace to be heated (or cooled) again and circulated back into the home. Typically, some of the return ducts are panned floor joists exposed either in the basement or crawlspace; other returns consist of wall cavities (stud spaces enclosed on two sides by drywall). While the International Building Codestill allows this leaky approach to be used as a method of providing air to the furnace system, Oregon and California now prohibit use of building cavities as return-air ducts.
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How do you know if your house “sucks”? One method is to measure if the amount of air being delivered to the home through the supply ducts is greater than the amount being returned through the intake grills of the return ducts. Using a flow hood on each return and supply grill while the furnace is running can measure this. Recently, homes tested by Pie Consulting & Engineering (“PIE”) indicated that the flows out supply registers were twice as large as the flows into the return-air grills.

How does this “suck” affect your home? The return air must equal the quantity of air that the supply side is delivering. Air not drawn in through intentional openings is pulled in through cracks, unsealed joints, and holes in the return building cavities. These holes and cracks would typically be the result of electrical line holes in the wall, sheathing, joist interfaces, and many other sources. Most of these leaks are either located in or connected to the basement and/or crawlspace. When the HVAC blower turns on, more air is drawn from the below-grade spaces than the above-grade rooms. This leads to negative pressure.

How do you know whether you have it or not? One way to find out is to close the basement door and stand on the steps with the HVAC blower on. Feel whether house air is rushing underneath the door into the basement. If the air is leaving the room, it would be positive pressure, if the air is coming into the room, the pressure would be negative. Similar to filling a balloon or letting the air in the balloon out.

Depending on the direction of the air flow, and the movement of moisture in the air can result in areas of high humidity. The use of vapor retarders on the inside wall are meant to minimize the humid air from entering areas that the humid air could condensate. If condensation occurs, one of the needs of fungal growth is met.

When mold begins to grow it rapidly expands its territory. Mold regenerates itself by propelling spores into the air. Mold spores are “spit” out away from the parent organism, land on a surface, and wait to see if water, food, air, and temperature are favorable to allow its growth.

If a home has a water leak, high humidity, wet soil exposure, or some other element that provides the mold a source of water, it will proliferate. High concentrations of humidity will equalize by migrating to lower humidity areas. So if an attic is heavily laden with moisture or a crawlspace has moist air, it can be drawn into the wall or floor cavities. The difference in temperature between the outside and the inside of the home can allow the humid air to condensate on the cold side of the wall surface. This can provide the water needed for the mold to proliferate in the space.

Thus, when the furnace system creates localized negative pressures, and the air is moved into the home via the cracks and holes, it travels through the cavities such as walls, floors, attics, or crawlspaces. Then, the small spores produced from the parent organism become airborne and can enter the ductwork system. Ductwork provides the pathway for the mold spores to be moved into the other areas of the home. If spores drawn from a crawlspace are delivered to a cold spot (a rim joist on the home’s north side), the combination of temperature, localized humidity, food (wood), moisture, and mold spores can lead to mold growth on those surfaces.

A thorough investigation of the duct system should be made to determine its tightness. Ducts should be sealed tightly, flexible ducts should be properly secured, and access areas should not be blocked by ducts. Visible holes in the ductwork are a clear indicator that the system cannot properly perform and may be inviting a problem.

What about the use of “duct tape”? The only use for duct tape not shown on the calendar of 1000 uses of duct tape is that of sealing an air duct. The Building Code states that the “Joints of duct systems shall be made substantially air tight by means of tapes, mastics, gasketing, or other approved closure systems.” The system used to close the tape shall be marked 181-FX for pressure-sensitive tape. However, the only tapes shown for this system do not include “duct tape.” Experience has shown that duct tape de-bonds from the ducts after installation, allowing leakage to occur.

So, recall the creation of the more energy-efficient home in the 1970s. Because cracks and holes are smaller and negative pressures are greater, the return air has to be pulled from smaller areas at a greater velocity. Consequently, the energy-efficient home is more efficient at transporting the mold spores from the cavities into the furnace and throughout the home. When indoor humidity levels begin to rise without mixing fresh outside air with stale indoor air, the result will be what we would expect: an increase in the problems associated with molds in the indoor environment.


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