How to prevent mold and mildew
How to prevent mold and mildew Sources for the information contained in this report: • U.S. EPA – "A Brief Guide to Mold, Moisture and Your Home" http://www.epa.gov/iaq/molds/moldguide.html • Texas A&M University - "Mold Mini-Course" provides 5 learning modules on mold: http://fcs.tamu.edu/housing/iaq/Mold/web/index.htm • Building Science Corporation – online articles on mold and moisture control: http://buildingscience.com/resources/resources.htm#Mold • National Association of Home Builders Research Center – ToolBase Services site: "Mold in Residential Buildings" (type ‘mold' into search box if necessary): http://toolbase.org/index-toolbase.asp • American Red Cross – "Repairing Your Flooded Home" provides excellent advice for cleanup of major water damage in homes: http://www.redcross.org/services/disaster/afterdis/reptoc.html . The Florida Solar Energy Center - is a research institute of the University of Central Florida http://www.fsec.ucf.edu/en/ Indoor space conditioning guidelines for vacant homes to avoid mold problems while minimizing both energy usage and electrical load in hot humid climates. • www.fsec.ucf.edu/en/publications/html/fsec-cr-1487-04/ The Short Version How to reduce or eliminate the risk of mold in the Southeast USA and other areas of the world with the same type of hot humid climate. Mold prevention measures are not difficult or costly; however, the reality is that once you have a mold problem, it then becomes costly and difficult to remedy. You cannot reliably control mold and mildew growth using only the air conditioning (cooling) system. To prevent mold and mildew growth the indoor RH (relative humidity) should not exceed 55%. In hot weather the air conditioning system does this fairly well. However the highest relative humidity in the home is likely to occur during mild weather when your air conditioner is not needed much during the day. In mild, cool but damp weather the cooling system is on very little or not at all; this is the most likely weather to promote mold growth. In Florida, the outdoor RH reaches nearly 100% at night, regardless of daytime temperatures. This moisture needs to be removed from the home during the day (while the temperature is higher) by the air conditioner or at night with a dehumidifier. By far, the best method is to use a dehumidifier. For best results use a dehumidifier with a humidistat control, a timer and a permanent drain. Preventing mold by lowering Relative Humidity while maintaining a comfortable environment and reducing the cost of cooling. Thermostat Settings for hot humid weather: Every Fahrenheit degree increase in your thermostat setting decreases air conditioning costs by about 10%. Set the thermostat to the highest temperature that is comfortable for you. A temperature of 78 degrees Fahrenheit or greater is recommended. Set the thermostat to the “cool” mode and not the “auto” mode (auto / heat / cool). Set the fan mode of operation to the “auto” mode not the “on” (continuous) mode. Do not try to control mold by lowering the thermostat setting as this will not work. Never leave outside doors or windows open in hot, humid weather while running the air conditioner as this not only fosters mold growth, but will also waste electricity and possibly damage the AC. Ceiling fans can keep you comfortable while saving money. Change air filters regularly and get professional service once a year to clean the coils. Make sure sprinkler systems do not hit exterior walls of the building. Route bathroom exhaust fans and dryer vents outside of your home, not into some other part of the home (garage, attic, utility room, etc). Minimize live indoor house plants. Eliminate any and all water leaks and standing water in or under your home. For example: water heater overflow pans, air conditioner overflow pans, drips under sinks or other hidden places. The Vacant Home If a home is to be vacant for an extended period of time then install a dehumidifier controlled by a humidistat. This is the best and most cost-effective solution to the high relative humidity and mold problem. If you are going to leave the heating or cooling system on in a vacant home set the thermostat fan selection switch to “auto” in order to save energy and avoid clogging the air filter. Open all interior doors to allow unrestricted air circulation. Keep all exterior doors and windows closed. Mold can be eliminated effectively in most cases. There are four elements needed to create an optimal environment for mold growth: available mold spores, available mold food, appropriate temperatures and considerable moisture. The removal of any one of these items will prohibit mold growth. Mold is basically everywhere so negating the availability of mold spores is impossible. Mold spores eat anything but especially love paper, wood and other organic fibers (the main items found in a living environment), so reducing their food source is highly unlikely. Regarding temperatures, mold grows in our refrigerators as well as hot tropical climates. So the comfortable living temperature that humans enjoy is also an excellent environment for mold growth. Moisture level (relative humidity) is the component that can most easily be controlled and is the factor we will focus on in our effort to control the growth of mold. Air Conditioner Fan Mode of Operation. Keep the air conditioner fan switch set to “auto” mode instead of the “on” mode. When set to the “on” position the blower fan runs continuously and the moisture which has condensed on the evaporator coil during the cooling cycle is re-evaporated and blown back into the home before it can drain off the coil and out of the home. This will cause the RH in your home to be significantly greater than using the “auto” mode. Match the AC capacity to the house