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November 16, 1993
The issue becomes even more complex when you realize that you can replace the word humidity in the previous sentences with the words "indoor air quality " and not change the meaning or impact. Dilution is often used as the solution to indoor pollution in heating climates. Unfortunately, in humid, air conditioning climates, the greater the rate of dilution, ventilation or air change, the greater the rate of moisture entry with the exterior air. Therefore, the greater the likelihood of mold and other biological growth problems, particularly if the moisture in this incoming air is not removed.
Humidity concerns in the southern humid climates are particularly difficult to resolve. This is because one of the most effective approaches to dealing with humidity in heating climates, ventilation, can cause major humidity problems in the humid south. The issue becomes even more complex when you realize that you can replace the word humidity in the previous sentences with the words "indoor air quality " and not change the meaning or impact. Dilution is often used as the solution to indoor pollution in heating climates. Unfortunately, in humid, air conditioning climates, the greater the rate of dilution, ventilation or air change, the greater the rate of moisture entry with the exterior air. Therefore, the greater the likelihood of mold and other biological growth problems, particularly if the moisture in this incoming air is not removed.
Air conditioning buildings (mechanical cooling) is the major source of both humidity and indoor air quality concerns in the humid south. When exterior hot, humid air is cooled its relative humidity is increased. If it is cooled sufficiently, condensation occurs. High relative humidities and condensation can lead to mold and other biological growth. Interior relative humidities at surfaces and within building cavities need to be controlled to prevent condensation and biological growth.
An ideal approach to control indoor humidity and indoor air quality in the hot, humid south is to minimize the need for outside air. The air should be obtained in a controlled manner (mechanically with a fan). The air should be conditioned where it comes into the building. It should be dehumidified by cooling it below its dew point, and used to maintain the enclosure at a slight positive air pressure relative to the exterior. By doing so, it can be used to control the infiltration of exterior hot, humid air. Furthermore, the building envelope should be built in a manner that aides in the pressurization of the building. Tight construction is recommended. The building envelope should also exclude rain, control rain water absorption and control vapor diffusion. Vapor diffusion retarders should be installed on the exterior of building envelopes in the humid south as compared to the practices in northern heating climates. Finally, the building envelope should be forgiving so that if it gets wet, it can dry to the interior. Interior vapor diffusion retarders such as impermeable wall covering should be avoided.
This approach has implications with respect to building envelope tightness and moisture permeability/resistance, air consuming devices, interior activities, interior pollutant source strengths, housekeeping practices, operating costs for air conditioning equipment, and air conditioning loads.
Mold and Biological Growth Problems
When problems from mold and biological growth do occur in the humid south, they can be divided into three
- problems on interior surfaces due to elevated levels of moisture in the interior conditioned air (high interior air relative humidity);
- problems on interior surfaces due to surfaces being too cold leading to high relative humidities at the cooled surfaces; and
- problems within building cavities due to high cavity moisture levels or moisture passing through building cavities causing high relative humidities on material surfaces as the moisture migrates into the conditioned space
Although these problems can occur independently of each other they often occur in combination. For example, elevated levels of interior moisture are usually due to moisture passing through building cavities from the exterior resulting in both cavity moisture problems as well as problems on interior surfaces once the moisture has gotten into the conditioned space. Overcooling of the space just magnifies both problems thereby creating a third.
Interior Surface Related Problems Due to Elevated Levels of Moisture
When interior moisture levels are high, relative humidities at surfaces also are high. Where relative humidities at surfaces are greater than 70 percent mold and other biological growth can occur. In the humid south, the moisture source for these problems is almost always the exterior air. Moisture must be removed from the air within conditioned spaces such that relative humidities at surfaces remain below 70 percent. Where conditioned spaces are cooled to 75 degrees Fahrenheit, relative humidities in the air within the space should not exceed 60 percent.
Air which is brought in from the exterior to supply ventilation needs and make-up air needs should be conditioned to "dew point 55". In other words, this air should be cooled to at least 55 degrees Fahrenheit in order to dehumidify it. At dew point 55, the temperature of the air is 55 degrees Fahrenheit and its relative humidity is 100 percent. Once this air is warmed up to 75 degrees Fahrenheit, the temperature of typical air conditioned spaces, its relative humidity will be approximately 50 percent. This air now mixes with the air in the space diluting/reducing the conditioned space moisture levels/relative humidity. The rate of dilution or mixing is determined by meeting the 60 percent relative humidity limit within the conditioned space.
The dehumidification capabilities of air conditioning systems are typically used to remove moisture from the air within the conditioned space. Unfortunately, the latent cooling loads (the energy required to cool/remove the moisture in the air) are usually higher than sensible cooling loads in the humid south. This means that air conditioning systems should be sized for their latent loads, rather than their sensible loads. Sizing of equipment becomes critical. Undersizing of air conditioning equipment can lead to to obvious comfort and humidity problems. However, oversizing of air conditioning equipment can also lead to high interior humidity problems since oversized equipment will not operate as often, and therefore will dehumidify less than properly sized equipment.
Concerns about energy conservation has lead to the development of energy efficient mechanical cooling systems. Unfortunately, this has also reduced the ability of many of these systems to dehumidify air. In many cases, the exterior air is not cooled sufficiently to remove sufficient quantities of moisture.
Air cooled to 55 degrees Fahrenheit is usually too cold to introduce into a space. In the past this cooled air was heated after it was cooled ("reheat") prior to use. Reheat results in a significant energy penalty, and is not allowed in many jurisdictions for this reason. To avoid reheat requirements, some systems do not cool air down to "dew point 55". Unfortunately, this can result in insufficient moisture removal and subsequent high interior moisture levels.
