Psychrometrics - The Study Of Air-Water Vapor Mixtures

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1 One of the largest obstacles when dealing with building moisture complications is the confusion surrounding moisture properties and dynamics. Part of the challenge is the elusive nature of moisture characteristics in buildings. Many of the dynamics which influence building moisture levels are not necessarily recognizable or measurable without the aid of specialized equipment and a working knowledge of the psychrometric process. We often hear a reference to a whole house approach for diagnosing and treating homes. Understanding and combining the relationship of energy improvements, and moisture control is a great example of this whole house concept. When the appropriate approach is used multiple and mutual benefits are obtainable, including energy efficient, comfortable, and healthy homes. Ignoring the interactive dynamics of moisture and other building performance issues is a risky approach, resulting in unhealthy situations, building durability problems, and comfort complaints. Psychrometrics - The Study Of Air-Water Vapor Mixtures Dry Bulb Temperature - The temperature of air as sensed by a thermometer. Wet Bulb Temperature - The temperature of air as sensed by a wet bulb, influenced by the evaporative cooling process. Specific Humidity - The weight of water vapor per pound of dry air. Relative Humidity (RH) - The amount of moisture air holds relative the maximum amount it can hold, at any given temperature Dew Point Temperature - The temperature at which the water vapor in air would begin to condense. The above psychrometric criteria have a constant relationship which is demonstrated on the Psychrometric Chart. Using the chart we can determine temperatures, dew point, relative and specific humidity. To predict whether or not moisture

2 condensation will occur on a given surface you need three pieces of information; the temperature of the air, the relative humidity of the air, and the surface temperature. To avoid surface condensation raise the surface temperature, lower the moisture content of the air, or employ some combination of both. Relative humidity is a function of the amount of moisture content in the air relative to the air temperature. Warm air holds more moisture than cold. If the air temperature cools to the point where the air can no longer hold the moisture, condensation occurs. If a window is colder than the surrounding air, the moisture in that air condenses when it comes into contact with the cold glass surface. Some condensation can be expected in cold weather since glass temperatures readily conduct cold exterior temperatures, even with today s advanced glass technologies. The colder the outdoor temperature, the more likely condensation will occur. The higher the relative humidity of the air near the window, the more likely condensation will occur. The occurrence of condensation is the specific intersection of a particular air temperature and air moisture content. This is a very predictable constant and proven equation as demonstrated on the widely accepted Psychrometric Curve. Using these humidity/air temperature equations building scientist can readily predict when condensation will occur. With a thermostat set at 70 degrees, and indoor relative humidity below 35 percent condensation will not be a problem on double-glazed/ Low-E windows until outside temperatures drop close to zero. If condensation occurs on the double-glazed/ Low-E windows it s an indication that relative humidity is too high for the temperature conditions. The best way to deal with condensation on high-quality, double-glazed windows is to reduce indoor relative humidity to no more the 35 percent. Additional Considerations During the first heating season, newly constructed homes can release overwhelming amounts of moisture into the air as part of the curing and drying process related to green concrete, plaster and wood. The tighter the home the less need (if any) there is for additional added humidification Since the human body does not often readily react to small shifts in humidity it can be difficult to monitor and control whole house humidification systems. Over humidification caused be these systems is a very common occurrence.

3 Applying the psychrometric process can be useful in helping to understand wet attics, attic condensation and attic mold. If warm moist air from the heated space escapes into the attic during the cold winter season, condensation will occur when that moisture contacts the cold roof plywood. In some attics condensation will only be apparent on the cold north side of the rood as the solar gain from the sun keeps the south side above the dew point. In any event air escaping into the attic during the heating season is bad news. The multiple benefits of sealing air leaks into the attic increase comfort, save energy, and can help reduce the potential for wet attics. This ceiling light fixture only leaks on cold winter nights. A symptom of severe attic condensation and heat loss. The attic roof plywood is saturated with water resulting in mold growth. Moisture Transfer Methods 1. Diffusion - The transport of water molecules through a surface due to a difference in vapor pressure. Various building materials allow different amounts of moisture to pass through them. The measured perm rating of these materials refers to the grains of water vapor passing through a square foot per hour. Materials with low perm ratings ( ) are considered good vapor retarders. Materials such as exterior house wraps have high perm ratings 2. Convection - The movement of air and the water vapor in it, due to stack effect, wind effect, and mechanical effect. 3. Capillary Action - Wicking action of wood or masonry Recent building research suggests that air leakage (convection) is the main factor contributing to condensation problems in insulated building cavities such as attics, kneewalls, and to a much lesser extent walls. Vapor diffusion plays a much