size. Air conditioner units are commonly oversized. The larger and more oversized the unit is, the poorer it will be at removing humidity. This is because, during each air conditioning cooling cycle, moisture removal does not reach full effective capacity during the first three minutes of operation. The larger oversized system has a shorter on cooling time during which interior humidity is removed. Do not try to control mold by lowering thermostat setting as this will not work. Two things result from lowering the temperature in the home, first relative humidity increases when the temperature decreases. Second, lowering the temperature decreases the temperature of the materials in the walls, floors and ceilings of the home, thereby significantly increasing the potential for actual moisture condensation on these items. A side benefit of setting your thermostat at higher temperatures is that it significantly decreases cooling energy costs. This is exactly the news you were hoping to hear. This Chart Shows the Impact of the Thermostat Set-Point Temperature and The Fan Mode of Operation on Interior Relative Humidity. Indoor Temp Avg RH % degrees F Fan= Auto Compressor Run Time Fraction Avg RH% Fan=On 80 55% 0.27 65% 77 60% 0.35 70% 76 67% 0.60 70% 71 74% 0.80 75% The AC was operated alternatively for one week in each mode. Based on experiments conducted by The Florida Solar Energy Center. If you would like to learn more about Mold and Mildew prevention and control, Then please continue and read the detailed technical explanation that follows. DETAILED TECHNICAL EXPLANATION What is Mold? Mold and mildew are fungi. Fungi are neither plant nor animal but, since 1969, have their own kingdom. The fungi kingdom includes such wonderful organisms as the delicious edible mushrooms, the makers of the "miracle drug" penicillin and the yeast that makes our bread rise and our fine wines ferment. Biologically, all fungi have defined cell walls, lack chlorophyll and reproduce by means of spores. Approximately 100,000 species of fungi have been described and it is estimated that there are at least that many waiting to be discovered. The vast majority of fungi feed on dead or decaying organic matter – they are one of the principle agents responsible for the natural recycling of dead plant and animal life. The most common fungi are ubiquitous within our environment and we are constantly exposed to them. For the most part, however, diseases caused by these common fungi are relatively uncommon and are rarely found in individuals with normally functioning immune systems. Mold can be managed effectively in most cases. Critical Requirements There are 4 critical requirements for mold growth – available mold spores, available mold food, appropriate temperatures and considerable moisture. The removal of any one of these items will prohibit mold growth. Let's examine each requirement, one-by-one. Mold Spores. Ranging in size from 3 to 40 microns (human hair is 100150 microns), mold spores are ubiquitous – they are literally everywhere. There is no reasonable, reliable and cost-effective means of eliminating them from environments that humans inhabit. So, trying to control mold growth through the elimination of mold spores is not feasible. Mold Food. If all three other requirements are met, almost any substance that contains carbon atoms (organic substance) will provide sufficient Magnified sporangiophore of the nutrients to support mold growth. Even the oil from your skin that is left Pilobolus — "the shotgun fungus." The small dark knobs at the top of when you touch an otherwise unsuitable surface, like stainless steel, or the each stalk contains spore sacks that soap residue left from a good cleaning will provide sufficient nutrients to are "shot" up to 6 feet into the air support the growth of some molds. And many of the most common toward the sunlight by this unique materials found in homes like wood, paper and organic fibers are among fungus. the most preferred of mold nutrients. Thus, eliminating mold food from your environment is a virtually impossible task. Appropriate Temperatures. Unfortunately, most molds grow very well at the same temperatures that humans prefer. In addition, anyone who has cleaned out their refrigerator quickly realizes that temperatures close to freezing are not cold enough to prevent mold growth and temperatures that are much warmer than humans prefer, like those of the tropics, will grow abundant quantities of mold. Therefore, it is not feasible to control mold growth in our home environment through the control of temperature. Considerable Moisture. Most molds require the presence of considerable moisture for growth. Obviously, the word "considerable" is key here. The mycologists (fungi scientists) refer to "water activity" when describing the required conditions for mold growth. The various species of mold have different water activity requirements. A material's "water activity" is equivalent to the relative humidity of the air that would be in equilibrium with the material at that material moisture content. The vast majority of mold species require "water activity" levels that are equivalent to material equilibrium moisture contents corresponding to a relative humidity of at least 70%. In fact, the great majority of serious, large mold outbreaks inside buildings occur where porous, cellulose-type materials have literally been kept wet by liquid water or sustained condensation. Human beings prefer a humidity level that is below the critical relative humidity for mold growth. Thus, of the four basic requirements for mold growth, moisture availability is by far the easiest mold growth requirement to control in environments that humans like to inhabit. As you will see from the remainder of this guide, and from the vast majority