Two approaches have been successfully used to address the issue of reheat. One is a new technology: heat pipe heat recovery. The other dates back to the 1930's and has been recently "rediscovered": run-around coils. Both of these approaches use heat removed during the mechanical cooling process to "reheat" the cooled air once it has shed its moisture thereby reducing the energy penalty of standard reheat.
Interior Surface Related Problems Due to Overcooling of Surfaces
When surfaces become too cold, surface relative humidities rise above 70 percent. When they rise to 100 percent, condensation occurs. If air conditioned air is supplied at too low a temperature, the diffusers can be extremely cold leading to condensation ("sweating"). Where diffusers are located poorly or adjusted incorrectly, cold air may be "blown" against surfaces creating cold spots and localized areas of high relative humidity and mold growth.
Supply ducts enclosed in interior walls and dropped ceilings often are not sealed and leak supply air. This supply air is typically under substantial positive air pressure and the resulting "jet" of air can "blow" against a surface leading to localized cooling. The cooling happens from the building cavity side, whereas the mold growth usually appears on the room side.
In cooling climates, condensing surfaces of exterior walls are typically the interior gypsum board. If interior spaces are "overcooled", the interior surfaces fall below the dew point temperature of the exterior air and condensation occurs. Figure 1 illustrates the case of a wall experiencing condensation as a result of overcooling. By raising the interior conditioned space temperature, the temperature of the first condensing surface is raised. Consequently, as the graph in Figure 1 shows, the potential for condensation is eliminated in this climate.
Figure 1: Potential for condensation in a masonry wall cavity in Tampa, Florida—By raising the temperature of the interior conditioned space from 70°F, the temperature of the first condensing surface (the outside surface of the gypsum board is raised above the mean daily ambient dew point temperature so that no condensation will occur.
Figure 2 illustrates the case of a wall experiencing condensation as a result of diffusion in a particularly severe cooling climate (Miami, Florida). By using impermeable rigid insulation (approximately R-7) on the interior of the masonry wall, the temperature of the first condensing surface is raised. As shown in the graph in Figure 2, condensation potential is eliminated since the temperature on the exterior face of the rigid insulation (the first condensing surface) is above the ambient range throughout the year.
Figure 2: Potential for condensation in a masonry wall cavity in Miami, Florida—Placing rigid insulation on the interior of the masony wall raises the temperature of the first condensing surface above the mean daily ambient dew point temperature so that no condensation will occur. The outside surface of the rigid insulation becomes the first condensing surface in this assembly.
Overcooling of spaces often occurs as a result of ignorance and/or poor design. Most people in the humid south are aware of the fact that the more an air conditioner operates the greater the removal of moisture from the interior air by the air conditioner. System controls are therefore often adjusted to provide frequent operation of the system. To keep systems operating more frequently, thermostat settings are lowered below recommended levels. In addition, when systems are oversized, in order to operate for longer periods of time, thermostat settings have to be lowered resulting in overcooled spaces.
In a final, unfortunate irony, overcooling a space can actually increase latent loads rather than result in a net moisture removal. Moisture flow is generally from warm to cold. The colder the space the more moisture is drawn into the space from the exterior.
Building Cavity Moisture Problems
In the humid south, moisture flow is typically from the exterior to the interior or from the warm to the cold. If the rate of moisture entry into the building envelope from the exterior is greater than the rate of moisture removal from the building envelope into the conditioned space, accumulation occurs with building envelope cavities and serious problems result.
The general control strategy for building envelopes in the humid south is quite straightforward. Make it difficult for moisture to enter the building envelope from the exterior, and make it easy for moisture to leave the building envelope to the interior.
Moisture enters building envelopes from the exterior in the humid south three major ways:
- rain leakage
- air leakage
- vapor diffusion
Controlling these mechanisms means keeping the rain out of the building envelope, keeping exterior air out of the building envelope and keeping exterior moisture from vapor diffusion out of the building envelope. In addition it also means being realistic about the probability of success at controlling these mechanisms and therefore providing a means of drying to the interior. In practice, this usually means not preventing the normal/typical drying to the interior conditioned space by installing an impermeable wall covering. Rain is a particularly severe mechanism of moisture transport in the humid south. When rain wets the exterior of a building the exterior surfaces typically absorb the rain water. For instance, brick cladding is a powerful rain water "sponge". Recall that moisture flow is from warm to cold. When wet brick is warmed by the sun, a significant temperature differential is created. The sun serves to "drive" rain water into a building envelope. If the interior space is also air conditioned or cooled, the air conditioning serves to "suck" the rain water inwards as a result of a temperature differential.
The effect of incident solar radiation on a rain saturated cladding is dramatic. Consider that a brick veneer or stucco coating can be readily warmed by the sun well above 120 degrees Fahrenheit. The air contained in an airspace behind a brick veneer can be similarly warmed and can be considered to be at saturated conditions (vapor pressure of 11.74 kPa). This results in an increase of almost 500 percent in the effective exterior vapor pressure (Figure 3). Solar radiation is a powerful force that drives moisture in rain-saturated cladding inwards. This force can be ten times greater than the vapor diffusion driving moisture outwards under the most hostile conditions experienced in heating climates.
Exterior sprinklers can excacerbate problems by wetting exterior claddings on a regular basis. The normal southern climate temperature differential then serves to move this sprinkler deposited water into the conditioned space. . .
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