4 smaller role. It is important to note that low rated perm materials (vapor retarders) should never be used on the cold side of any insulation or building component where condensation may occur. To stop moisture migration into insulated building cavities the most important consideration is an air tight barrier to stop air movement and the moisture carried with it. Typical Moisture Sources in the Home Source LBS/HR Respiration 0.1 per person Bath 1.5 Shower 2-4 Washing Machine 1-2 Hand washing 0.8 Gas Stove 0.4 Gas Oven 0.8 Boiling Water 1.4 Typical Household Moisture Generation = 22 LBS Per Day 3-4 Family members 14 LBS Bathing showers 3 Cooking 2 Dishwashing 1 Plants 1 Moisture Balance in Buildings Four factors influence moisture levels in the home. Appropriate control and management of these factors is the key to a healthy moisture balance. 1. Source strength is the largest factor influencing moisture levels in the home, including moisture production from normal household activities. Additional sources from leaks, standing ground, and other incidental complications can easily increase indoor moisture to unacceptable levels. Treating these extraneous sources are the most effective way to reduce moisture complications.

5 2. Temperature of surfaces below the dew point will allow condensation to occur. Treating cold surfaces eliminates condensation while improving comfort and energy efficiency. 3. Moisture transfer includes moisture intrusion from outside and interior situations that allow moist air to leak to cold surfaces. Treating the building shell to prevent leaks and capillary action can eliminate exterior bulk moisture intrusion. Sealing heated spaces from cold surfaces and the use of vapor retarders eliminates conditions where condensation can occur. 4. Ventilation rate can determine the potential to flush out excess moisture and improve circulation. It is important to note that in the heating season outside air has less specific humidity than inside air, even if it has a higher relative humidity. When cold outside air is brought inside and heated, the relative humidity drops. Consider the following example; Assume a scenario of outside air temperature at 40 F and a relative humidity of 60%, bring that air inside and heat it to 75 F. The relative humidity for that volume of air drops to 18%, the specific humidity does not change. The Drying Effect of Outside Air: Moving high relative humidity air out of the building and replacing it with dry outside air lowers the inside relative humidity. Homeowners often complain of dry homes. Typically these are drafty homes. Often occupants will respond to dry homes with humidifiers. This approach will add moisture to the air but may result in condensation if surface temperatures are below the dew point temperatures. The best treatment for dry homes is to reduce excessive, uncontrolled infiltration of dry outside air. Mold, Mildew, and Wood Decay There are tens of thousands of different varieties of molds and mildews. Found in many common environments, their growth is encouraged by warmth and high humidity. Molds which are produced in the home are of particular concern especially to individuals who are sensitive to the microorganisms that cause allergies, asthma, and other health related problems. People should not live in moldy houses!

6 Indoor mold growth typically occurs when indoor relative humidity exceed 55% in the winter and 70% in summer. Cold surfaces in the heating season such as thermal defects and around windows are good candidates for mold growth. Damp basements or leakage areas are also common mold sites. Wood moisture content is defined as the weight of water in wood, relative to the weight of dry wood. Dry lumber has a moisture content of less than 19%. Kiln dried lumber is less than 15%. The moisture content of wood installed in buildings is influenced by the relative humidity of the building. Typically wood moisture content will not exceed 20% unless subject to standing water, condensation, or leaks. Wood decay is sometimes called dry rot because of the crumbly effect on the wood. When wood temperatures are above 50 F and moisture content reaches the fiber saturation point (30%) decay fungal growth begins. Moisture Sources And Symptoms & Solutions Many different factors and variables determine the moisture levels in each home. Normal daily activities can produce significant amounts of moisture. Additional moisture sources can overcome the natural ventilation of the home raising the indoor moisture to unacceptable levels. The volume of the moisture source is the largest factor in determining the potential for problems. Moisture symptoms can help building practitioners identify and evaluate the extent of potential problems. Some symptoms are visible for example mold, rust, and window condensation. Other factors must be measured with diagnostic tools primarily a psychrometer, blower door, and moisture meter. A history of the home and some lifestyle considerations of the occupant can also be important indications for determining the optimum moisture control strategy for the home. Modest lifestyle changes may be necessary, decrease or stop humidifier use, cook with covered pots, avoid drying laundry inside, and cover aquariums. Simple home repairs can help. Cover and seal any exposed openings in basements floors or walls, repair leaky plumbing, improve drainage, make sure dryer and exhaust fan vents are vented to outside.

