Managing Building Moisture

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1 Managing Building Moisture, yz{ } ~ } ~,,,, y zz {{ }} ~~,

2 Managing Building Moisture by Dennis Stanke, staff engineer La Crosse, Wisconsin Bruce Bradway, senior airside applications engineer Lexington, Kentucky with Art Hallstrom, airside applications engineering manager Lexington Kentucky Nan Bailey, information designer La Crosse, Wisconsin Special thanks to: J. David Odom III, vice president CH2M Hill and his staff in Orlando, Florida, for permission to adapt text and illustrations from the CH2M Hill manual Preventing Indoor Air Quality Problems in Hot, Humid Climates: Design and Construction Guidelines, CH2M Hill, SYS-AM-15 i

3 Contents Preface... iv Introduction... 1 Good Reasons for Dry Buildings... 2 Better Indoor Air Quality/Better Health... 2 Reduced Building System Deterioration... 3 More Comfortable Space Conditions... 3 Moisture Sources... 4 Liquid-Water Sources... 4 Weather... 5 Ground Water... 5 Interior Leaks... 6 Cleaning... 6 Water-Vapor Sources... 6 Vapor-Pressure Diffusion... 7 People Evaporation Combustion Infiltration Ventilation Condensation Moisture and Building Envelope Prevent Liquid-Water Intrusion Minimize Vapor-Pressure Diffusion Minimize Infiltration Summary Moisture and Occupied Spaces Minimize Liquid-Water Sources Prevent Unplanned Condensation Dehumidify Spaces Account for All Loads Part-Load Control Unoccupied Control System Monitoring Summary ii SYS-AM-15

4 Contents Equipment-Room Moisture Minimize Moisture Sources Prevent Unplanned Condensation Raise Surface Temperature and Vapor Seal Lower Equipment-Room Dew Point Dehumidify Ventilation Air Equipment-Room Design Examples A Poor Design: Negative Pressurization A Better Design: Positive Return-Air Pressurization Best Design: Positive Supply-Air Pressurization Summary Moisture and Chillers Moisture and Air-Handling Units Condensate Collection Pans Size Coil to Limit Carryover Slope to Prevent Standing Water Drain to Prevent Flooding Drain-Line Seals External Condensation Seal Penetrations and Joints Improve Unit Insulation Internal Condensation Seal Unit Penetrations and Joints Lower Equipment-Room Dew Point In Conclusion Acknowledgments SYS-AM-15 iii

5 Preface Uncontrolled moisture within a building can contribute to structural damage, occupant discomfort and unacceptable indoor air quality. Moisture is often overlooked or underestimated during HVAC system design and operation, and it can cause significant problems in the building envelope, occupied spaces and mechanical-equipment rooms. This manual helps HVAC system designers identify and quantify moisture sources. It also presents moisture-management techniques related to the building envelope, the occupied space and the mechanical-equipment room. Moisture problems can occur in buildings in any geographic location. The solutions identified in this manual apply to building-moisture management in any climate; however, the concepts are especially applicable for buildings located in humid climates. The Trane Company, in proposing these system design and application concepts, assumes no responsibility for the performance or desirability of any resulting system design. System design is the prerogative and responsibility of the system designer. iv SYS-AM-15

6 Introduction Uncontrolled moisture in buildings can cause serious problems for building occupants, furnishings and structure. Microbial growth, encouraged by high relative humidity, leads to poor indoor air quality and building deterioration. Poor IAQ results in discomfort, health problems and could ultimately lower productivity and spawn lawsuits. Uncontrolled moisture can also stain wall surfaces, damage paint, corrode metal surfaces and accelerate the deterioration of building furnishings and structural materials. Buildings located in climates with long periods of high ambient temperature and high, absolute humidity (hot, humid climates) are particularly prone to uncontrolled moisture. Figure 1 illustrates humid climate and fringe climate regions within the U.S. Figure 1 Hot, Humid Climates This manual discusses design considerations and HVAC operating techniques that help control moisture within occupied buildings once it enters the building. l SYS-AM-15 1

