Several building investigators and biologists have contributed to this list of commonly-heard, but misleading statements about mold.
These are correct enough that they cannot be eliminated entirely from discussions—and yet they are incorrect enough that they have often led to ineffective or even counterproductive decisions by designers and owners.
“Mold won’t grow until the rh rises above 70% ”
This statement is false when it refers to rh measurements of air in the middle of a room, or air inside a duct, and indeed all measurements taken in air which is in any way separated from the immediate surface of the potential food source.
It is only correct when it refers to the air inside the microscopic crevices of the surface of that potential food source. Since measuring rh in micro-crevices on all surfaces is not practical with HVAC sen-sors, it’s best to avoid using 70% rh as the threshold of concern. It creates the misimpression that when the building automation system or hand-held instruments report that the rh is less than 70%, the risk from mold is small. That’s not true. There’s a great deal of risk at that level. Here’s why.
If the surface of any material is colder than the air—such as when cold air from the air conditioning system blows on a wall surface—the rh inside the crevices of the cold wall surface will be far higher than 70%. This fact is explained by figures 5.5 and 5.6.
So in place of the 70% rh threshold for concern, it’s more productive for HVAC designers, architects, building owners and building operators to focus on keeping the indoor dew point below 55°F [12.8°C]—at least for building-related mold in hot and humid climates. That’s a value which, when reported by the building automa-tion system or by a handheld instrument, is more informative than
“70% rh” about the risk of high surface rh and therefore about the potential for mold growth in hidden places.
“Paper-faced gypsum wall board is a high mold risk”
This statement is not correct... until one adds more qualifiers. Specifi-cally, this statement is true: ”Wet paper-faced gypsum board, which has no anti-microbials in it and which does not dry out for days or weeks, is a high mold risk.”
Conventional paper-faced gypsum board provides an economical-ly-attractive combination of fire resistance, sound attenuation, ease of finishing and durability even during prolonged periods of high humid-ity. And in its unfinished state, paper-faced gypsum board will dry very quickly. So it can retain its structural and other beneficial properties and resist mold growth even with small amounts of periodic wetting, or short periods of actual flooding. That’s why nearly all buildings in the US and Canada use it for the interior of exterior walls, and for both sides of internal partitions. Several billions of square feet are installed every year, and the product performs admirably.
On the other hand, it is quite true that when paper without anti-microbials gets wet—and stays that way—it will grow mold rather well. And unfortunately, one type of mold that competes and grows well on saturated paper is the notorious stachybotrys chartarum, a fungus which will produce toxic defenses when threatened by bacteria or other fungi which are competing for the same food source.
Also, it is true that high humidity makes paper-faced gypsum board rather fragile from a mold perspective. High humidity puts the paper closer to the edge of a mold problem. Long periods of high surface humidity and/or intermittent condensation allow a nearly invisible layer of fungus to grow on the paper facing and backing. This thin layer “preconditions” the paper facing for a rapid increase in fungal growth as soon as more water becomes available. That’s one reason why floods or rain leaks or after-hour spikes in humidity in buildings along the Gulf Coast seem to produce ‘explosions” of fungal growth on paper-faced gypsum board within just a few days of the event.
And finally, using vinyl wall covering on the indoor surface of exterior walls made of paper-faced gypsum board is very commonly
associated with lawsuits and mold growth. The vinyl traps moisture in both the gypsum and in the paper face, allowing mold to grow in the paper or in the adhesive for the wall covering.
But the fact is that paper-faced gypsum board is and will prob-ably remain the preferred indoor surface for buildings in the US and Canada, and it is becoming much used more widely in the rest of the world as well.
A prudent response to this fact is to avoid using unprotected paper in parts of the building which one would reasonably expect will get damp. For example, bathrooms, kitchens, and laundry rooms will have occasional water spills. Also, when paper-faced gypsum board is used on the interior surface of exterior walls, it would be a very, very risky decision to cover that wall with any vinyl or vapor retardant paint.
Owners and interior designers are specifically warned against this practice, which has led to many agonizingly expensive lawsuits about mold in both commercial and residential buildings in hot and humid climates.
Further, for all exterior walls, and in institutions where floor mop-ping and carpet cleaning are frequent, it is wise to install paper-faced gypsum board with a narrow air gap, to serve as a capillary break between the top of the finished floor and the bottom of the gypsum.
With an air gap, the gypsum cannot soak up moisture from the carpet or wet tile. Figure 5.13 shows such a gap.
“When you smell musty odors, you need more outdoor air to improve the indoor air quality”
This statement is not correct, but it is very widely believed. This statement is often the reason that:
1. Ventilation systems are incorrectly assumed to be at fault, during the early stages of investigating an indoor air quality complaint, and that;
2. A typical response of building operators to musty odor complaints is to increase the volume of ventilation air, beyond the ability of the HVAC system to dry it, making the problem immediately worse.