7 Additional moisture sources can include exposed dirt floors, green firewood stored inside, unvented (or faulty) combustion heaters, spas, and expansive house plant collections. Removal or reduction of excessive moisture sources is the most desirable and most effective solution. The elusive characteristics and seasonal nature of moisture in buildings make it difficult to predict which homes may be susceptible to problems. Typically there are a combination of key sources and symptoms that can be used to help quantify moisture levels. Consider the following data resulting from the study of 101 homes with various moisture levels. A level 1 indicates a home with little or no problem, level 2 - moderate and potentially worsening problem, level 3 - severe problem home. humidifier humidifier occasional use frequent use dryer vented indoors clothes dried inside crawl space without ground cover basement foundation seepage sump pump opening or equivalent Level 1 6 % 0 % 3 % 0 % 3 % 0 % 6 % Level 2 2 % 5 % 12 % 8 % 10 % 15 % 17 % Level 3 8 % 12 % 24 % 6 % 28 % 36 % 20 % gutters/ plumbing other fan use fan use fan use fan use

8 downspouts leaks none seldom occasional often ineffective >20 min. Level 1 26 % 3 % 3 % 41 % 26 % 24 % 9 % Level 2 22 % 5 % 2 % 46 % 41 % 12 % 0 % Level 3 32 % 4 % 8 % 56 % 32 % 4 % 8 % The information from this study can serve as a guideline to help quantify moisture potential and categorize homes for appropriate solutions. The symptoms and sources found should be assigned a value by the practitioner based on degree and potential. It is important to remember that the total combination of conditions will determine the potential for any individual building. No data or numbers can be as intuitive and flexible as the practitioner's experience and common sense. Do not let the numbers supersede your own best judgment. Some homes may have obvious problem symptoms but no apparent sources. It is important for the practitioner to use good judgment in these cases and proceed with the appropriate caution. A thorough customer interview, including a history of the home and pertinent symptoms is imperative. It is important to fully document specific details or any extenuating conditions which effect the moisture conditions of the home. Ventilation - Don t Stay Home Without it! In cases where sources reduction is not sufficient or possible controlled ventilation systems must be used to move moisture laden air from the building. Controlled ventilation has an advantage over natural infiltration. Natural drafts are unreliable subject to temperature and wind change. Unfortunately many of the existing fans found in homes today are not powerful enough or used long enough to provide adequate air exchange. Typically these fans are moving air at about one half the rated capacity. Many are too loud and use to much energy for long term use. It is not uncommon to find bath exhaust fans that dump moist air back into the building due to faulty venting and ducting. A wide variety of powerful yet low wattage quiet fans are on the market today. Properly installed with a timer switch or dehumidistat control the fans can provide spot ventilation where it is needed most - typically from showering and cooking. In some very tight or problem homes systems can be designed to run continuously. Heat recovery systems usually seen in new tight construction, preheat fresh air to minimize heating costs Often homeowners and some builders confuse attic ventilation with moisture control in the heated area of the building. It is important to point out that these are

9 two different systems that deal with two different zones of the building system. Attic ventilation will not address moisture sources originating from the living space. Procedural Issues Increasing the range of buildings that can be retrofitted while limiting potential liabilities is a balance that can sometimes take considerable good judgment. Proceeding in a consistent standardized fashion eliminates guess work. Assessment procedures that evaluate and document conditions are the basis for finding problems before they get out of control. Moisture audits can be an important part of any energy/comfort improvement initiative. The key is to employ a pre-emptive approach to avoid problems. Identify potential problems before any work begins. Quantify moisture sources and symptoms to determine the potential for problems. Bring the customer on board as part of the audit procedure. Document existing conditions and follow-up to track post retrofit results. Customer education is critical due to the many myths surrounding moisture issues. The occupant is inevitably the one responsible for source reduction and ventilation control. Use your professional integrity and diagnostic tools to increase credibility. Present the science involved by referring to graphics, psychrometric charts. Use simple examples, photos, and educational brochures to explain confusing concepts. User friendly solutions will be more readily accepted by the occupant. This is especially true for ventilation controls. Ask unconvinced homeowner s to run their existing fans for a trial period. Even if the fan is loud leave them on continuously for 24 hours during a period of high moisture. Follow up to track the effect. Does Weatherization Cause Indoor Air Quality Problems? No one has all the answer on dealing with IAQ issues. However many experts now agree that tight homes don t necessarily have worse IAQ than drafty homes. Many energy improvements actually help moisture control such as treating cold areas, adding storm windows, sealing air leakage path, and duct leaks. House tightening reduces air exchange rates which may have previously kept indoor humidity low. In this case the retrofit did not create the moisture but lead to conditions that made an existing situation more obvious.

10 Building tightening work should not proceed if there is a any significant indoor air quality problem that is not treatable. To predict moisture problems we have science, audit procedures and diagnostic tools. To prevent problems we must treat the building as a whole system. This whole house challenge includes treating the most important, sometimes erratic component of the system - the occupant.