7 Good Reasons for Dry Buildings Controlling building moisture takes time and money. How do building occupants and building owners benefit from these expenditures? Better Indoor Air Quality/Better Health Building components (walls, floors and ceilings) and building furnishings (wall coverings, carpets, furniture and stored materials) provide ideal amplification sites for microbial growth. Microbial growth fungi, bacteria and dust mites, for instance can produce odors, allergens, and in some cases, toxins. Odors cause discomfort and long-term exposure to allergens and toxins can lead to health problems such as asthma and lung disease. Also, musty-smelling buildings cannot command high rent or a high resale price. For microbial growth to occur, certain conditions must be present w A source of food w Temperature between 40 F 100 F w Adequate moisture, usually 70% RH or higher w A source of mold or mildew spores These conditions can certainly be present in a building. Materials used in building construction, building furnishings, stored materials (books and papers) and accumulated dirt can all become food sources for microbial growth. Typical indoor temperatures fall in the middle of the microbial growth temperature range. Uncontrolled, indoor relative humidity can easily rise above the 70% RH needed to encourage microbial growth, especially in hot, humid climates. Spores, of course, are present everywhere in both indoor and outdoor air as well as in building materials and furnishings. Of the four conditions for microbial growth, relative humidity is most easily controlled. Maintaining indoor relative humidity below 60% RH, as required by ASHRAE (Figure 2), limits the potential for microbial growth in buildings. Figure 2 ASHRAE-Recommended Humidity Levels 2 SYS-AM-15

8 Good Reasons for Dry Buildings Reduced Building System Deterioration The same fungi (mold and mildew) that cause people discomfort and/or harm can also cause building materials, furnishings and structure to prematurely corrode and/or degenerate. Premature failure of walls, ceilings and floors and irreversible damage to carpets, wall coverings and furnishings can result. The same rationale holds true for the air handling equipment and duct system. Wet insulation can lead to corrosion and/or deterioration, shortening the useful life and effectiveness of the air-distribution system. Deterioration in buildings increases maintenance and operation costs. Figure 3 illustrates the relationship between the effects of relative humidity and building operation, maintenance and repair costs. Maintenance includes normal cleaning and periodic replacement of damaged furnishings, such as moldy carpet and wallpaper. Building operational costs include the cost of energy. Figure 3 Humidity and Building Costs More Comfortable Space Conditions Controlling indoor relative humidity to an acceptable level results in consistent thermal comfort within the occupied spaces. Thermal comfort reduces occupant complaints and improves worker productivity, and increases both rental potential and market value. l SYS-AM-15 3

9 Moisture Sources Moisture enters the building as liquid water or as water vapor. It causes trouble in either form and changes readily from vapor to liquid through condensation. It must be properly managed to avoid trouble. Let s look at possible moisture sources and techniques to minimize the impact of each. Liquid-Water Sources Liquid water damages furnishings and building structure, supports microbial growth and provides surface wetness for evaporation, a source of water vapor and increased indoor moisture load. Common liquid-water sources include the weather (rain, fog and snow), ground water, leaking pipes and equipment, and wet cleaning processes (see Figure 4). Perhaps the most troublesome source of liquid water, condensed water vapor, is discussed separately on the following pages. Figure 4 Common Liquid-Water Sources in Buildings 4 SYS-AM-15

10 Moisture Sources Weather During building construction, prior to completion of the roof and walls, building materials and the partially completed building structure may become saturated with rainwater or snow. Protect building materials from rain, snow and condensation (which can form inside equipment wrapped with a vapor retarder such as plastic) during construction. For best results, store materials within covered structures. If materials become wet, dry them quickly and completely or replace them. Mold can grow within 24 hours on wet materials. Discard visibly moldy materials and replace with new, dry materials. During normal operation, rain may enter the building structure through roof and wall leaks or openings. Design windows and walls to minimize leakage and control water with internal drainage schemes. Roof design and construction practices must result in a leakproof membrane that drains properly. Roofs need proper maintenance to assure long-term integrity. Repair leaky walls and roofs quickly to prevent water damage and to avoid high latent loads indoors. The outdoor intake airflow may entrain rain droplets or snowflakes and carry them into the duct system. Design outdoor air intakes to limit rainwater entrainment, using rain hoods and moisture eliminators sized to avoid high intake velocity. If the design allows water droplets to penetrate the intake, provide for indoor drainage (drain pans for instance); in other words, manage the water flow once it enters the building. Rain hoods and moisture eliminators cannot stop entrained snow. Prevent possible filter damage or internal flooding using a heating coil to melt the snow or a large plenum to allow the snow to settle, melt and evaporate before it reaches the filters. Fog also enters the building through the outdoor air intake. Fog droplets, too small to stop at a louver or moisture eliminator, usually evaporate quickly within the HVAC system, causing little or no surface wetting. However, the evaporated droplets certainly add to the indoor moisture load and must be accounted for in the design and operation of the system. Ground Water Ground water may seep into the building through very small cracks in basement walls and floors. Be sure that surface water and subsurface water drains away from the building, not toward it. Design the floor to limit water intrusion via cracks and joints. Manage any water that penetrates the floor using proper slopes and drains. SYS-AM-15 5