of the literature on mold control, the consensus regarding effective mold control strategies consists of the combination of reducing the availability of moisture and killing and removing active mold growth colonies. Determining If You Have Mold Common household molds have a characteristic "musty" or "earthy" smell, somewhat like the forest floor deep in the woods. Growing colonies of mold can also be visually observed in many cases. Most people are familiar with moldy bread or mold growth on cheese or other food products that have been kept too long, so the "green fuzzy" characteristic of most mold growth is familiar. And those who have lived in Florida have heard the expression "green shoe syndrome" which refers to the fact that mold is particularly fond of leather products left unused for periods of time in dark humid places. Although most active mold colonies appear greenish to black (typical of mold growing on bathroom tile grout) in color, the characteristics of mold colonies growing behind vinyl wall covering in buildings takes on very different characteristics. These mold outbreaks typically result in pinkish to yellowish staining of the wall covering. They are quite important because they indicate serious, detrimental moisture accumulations within the gypsum wallboard behind the wall covering that cannot be removed by your air conditioning or dehumidification systems. Where these problems appear, they usually require the assistance of a professional equipped with pressure measurement and other diagnostic equipment to determine the Note the pink "splotch" at the bottom-center source of the moisture causing the problem. of this photo. It is the telltale warning sign that there is a likely mold "bloom" behind Minimize Mold Growth the vinyl wall covering. The following practices will help minimize the growth of molds inside homes located in hot, humid climates. Mold growth on the outside of buildings is not covered here and readers are cautioned that these practices may not be applicable in other climates. Air Conditioner Operation: Always set the fan mode switch of your air conditioner thermostat in the AUTO position, never in the ON position. When set to the ON position the blower fan runs continuously and the moisture which has condensed on your air conditioners’ evaporator coil during cooling is re-evaporated and blown back into your home before it can drain off the coil and out of your home. This causes the relative humidity in your home to be significantly greater than if the air conditioner thermostat fan mode switch is set to the AUTO position. Even in the "auto" position, some air conditioners run the blower for 1 to 3 minutes after the compressor shuts off. To maximize dehumidification, it is best to disable this feature completely or at least limit the purge time to just one minute. A qualified AC contractor should be able to make these changes for you. Air Conditioner Selection: If you are building a new home and can choose, then choose an air conditioning system with a variable speed air handler and an operating selection mode for "enhanced moisture removal" (dehumidifier). This is a good option for multiple reasons: the units are SEER 14+, they are quiet and they do a better job removing moisture, particularly under part load conditions. They accomplish this by starting the air handler fan at a lower speed during each cycle, which improves moisture removal. The variable speed fan motors are intrinsically more energy efficient-- they use as little as 270 watts of electricity per 1000 cfm of air flow as compared with the typical 450 watts per 1000 cfm for standard blower units. Each major manufacturer has them. When used properly, they are ideal for use in the Southeast USA and other places around the world with the same hot humid climate. Air conditioner sizing: Oversizing of air conditioners is common. The more an air conditioner is oversized, the poorer its humidity removal performance, especially at higher thermostat settings. This is because, during each air conditioning on cycle, the moisture removal does not reach full capacity for about the first three minutes of operation. The more the system is oversized, the shorter the on-cycle during which moisture is removed. Thus, if a home is properly sized with a 2-ton air conditioner and a 4-ton system is installed, the 2-ton machine would do much better job removing moisture even though the 4-ton machine had twice the nameplate humidity removal capability (Btu/hr). Remember, the shorter the air conditioner on-cycle, the less chance for effective moisture removal. This fact can be clearly seen in the figure below, which is taken from FSEC test data. Ceiling Fans: Use ceiling fans in the summer; they allow you to be comfortable at higher temperatures. They also save air conditioning costs if you use the most efficient fans and turn them off when no one is in the room. Thermostat Fan and Temperature Settings Affect Mold Growth. Thermostat Set Point: Set the summertime thermostat to the highest temperature that is comfortable for you. A temperature of 78o Fahrenheit or greater is recommended. Never lower the thermostat set-point in an attempt to control humidity as this will not work. Setting the thermostat temperature lower does two things that are counter to your goal of reducing the moisture content of the materials in your home. First, contrary to what you might have been told, it actually slightly increases the indoor relative humidity in your home! Second, and more important, it decreases the temperature of the materials in your walls, floors and ceilings of your home, thereby significantly increasing the potential for actual moisture condensation on these elements of your home. The good news: a side benefit of setting your thermostat to a higher temperature