It is certainly wise, when investigating an indoor air quality com-plaint, to make sure that the space where the complaints originate has an adequate amount of outdoor air. But recognize that adding more ventilation air might simply disguise a problem, and even make that problem much worse—if the ventilation air is not dried.
Musty odors indicate that excess moisture is accumulating so that bacteria and fungus can grow, or that humidity absorption is causing materials to react chemically, releasing volatile organic vapors. To solve a musty odor problem, find the moisture and fix the problem that led to the moisture accumulation.
The problem may indeed be inside the cooling coils or an air handler cabinet or the systems’ air distribution duct work. Dirt plus water equals bacteria and fungi, which generate musty odors.
But in a hot and humid climate, adding more ventilation air than what is necessary to meet local codes and ASHRAE standards adds an extra and very unwelcome humidity load. And musty odors are usually an indicator that the system is already not removing the cur-rent humidity load. Adding a larger humidity load will not improve that situation. Removing more of the humidity load may well be part of the answer, but that will cost more money. So it’s important to understand where the moisture is accumulating and why, before investing in any additional ventilation.
“To prevent mold, keep the AC system running even when the building is unoccupied”
This suggestion often creates mold growth rather than preventing it, and it certainly wastes a tremendous amount of energy. The more accurate and helpful suggestion is: “To prevent mold, keep the indoor air dry, even when the building is unoccupied.”
Many HVAC designers and building owners assume that running the air cooling system will keep the building dry—but that’s usually not the case.
Usually, the operators raise the thermostat set point when the building is unoccupied. But often the outdoor air dampers remain open. Unless the outdoor air dampers are shut, highly humid outdoor
air can flood the building. Then, from time to time during unoccupied periods, the cooling system switch on and chill the building briefly—
just long enough to create very high surface rh on any wall or ceiling washed by cold supply air. The cool surfaces then condense moisture from the humid air, creating a high risk of mold growth.
Schools, which have long unoccupied periods during nights, weekends and vacations are especially vulnerable to this mecha-nism of mold growth. This risk is illustrated by the photo in figure 5.22.24
Rather than “just running the AC system,” keep in mind that the goal is to make sure the indoor air is dry in absolute terms (a low dew point). Often, an AC system can indeed be operated to achieve this goal. But it requires thought, and the specifics depend on the sort of system which is installed in the building.
Keeping the building dry after hours and during shut-downs is rather simple if the system is equipped with a dedicated ventilation air dehumidification system which has a return air connection. The operators simply set the dehumidification system to recirculate rather than to ventilate, and then control that system based on indoor air dew point. When the dew point in the building raises above 55°F (12.8°C], turn on the ventilation drying system to recirculate and dry the indoor air. When the dew point falls below 52°F [11°C], turn the dehumidification system off. Set the thermostat to a higher temperature to save energy, or don’t operate the cooling system at all until the occupants return.
For commercial buildings without dedicated ventilation dehumidi-fiers, and which are not equipped with cooling equipment which has a dehumidification mode, the cooling system might still be able to keep the air dry. It takes more care and thought. The operators and/
or the control system will need to:
1. Close the outdoor air dampers, or reduce the amount of outdoor air ventilation to the absolute minimum required by local codes for unoccupied operation. This is the essential first step. If this is not done, or if it cannot be
accomplished given the limitations of the system’s air flow controls, then it is best not to operate the cooling system at all. Flooding the building with humid ventilation air will greatly raise the risk of mold.
2. Operate the cooling system in periods of at least an hour, continuously, without regard to temperature, or...
3. Operate the cooling system in response to a dew point signal rather than a thermostat. If the indoor dew point is above 55°F, run the system until the dew point falls below 52°F. [12.8°C and 11°C]. Then turn off the system.
One caution is appropriate for these cooling-based drying strate-gies. One must be careful not to overcool the space. If the walls and ceilings get too cool, humid air behind walls and above ceilings will condense, feeding mold growth.
For example, if two apartments share a wall and one is cooling down at night while the other stays hot, humid air from the uncooled apartment could condense in the common wall each night. The same problem can happen in a commercial building, where different parts of the building are often served by different systems. When in doubt, monitor surface temperatures vs. the indoor air dew point. If the air dew point is not at least 12°F [5.5°C] below the surface temperature, then the operational strategy needs to be adjusted, because the rh at the surface may be above 80%.
There are many types of cooling systems. So these three sug-gestions are just the beginning of a long list of alternatives. But the principle remains the same: keep the building dry during unoccupied periods. (Below a dew point of 55°F [12.8°C].)
Apartments and condominiums which serve as vacation homes often have long unoccupied periods. Mold can grow during these long owner absences. For vacation homes built with cooling units which do not have a dehumidification mode, one way to reduce risk is to place one portable dehumidifier inside the bathtub in the bathroom, and another unit inside the sink in the kitchen. Placing the dehumidifiers
then the owner and designers must accept the mold risk inherent in humid ventilation air.