11 Moisture Sources Interior Leaks Leaking appliances, valves and pipes can quickly wet large areas of interior structure. Liquid water within walls and concealed areas, through capillary action and surface tension, can travel long distances and result in widespread, longterm wetting. Plan and follow maintenance procedures to assure speedy location and repair of any water leaks within the building. Accidental spills and floods should be cleaned up quickly. If porous materials become wet, dry them completely within 24 hours or consider replacing them. Cleaning Cleaning processes, such as floor mopping and carpet shampooing, result in large wet areas. Carpet shampooing in particular increases moisture content in carpet fibers (and in the accumulated dirt beneath the carpet). Use wet cleaning processes cautiously (or not at all). Take steps to dry wetcleaned surfaces within 24 hours. For shampooed carpets, assure speedy evaporation by providing adequate air motion and dehumidification during drying. If a cooling coil controlled by a thermostat provides the necessary dehumidification, auxiliary heat may be needed to maintain a sufficient cooling coil load for continuous dehumidifying capacity. Water-Vapor Sources A high indoor water-vapor level elevates the indoor dew point and relative humidity, and it contributes to the latent load on HVAC equipment. High dew point increases the likelihood of unplanned condensation; high relative humidity can result in occupant discomfort, increased dust mite population, and accelerated microbial growth. HVAC-equipment capacity must match total (latent plus sensible) load. Common water-vapor sources in buildings (see Figure 5) include vapor-pressure diffusion from outside, evaporation from people, wet surfaces and processes, generation from combustion, infiltration from outside and introduction from outside via ventilation airflow. 6 SYS-AM-15

12 Moisture Sources Figure 5 Common Water-Vapor Sources Vapor-Pressure Diffusion Water vapor moves through solid materials in direct proportion to the difference in vapor pressure between the opposite sides of the material and the resistance of the material to water-vapor flow (the permeance of the material). SYS-AM-15 7

13 Moisture Sources The moisture load (grains of water per hour) added to a building from outside by vapor-pressure diffusion can be found using Equation 1. Equation 1 Wp = P A (VPo VPi) Where Wp = moisture load (gr/h) P = permeance factor (gr/h ft 2 in. Hg) A = surface area (ft 2 ) VPo = outdoor vapor pressure (in. Hg) VPi = indoor vapor pressure (in. Hg) The table below (from 1997 ASHRAE Fundamentals Handbook) shows permeance factors for some common construction materials. The permeance factor for a composite wall (Pc) can be found as the reciprocal of the sum of permeance reciprocals. Common Construction Materials Permeance* hardwood siding (1/8") air space (1.0") polyethylene vapor retarder (0.002") 0.16 insulating board sheathing (1.0") fibrous insulation (6.0") gypsum wallboard (3/8") paint, commercial latex 6.28 vinyl wallpaper 0.23 *Permeance factor = gr/h ft 2 in. Hg Consider a composite wall that includes wallboard (P = 50), fibrous insulation (P = 19), exterior sheathing (P = 50), and hardboard siding (P = 11), and has a high composite permeance [Pc = 1 ( ) = 5.448]; i.e. water vapor can diffuse through the wall quite readily. The same composite wall with a polyethylene vapor retarder (P = 0.16), has a much lower its composite permeance (Pc = 0.155); i.e. water-vapor diffusion through this wall is much more difficult. 8 SYS-AM-15

14 Moisture Sources Figure 6 shows the composite walls described above. On a hot, humid day in Jacksonville, outdoor water-vapor pressure exceeds indoor water-vapor pressure more significantly (VPo VPi = = 0.64 in. Hg) than on a cooler day (VPo VPi = = 0.21 in. Hg). Applying the permeable wall structure (Pc = 5.435) to a small building with 4000 ft 2 of exterior wall surface, and solving Equation 1, we find that the moisture load due to vapor-pressure diffusion can be quite high (Wp = = 13,900 gr/h) on the hot day and lower on the cooler day (Wp = = 4580 gr/h). Adding a vapor retarder results in a less permeable wall structure (Pc = 0.155) and significantly lower moisture load on both the hot day (Wp = = 400 gr/h) and on the cooler day (Wp = = 130 gr/h). As this example illustrates, moisture load due to vapor-pressure diffusion can be significant through a simple wall. The addition of an exterior vapor barrier reduces it considerably but cannot eliminate it. Use a vapor retarder in floors, walls and ceilings to minimize moisture transfer due to vapor-pressure diffusion. Locate the vapor retarder within walls and ceilings, near the warm side, to avoid unplanned condensation within the structure. Figure 6 Vapor Diffusion Through Wall Structure SYS-AM-15 9