is that it significantly decreases cooling energy costs. In Florida, each one degree Fahrenheit increase in your Thermostat Temperature Setting decreases Air Conditioning Energy costs by about 10%. This Chart Shows the Impact of the Thermostat Set-Point Temperature and the Fan Mode of Operation on the Interior Relative Humidity. Indoor Temp Avg RH % Compressor Avg RH% degrees F Fan= Auto Run Time Fraction Fan=On 80o 55% 0.27 65% 77o 60% 0.35 70% 76o 67% 0.60 70% 71o 74% 0.80 75% The AC was operated alternatively for one week in each mode. Based on experiments conducted by The Florida Solar Energy Center. Space Pressurization: It is important that homes in hot, humid climates be pressurized slightly with respect to outdoors. The reason is fairly straightforward but not very obvious. If homes are depressurized with respect to the outdoors, then hot, humid outdoor air will be pulled through the very small air pathways that exist in all building envelopes (walls, ceilings, floors, etc.). To get from the outside of the home to the inside, this air often must follow circuitous pathways. For example, the air may enter the wall system high on the exterior where an outdoor light fixture is mounted and exit the wall system low on the indoors where an electrical outlet is located. If the home is air conditioned, the gypsum wallboard will be relatively cold — often colder than the dewpoint temperature of the humid outdoor air that must flow along that gypsum wallboard to that indoor electrical outlet. In Florida, it is not uncommon for summertime outdoor air dewpoint temperatures to be greater than 80 F! When this occurs, the colder gypsum wallboard can act just like that ice tea glass that "sweats" like crazy when you take it outdoors — it can condense the moisture out of the air that is flowing along its back surface on its way to the electrical outlet that is serving as its pathway into the air conditioned home. As illustrated in the figure below, this can result in moisture accumulation within the wallboard, which, in turn, can result in significant mold growth. Interior Doors: Interior doors should be kept open when air conditioning unless your heating and cooling system has a fully ducted return air system from each room of the home or sufficient return air transfer pathways have been installed to ensure that closed interior doors do not result in space depressurization problems in the home. Exterior Doors and Window: Never leave outside doors or windows open in hot, humid weather while running the air conditioner as this not only fosters mold growth, but will also waste electricity and possibly even damage the Air Conditioning system. Interior Air Space Depressurization: . The diagrams below (from detailed computer simulations) show the combined impacts of heat, moisture and air transport and illustrate the importance of this air flow phenomena. The wall on the left bounds a space that is pressurized with respect to the outdoors and the one on the right bounds a space that is depressurized. The Pascal pressure gradient of two is very, very small; one atmosphere equals 101,325 Pascal. Clearly, it is the direction of the pressure gradient rather than its magnitude that is critically important here. You very much want your home slightly over pressurized in hot, humid climates so that dry, cool indoor air is pushed out of the home through the walls (figure on left) rather than have hot, humid outdoor air sucked into the home through the walls (figure on right). Fortunately, it is relatively easy to pressurize a home; all that is necessary is that slightly more air is brought into the home than is exhausted. This normally requires a positive mechanical ventilation system. . Things that may cause space depressurization in homes: >Exhaust fans - bathroom, kitchen, attic, crawlspace, etc. >Cloths dryers. >Supply duct leaks. >Insufficient return air pathways due to interior door closure. Measure the RH in Your Home: Invest in a digital temperature and relative humidity (RH) sensor (about $20) and observe your indoor relative humidity. Two sources for these sensors are Radio Shack and Therma-Stor Products. During the hot summer months, with the air conditioning on, the RH should not exceed 55% during the day on a regular basis. If it does, you probably have problems either with leaks in your duct system or with your air conditioner unit itself; it could be too large, improperly charged or have insufficient air flow across the condenser coil. Consult with a qualified air conditioning or mechanical engineer to determine the problem. The highest relative humidity in the home is likely to occur during mild weather when your air conditioner is not needed much during the day. In Florida, the outdoor relative humidity reaches very near 100% on most nights, regardless of daytime temperatures. If your home is open to the outdoors during these periods, the materials in your home will absorb moisture from this very humid air, again regardless of the temperature. If this moisture is not removed during the following day, the "water activity" of the materials in your home can stay at or above 75% for extended periods and mold is likely to grow on these surfaces. If you have RH levels exceeding 70% to 75% for extended periods, and find it difficult to control mold growth on surfaces in your home in spring, fall and winter, you may need to invest in some type of dehumidification system and should consult with a qualified mechanical system expert for advice. Vinyl Wall Covering: Impermeable interior surfaces like vinyl wall coverings can result in severe mold problems in hot humid climates such as Florida's. Moisture coming from outdoors can accumulate within the gypsum wallboard that is behind the vinyl wall covering. This normally occurs as a result of