To mitigate the risk of humid ventilation air, one can reduce the volume of that air to the code-mandated minimum with a two-stage or variable volume ventilation system. A two-stage system can switch between occupied and unoccupied ventilation air volumes based on occupancy schedules programmed into a building automation system, or based on room occupancy sensors.
For an even closer match between actual occupancy and air volume, CO2 sensors and dampers can varies the volume of ventila-tion air. Indoor CO2 concentration is a useful way to quantify human occupancy. As the indoor CO2 concentration rises, more ventilation air is allowed into the building. As the concentration falls, the ventilation air volume is reduced.
Such two-position dampers or variable-volume dampers and sensors do add cost and complexity to the system design. But benefits balance those costs. Minimizing the ventilation air flow and drying the air are the best ways to reduce the mold risk while also reducing operating costs.
Again, it’s not the ventilation air that causes mold—it’s the amount of humidity brought into the building which can cause mold.
So dry out the ventilation air, and minimize its flow rate.
“All you need is air movement and light to prevent mold”
This statement is accurate, but incomplete, and therefore not useful.
It creates the false impression that mold will not grow if the lights in the building are on, and if fans are circulating air. The completely correct statement would be: “If you have enough dry air movement and energy to dry the surface, mold will not grow.”
If enough infrared energy (sunlight, usually) is falling directly on a surface, chances are that it will provide enough heat for some of the moisture at the surface to dry out—as long as there is also enough dry air flowing across the surface to carry away the moisture.
inside the bathtub and kitchen sink reduces any risk of condensate overflows. Then, connect a tube to each condensate collection tank, so the condensate flows out of the tanks and into the drains without the need to physically remove and empty the tank. Set the humidistat on the units so they switch on when the relative humidity rises above 50%. Then, the cooling system can be reset to a much higher tem-perature to save energy costs.
It will take energy to run the dehumidifiers, but not nearly as much energy as cooling the space as if it were occupied, in order to keep the humidity under control. And warmer temperatures reduce the potential for condensation, especially when the dehumidifiers are keeping the humidity low.
“In a hot and humid climate, all that code-required ventilation air causes mold”
This statement is false. It becomes true only when twelve critical words are added. Specifically: “In a hot and humid climate, all that code-required ventilation air causes mold if it is not dried before it is supplied to the space.”
Dry ventilation air does not cause mold; it helps prevent it. At the same time, it is quite true that humid ventilation air does indeed raise the risk of mold.
The unstated assumption behind this common misconception is that building owners simply will not pay to install drying equipment such as that shown in figure 5.14, or will not choose to invest in a cooling system which can also dry ventilation air.
It is quite true that this equipment costs more money to install than equipment not designed to handle ventilation air. But 100 years of ASHRAE experience and millions of dollars of research all over the world strongly suggest that without adequate ventilation, build-ings and occupants both have problems. That’s why building codes require ventilation air. The appropriate response to the mold risk inherent in ventilation air in a hot and humid climate is to dry that air before it is delivered to the space. If that is not deemed affordable,
But it’s not true that mold will stop growing when fluorescent lights are switched on while humid ventilation air is brought into the space. In fact, bringing more moisture to a cool surface by flowing humid air across that surface will increase the mold growth rate rather than reducing it.
The key is the moisture content of the food source. More moisture allows more mold. Less moisture allows less mold.
To the extent that adding heat from lights and removing moisture with dry air movement reduces moisture content of the food source, the statement is true. But if the light source adds little or no heat, or if the particular air used to create air movement adds more moisture to the material, the statement is false.
“To prevent mold in basements and crawl spaces, ventilate them with ventilation openings, or with fans.”
In hot and humid climates, this advice is only effective for buildings which have no air conditioning. In air conditioned buildings, this advice has been responsible for mold growth problems.29
In buildings with mechanical cooling, the surfaces of the structure which form the ceiling of the crawl space or basement are relatively cold. So when humid outdoor air is used to ventilate the crawl space below the ground floor, condensation forms on the cold ceiling of that crawl space, making it vulnerable to attack by mold.
Fans make the problem even worse, because they bring in humid air continuously—far more humidity than would drift in through intermittent wind pressure differences. More humid air means more condensation on any cool surfaces, which means more mold.
To prevent mold in basements and crawl spaces:
1. Keep moisture from getting out of the ground and into the air which fills the basement or crawl space. This requires a durable, well-sealed layer of vapor barrier material to cover any exposed earth or below-grade walls. (Note:
Adding gravel without a vapor barrier to cover the earth
increases the humidity load rather than reducing it. The gravel provides a greater surface area from which mois-ture can evaporate.)
increases the humidity load rather than reducing it. The gravel provides a greater surface area from which mois-ture can evaporate.)