15 Moisture Sources In humid climates and other predominantly cooling climates, use lowpermeance materials near the outside surface (the warm side) to keep summertime outdoor water vapor away from cold, interior surfaces. When the building includes an attic, be sure to include a continuous vapor retarder on the attic side of the insulation. In mixed climates, for buildings with low wintertime inside relative humidity (35% RH or less), use low-permeance materials near the outside surface, as recommended for cooling climates. In predominantly heating climates, use low-permeance materials near the inside surface (the warm side) to keep wintertime indoor water vapor away from cold exterior surfaces. People Building occupants produce water vapor at different rates, depending upon their activity level, via respiration and perspiration. According to the 1997 ASHRAE Fundamentals Handbook, an adult working at a desk introduces a latent load of 155 Btu/h, and an active athlete contributes 1000 Btu/h. Since water vapor contains approximately 0.14 Btu/gr, an office worker contributes 1100 gr/h (Wo = 155 / 0.14) to the indoor moisture load while a volleyball player contributes approximately 7100 gr/h (Wo = 1000 / 0.14). Although it can be significant, many designers erroneously consider respiration as the sole source of moisture load in buildings. Design the HVAC system and equipment with sufficient capacity to satisfy the total moisture load, including the people-related moisture load as one of many sources. Evaporation Wet surfaces add moisture to indoor air through evaporation. Evaporation occurs when the air vapor pressure (VPa) is less than the vapor pressure of the saturated air at the wet surface (VPs). Wet surfaces, found throughout the building, may be planned (pools, aquariums and fountains) or unplanned (such as leaky pipes and unplanned condensation). Cooking processes and live plants add water vapor via evaporation. On rainy or snowy days, building occupants carry a significant amount of moisture into the building on their shoes and clothing. Equation 2 can be used to calculate the moisture load due to evaporation from liquid-water surfaces. 10 SYS-AM-15

16 Moisture Sources Equation 2 We = H A (VPs VPa) 7000/1060 Where We = moisture load from evaporation (gr/h) H = latent heat transfer rate (Btu/h ft 2 in. Hg, see Figure 7) A = water surface area (ft 2 ) VPs = saturated vapor pressure of air at the water surface temperature (in. Hg) VPa = vapor pressure of space air (in. Hg) 7000 = definition of grains (gr/lb) 1060 = latent heat of vaporization at 75 F (Btu/lb) Figure 7 shows latent heat transfer rate (H) related to air stream velocity. For example, a large aquarium in an office with typical 50-fpm transverse airflow (perpendicular to surface) transfers latent heat to the air at a rate of (H = 250 Btu/h ft 2 in. Hg). If the space is 72 F, 50% RH (VPa = 0.39), and 8 ft 2 of water surface at 78 F (VPs = 0.96 in. wg) is exposed, the evaporation moisture load can be found using Equation 2 (We = / 1060 = 7500 gr/h). In another example, a 4-ft diameter puddle (A = 12.5 ft 2 ) of condensate at 80 F (VPs = 1.03) on the floor of an equipment room at 85 F, 50% RH (VPa = 0.60), contributes 8900 gr/h (We = / 1060) to the equipment room. Figure 7 Latent Heat Transfer from Water Surface (with respect to the research by W.H. Carrier in 1918) SYS-AM-15 11

17 Moisture Sources Depending on the situation, moisture load due to evaporation from water surfaces may be significant. Design the HVAC system with sufficient capacity to satisfy the moisture load from all sources, including evaporation from planned water surfaces (pools and fountains) and predictable liquid-water sources (rainwater in entryways, shampooed carpets). Minimize unplanned evaporation sources (liquid water from leaky pipes, leaky roofs, and spills) by quickly eliminating the source and removing the liquid water. Combustion Combustion liberates water vapor. Remember from high-school chemistry: the two products of complete combustion are carbon dioxide (CO 2 ) and water (H 2 O). Equation 3 can be used to calculate the rate of moisture generation from an open gas flame. Open-flame heaters, boilers and appliances, as well as openflame cooking surfaces, can produce significant water vapor. Indoor combustion processes must be considered when calculating indoor moisture load. If an unvented gas griddle consumes natural gas at a known rate (G = 6.7 ft 3 /h), the combustion moisture load can be easily found (Wc = = 4400 gr/h). Equation 3 Wc = G K Where Wc = moisture load from combustion (gr/h) G = gas firing rate (ft 3 /h) K = 650 gr/ft 3 for natural gas 1300 gr/ft 3 for propane If possible, use vented combustion processes to eliminate combustion moisture load and other products of combustion. Dehumidify to remove moisture from unvented combustion processes within the building. Infiltration No building is airtight. Outdoor air enters (infiltration) and indoor air leaves (exfiltration) via countless little openings in the building envelope as well as large intentional openings such as doors and windows. Driven by differential pressures imposed by mechanical ventilation, wind and stack effect, the infiltration (exfiltration) air carries water vapor with it. Air passes through any available penetration in wall, floor or ceiling. 12 SYS-AM-15