house depressurization; where outdoor air is being sucked into the home through the very small air pathways that exist in all normal wall systems. Where this problem occurs, outbreaks of mold often occur beneath the wall covering on the surface of the gypsum wallboard. This mold growth is normally characterized by pinkish to yellowish "splotches" on the vinyl wall covering. The moisture accumulation also can be severe enough to cause the gypsum wallboard to badly deteriorate and become "mushy." If you have noticed these symptoms, a building science professional should be consulted. Positive pressurization of your home is one method of minimizing the potential occurrence of this problem. Return Air Pathways: It is important that there be sufficient air flow pathways for the supply air that is delivered to each room of a home to return to the air conditioners’ air handler unit (the box with the blower fan). Otherwise, the part of the home containing the main return to the air handler unit will be "starved" for air, resulting in depressurization of this space with respect to the outdoors. If this occurs, outdoor air will be drawn through the small pathways that exist in the exterior building envelope. In hot, humid climates these air flows can result in the accumulation of moisture within the gypsum wallboard, especially if it has vinyl wall covering. This, in turn, can result in the rapid and abundant growth of molds; remember, the cellulose (paper) on gypsum wallboard makes an excellent, preferred mold food. If room doors are kept open, there will be sufficient return air pathways. However, if room doors are closed, the rule-of-thumb is that there should be about 50 square inches of "free" air transfer area for each 100 cfm (cubic feet per minute) of supply air to the room. In this case, the term "free" means a simple, clear hole in the wall between the room and the remainder of the home. If, for appearance and privacy reasons, this hole is to be covered by grilles on each side of the wall, then the overall return air pathway area needs to be increased by about 40% to account for the air flow resistance of the grilles, or about 70 square inches per 100 cfm of air flow. Bathrooms: Most bathrooms, particularly tile in and around showers and tubs is regularly wet. As a result, most bathrooms grow mold and require regular cleaning. A weak solution of water and common household bleach can be used to regularly clean these areas and keep them free of mold. Bathroom fans are also recommended to remove moisture when it is being generated by bathing or showering. Whole-House Ventilation Fans — Opened Windows: Avoid the use of these fans in humid weather, especially if you have noticed mold growth in your home or you are having trouble controlling the relative humidity in your home. In addition, avoid opening windows for long periods when it is humid outside (e.g. during nights and evenings) if you are experiencing mold growth problems in your home. Air Conditioner Maintenance: Change the air filters regularly and use pleated filters. Once a year get your airconditioners professionally serviced. At that time make sure coils are clean, the condensate drains properly and that the drain pan has no mold. Exterior Water Management: Redirect water away from the home's exterior; redirect sprinklers so that they don't spray on the exterior walls. Do not landscape with hills that direct water flow towards the building. Install gutters, direct outflow away from the building and keep down-spouts free of debris. Small Leaks: Even small water leaks will cause mold problems. Rainwater leaks from improperly flashed windows, wall and roof penetrations and plumbing leaks should be promptly repaired. Periodically inspect under sinks and vanities for signs of water leakage. Use your nose and smell for "musty" or "earthy" odors – they usually indicate the presence of mold. Fix all water leaks promptly. Water Damage: Water damage from flooding or other major water intrusion in homes should be dried within 24 hours if at all possible. For severe flooding and severe water damage for more than 48 hours, a trained restoration professional should be consulted regarding cleanup procedures. Readers are also encouraged to consult the American Red Cross web site for further information. Moisture Condensation: Single-pane, metal windows, which are common in Florida, generally condense water on the inside in winter. It is good practice to remove this condensation before it can run off and be absorbed by porous materials like wood casing or gypsum wallboard. Condensation can also occur on other surfaces in homes. If condensation is noticed on interior surfaces in summer, it may indicate a number of problems, including inability to control indoor humidity; air conditioner supply registers aimed directly at interior surfaces; duct leakage problems and pressure imbalances; or all of the above. If you notice indoor surface condensation during summer, you should contact a professional to help diagnose the cause. However, during early spring when the ground is still cool, it is quite possible to experience some condensation on tile floors on slab-on-grade homes that are open to the outdoors. This should not be a regular occurrence, but only something that occurs rarely. . . Exhaust Fans: Make sure the clothes dryer vent goes all the way to the outside of the home, not to the crawlspace or to the inside of the attic or the house. The same goes for bathroom vent fans. It is also important for the kitchen range hood to vent to the exterior as well. Re-circulating stove and kitchen vents provide no removal of stovetop moisture and inferior control of cooking related pollutants compared with venting completely to the outdoors. A major deterrent to the use of kitchen range hoods is noise. Choose an ultra-quiet, inline ventilation fan for your range hood. Kitchen and bath exhaust fans should only be used while cooking or using the bathroom to remove excess moisture generated by these activities. It is best practice to either have bathroom vent fans interlocked with the light switch so they do not get left on or have them switched by a manual timer that will shut them off after a period of time, or control them by humidistat. Closets: Fungi like the dark and closets are rarely supplied with conditioned air as a standard part of air conditioning systems. As a result it is not all that uncommon to have mold or mildew occur in closets, especially on leather. Leaving the closet doors open to provide more conditioned air circulation or leaving the closet lights on with the door closed so as to raise the temperature (which lowers the RH) can reduce these problems. House Plants: Minimize live house plants, especially if you have any trouble controlling the relative humidity in your home. The following Vacant Home Interior Space Air Conditioning Study was sponsored by Florida Power & Light (FPL) as part of its Conservation R&D Program and carried out by the Florida Solar Energy Center (FSEC). The Vacant Building If you leave either the AC or heating system on in a vacant home, be sure to set the thermostat fan switch to “auto” to save energy and avoid clogging the filter. Before leaving any home vacant, open all interior doors to allow unrestricted air circulation. Make sure all exterior doors and window closed. If a home is to be vacant for an extended period then install a dehumidifier controlled by a humidistat. This is the best and most cost effective solution to the high relative humidity and mold problem. Mold, Mildew and the Vacant Building Evaluation of the Vacant Home Interior Air Space Condition Considering Summer Relative Humidity, Energy Use, and Peak Utility Load. In the US, about one in four retiree "snowbirds" make Florida their destination to escape the northern cold. Snowbird residents number more than one million in Florida during the winter. In some counties of FPL's territory these seasonal residents compose up to 18% of the population. This pattern of seasonal occupancy creates a need for space conditioning guidelines for vacant homes to avoid mold problems while minimizing overall energy usage and electrical power usage during utility peak load hours. Three homes on Florida's east coast in Brevard County were equipped with monitoring equipment for this experiment. The first home was a 1200 sq ft, two bedroom condo built in 2000. The second house was a 12 year old 1,950 sq ft split-level home, with brick and frame construction. The third was a 25 year old 1,600 sq ft single-story, vinyl sided home on a concrete slab with a low pitch metal roof. All three test-homes had 2.5 ton central air conditioning (AC) systems with heat pump, gas, and electric strip heating, respectively. Five space conditioning strategies for vacant homes were assessed during hot, humid summer conditions and warm, humid fall weather conditions. The aim was to control relative humidity, minimize energy use, and limit peak load electrical demand. The five approaches were: (1) No air space conditioning as the ambient baseline, (2) AC (cooling) set at 83oF to 85oF, (3) Morning AC operation at 74oF, (4) Dehumidifier alone, (5) Space heating alone. Project staff identified several RH (relative humidity) targets for controlling mold. First, RH should stay below 70% essentially all of the time. Second, RH should stay below 65% most of the time. Third, the target for RH, when appliance control included a humidistat, was set at 62%. Mold and mildew grow when RH is high, say over 70%, for extended periods of time. In order to control these indoor air quality health hazards, it is important to understand the concept of relative humidity. Relative humidity, expressed in percent, is the measure of how much moisture is in the air compared to the maximum amount of moisture the air could hold at that temperature. There are two ways to control RH; (1) raising room temperature by adding heat and (2) removing moisture from the air using an air conditioner or dehumidifier. This research study explored both strategies. An important observation was that some homes require more aggressive action to control RH than others. Air infiltration was found to be an important factor in the struggle against high indoor RH. Air infiltration, usually measured in air changes per hour (ach), is the rate at which outside air enters the house. Homes that have higher infiltration rates require greater moisture removal rates to achieve RH control. Overview of Humidity Control Approaches The first control approach was to let the building “float”; that is with no AC, dehumidifier, or heating system operating. “Floating” yielded indoors conditions that were 4.8oF, 6.1oF, and 4.2oF warmer than outdoors, respectively, in the three houses. Whereas outdoor RH averages about 77% from June through October, indoor RH averaged 62%, 69%, and 73% in the three houses (see Figures A and B). The elevation of indoor temperature caused by solar radiation striking the houses produced this reduction in RH below the outdoor RH. Since the objective was to keep RH below 65% most of the time, only House 1 experienced acceptable RH without mechanical intervention. This does not mean, however, that houses 2 and 3 would experience mold problems without mechanical intervention. It just means that the risk of mold growth was higher. Figure A: Figure B: The second approach, AC at 85oF (fan AUTO), yielded generally poor results because the AC run time was too small to ensure adequate moisture removal. This approach works fairly well on hotter than average summer days, but on cooler and cloudier days, little or no AC run time lead to little or no moisture removal. Even setting the thermostat to 83oF did not ensure good performance, especially during the cooler months. During some test periods at some houses, neither AC at 85oF nor AC at 83oF caused the AC system to run at all. Furthermore, energy consumption tends to concentrate during the utility’s peak demand period. The third approach, AC set at 74oF from 3 to 5 AM (fan AUTO), yielded promising but mixed results. In House 1, this approach produced low RH (48%) in large part because of low natural infiltration (Figure C). In House 2, two-hour operation yielded marginal results with RH primarily in the range of 65% to 70%. Extending operation to four hours (3 to 5 AM and 10 AM to noon) yielded average 59% RH. In House 3, an RH of 55% even though the home reached the thermostat setpoint before the air conditioner had run the intended two hours each morning. In some homes, a thermostat setting lower than 74oF may be required. (Figure D). Figure C: Figure D: The fourth approach, a dehumidifier coupled with a floor fan to circulate the air, was effective in controlling RH in all the homes. Two control strategies were implemented. The first strategy involved setting the built-in humidistat to 62% (Figure E). This proved to be difficult because of erratic and undependable humidistat operation. The dehumidifier operation time required to control RH varied depending partly on the size of the house and more so on the infiltration rate. The second strategy, which involved operating the dehumidifier on a timer, provides reliable results with adequate operation time. Dehumidifier operation of 3 hours per day produced 56% RH in House 1 (small sq ft and tight; Figure F). 15 hour per day operation yielded 60% RH in House 2 (large sq ft and very leaky). 3 hours per day operation yielded 67% RH in House 3 (medium sq ft and leaky). 4 to 5 hour operation time would be required in House 3 to meet our RH control objectives. It is interesting to note that a dehumidifier is a high efficiency space heater (coefficient of performance [COP] ~ 2.0) and that more than 50% of the RH reduction from dehumidifier operation resulted from indoor temperature increase. The timed operation can be set for off-peak hours, and operation using humidistat control would result in the least run time during the hottest (peak) hours of the day. Figure E: Figure F: The fifth approach, which involved heating the house with the central space heating system, was also effective at reducing RH in all three homes. This result is not surprising. A typical Florida summer dew point temperature would be 74oF. Heating the indoor temperature to 88.5oF on this typical summer day increases the moisture capacity of the air and, as a result, reduces indoor RH to 62%. If the home is going to be vacant during the cooler months such as May and October when the dew point temperatures are lower, then maintaining this high indoor temperature will produce lower RH. In House 1, for example, indoor RH was 54% over a period from September 27 thru October 13 (Figure G; outdoor dew point temperature averaged 69oF during this period). In House 3, indoor RH was 59% over a period from September 30 October 14 (Figure H; outdoor dew point temperature averaged 69oF during this period). The difference in RH between house 1 and house 3 is due to the difference in air flow infiltration. House 1 being much tighter than house 3. Figure H: During periods with lower dew point temperature, the same RH can be achieved at a lower space temperature. If, for example, it is October and the outdoor dew point temperature is 68oF, the indoor temperature required to produce 62% RH would be 82oF. If possible, a friend or family member living nearby could manually set the thermostat to heat ON and fan AUTO with a setting of 88oF in June thru September and 82oF for May, October, and November. This analysis of energy use is based on the optimum length of system operation to achieve the desired RH control. In House 1, for example, we estimated that only 1 hour per day of AC on at 74oF would be required, and in House 3, we estimated that the dehumidifier would need to run for 5 hours per day to achieve acceptable RH control. Keep in mind that House 2 is larger than Houses 1 and 3. Monthly electricity cost (12.5 cents per kWh) to achieve 65% RH or lower HOUSE # --> 1 2 3 House size (square feet) 1200 1950 1600 Natural infiltration rate low high high o AC at 85 F (fan on auto) Average 1584 *1 $34.62 $64.44 $48.54 . $49.20 AC (ON at 74 F 3 to 5 AM) *2 $16.63 Dehumidifier (on timer) *3 $12.36 Space heating (at 88 to 90oF) *4 $25.11 $31.52 $26.32 $26.33 $18.93 $39.93 $29.44 $24.80 $19.20 $31.50 o *1 *2 *3 *4 Assumes Assumes Assumes Assumes 4 hours per day AC operation. 1 hour per day AC operation. 5 hours per day dehumidifier operation. heater is a heat pump and that the period of operation is June – October. Recommendations: Two methods have been identified as “not effective”. Floating. This approach cannot reliably achieve RH below 65% most of the time. AC at 85oFahrenheit. This approach does not work in most homes because AC run time is not sufficient, especially on humid, cloudy days that typically represent the worst-case conditions for indoor humidity control. Even setting the AC at 83oF did not result in reliable humidity control in all houses during fall weather. Furthermore, this approach imposes maximum demand during the utility’s peak period of 12 PM to 9 PM. Three methods were found to be “effective”. These methods show considerable promise, each showing the ability to control indoor RH with reasonable energy use and peak demand impacts. None of these methods stands out as being substantially better than the others within the limited sample of homes. AC at 74oF from 3 to 5 AM. Using a programmable thermostat to operate the cooling system during the early morning hours appears to work well in a majority of homes and under a wide range of weather conditions. In homes with high infiltration rates, the AC operation time may need to be extended to four hours to keep RH below 65% most of the time. A lower thermostat setting of say 70oF to 72oF may be necessary during the spring and fall months. Energy use is modest for this strategy, and electrical load is kept off-peak, as long as the thermostat clock retains its memory. The effectiveness of this approach assumes that the AC system can produce cold supply air. Warmer than normal supply air can result from improper refrigerant charge, excessive system airflow rates, or large return duct leakage. It also assumes that system operation does not induce high infiltration rates due to duct leakage or closed interior doors. For those considering this approach, diagnosis and repair of duct leaks of the central air conditioning system would assure best results. Also, whenever using the cooling or heating system in a vacant home, fan control should be set to “auto” to save energy, to avoid drawing humid air into the house through duct leaks, and to prevent the filter from fouling up too quickly. Dehumidifier. Dehumidifiers can be controlled by a humidistat or by a timer. Humidistat control. Setting a dehumidifier at 60% and letting it cycle ON and OFF to maintain the desired RH is generally effective. The control performance of the built-in humidistats was found to be imprecise, and there are questions about the reliability of available humidistats. If used with a more reliable humidistat, this approach can be quite effective. Energy use is moderate and favorable for utility peak impact, because the dehumidifier tends to operate more during off-peak periods. Timer control. Operating the dehumidifier at the lowest RH setting with a timer for a specific number of hours each day is effective and reliable in controlling RH at a reasonable energy cost. Dehumidifier run time must be determined based on the size and air tightness of the house, generally using a trial and error approach. A timer with battery back-up is preferred to ensure the timer stays synchronized with the correct hour. If a dehumidifier is operated by means of a timer, a floor fan could be plugged into the same timer to help circulate air to the parts of the home furthest away from the dehumidifier. Avoid operating the dehumidifier between the hours of noon and 9 PM when the electric utility experiences its peak load. Space heating. Heating the house to 88oF to 90oF lowers indoor RH quite effectively for nearly all hours of the summer without removing moisture from the room air. Energy use is moderate for the period June through September, but increases substantially in early spring and late fall. In general, this approach should not be used during the months November through April, since heating energy use could become excessive. The heating approach is nearly four times more energy efficient using a heat pump compared to resistive strip heaters. With either heat source, the peak hour demand to the utility is minimal since no space heating is required from 12 to 9 PM on hotter than average summer days. Humidistat control over the heating system can greatly minimize this higher energy use during cooler months. The premise of this approach is that during cooler months the outdoor dew point temperature declines. With lower dew point temperatures, the indoor temperature required to maintain a 62% or lower RH also declines. In this approach, a humidistat is used to turn on the space heating system when indoor RH exceeds 62%. A humidistat is installed to operate the heating system. The thermostat is switched to OFF (or set to HEAT at say 40oF in north Florida to prevent house freeze-up). Either the thermostat or humidistat can now turn on the system to provide heating. Energy savings result during cooler months because the indoor temperature required to meet our RH objective is automatically reduced (by the humidistat) as the outdoor dew point temperature declines. If you leave either the AC or heating system on in a vacant home, be sure to set the thermostat fan switch to “auto” to save energy and avoid clogging the filter. Before leaving any home vacant, open all interior doors to allow unrestricted air circulation. Make sure all exterior doors and window closed. If a home is to be vacant for an extended period then install a dehumidifier controlled by a humidistat. This is the best and most cost effective solution to the high relative humidity and mold problem. Sources: • U.S. EPA – "A Brief Guide to Mold, Moisture and Your Home" http://www.epa.gov/iaq/molds/moldguide.html • Texas A&M University - "Mold Mini-Course" provides 5 learning modules on mold: http://fcs.tamu.edu/housing/iaq/Mold/web/index.htm • Building Science Corporation – online articles on mold and moisture control: http://buildingscience.com/resources/resources.htm#Mold • National Association of Home Builders Research Center – ToolBase Services site: "Mold in Residential Buildings" (type ‘mold' into search box if necessary): http://toolbase.org/index-toolbase.asp • American Red Cross – "Repairing Your Flooded Home" provides excellent advice for cleanup of major water damage in homes: http://www.redcross.org/services/disaster/afterdis/reptoc.html . The Florida Solar Energy Center - is a research institute of the University of Central Florida http://www.fsec.ucf.edu/en/ Indoor space conditioning guidelines for vacant homes to avoid mold problems while minimizing both energy usage and electrical load in hot humid climates. • www.fsec.ucf.edu/en/publications/html/fsec-cr-1487-04/
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