18 Moisture Sources Elevator shafts act as chimneys, reducing pressure on lower floors. Gaps can be found at wall-floor or wall-ceiling joints, between wallboard and electrical fixtures on perimeter walls, at electrical wire and conduit penetrations, at pipe and duct penetrations, and at window and door penetrations. Air also comes and goes through open exterior doorways and windows. Whenever an exterior door opens in Florida, cooled indoor air spills out of the building at floor level, only to be replaced by an inflow of warm, moist air at door-top level. How much water vapor moves with the air? Building moisture load due to infiltration can be calculated using Equation 4. Equation 4 Wi = A r 60 Va (HRo HRi) Where Wi = moisture load due to infiltration (gr/h) A = area of opening (ft 2 ) r = density of outdoor air (lb/ft 3 ) 60 = minutes per hour (m/h) Va = air velocity through opening (fpm) HRo = outdoor air humidity ratio (gr/lb) HRi = indoor air humidity ratio (gr/lb) Equation 4 includes air velocity and the total area of all openings in the envelope. Values for these variables may be estimated, either at normal or extreme differential (inside-to-outside) building pressure. However, it may be more practical to estimate total envelope airflow (Qe = A Va) as follows. Building tightness specifications often rate building leakage at a differential pressure of 0.30 in. wc. At a positive differential pressure of 0.30, the 1997 ASHRAE Fundamentals Handbook, Chapter 25, estimates typical exfiltration airflow per square foot of exterior wall surface for tight, average and leaky walls (Q = 0.10, 0.30 and 0.60 cfm/ft 2, respectively). If we assume an exponential relationship between differential pressure and airflow (Q = k P 0.65 ), established statistically by various researchers, we can find the flow coefficient (k) for each wall construction category, as shown in Figure 8. SYS-AM-15 13

19 Moisture Sources Figure 8 Leakage Airflow Finding Worst Case Design Condition Most designers use the ambient weather data presented in the 1997 ASHRAE Fundamentals Handbook to determine total cooling load due to ventilation. Traditionally, designers used peak dry-bulb temperature and meancoincident wet-bulb temperature as worst-case design condition. A designer in Jacksonville, using the 0.4% annual peak dry bulb (96 F) and mean coincident wet bulb (76 F) from 1997 ASHRAE Fundamentals Handbook, can first find outdoor and indoor air enthalpy (ho = 39.2 and hi = 26.4 Btu/lb, respectively), then find the associated air conditioning load. For our example building, ventilation air adds 9.6 tons (Qt = ( )/ = 9.6) to the building air conditioning load at this design condition. Is this the worst-case ventilation air load? No! The 1997 ASHRAE Fundamentals Handbook also presents annual peak dew point and mean coincident dry-bulb temperatures. Using the 0.4% annual-peak dew point (76 F) and mean coincident dry-bulb (84 F) temperatures, both outdoor air enthalpy (ho = 41.6 Btu/lb) and ventilation load (Qt = 11.4 tons) increase. Ventilation air on a warm, rainy day represents more total cooling load than the same volume of ventilation air on a hot, sunny day. Assuming our example building in Jacksonville uses average wall construction and operates at a slightly negative pressure (DP = 0.05 in. wc.), we can calculate infiltration airflow (Q = 0.66 (0.05) 0.65 = cfm/ft 2 ) per square foot of exterior wall surface or total infiltration through the envelope (Qe = Q Ae = = 376 cfm). In Jacksonville, Florida, when outdoor conditions are hot (96 F, 80% RH, HRo = 156 gr/lb) and indoor conditions are comfortable (72 F, 50% RH, HRi = 58 gr/lb), we can use Equation 4 (with A Va = Qe) to calculate the infiltration moisture load (Wi = (156 58) = 153,000 gr/h). Even very low infiltration airflow can contribute significantly to moisture load and must be considered during design. Seal air leaks due to cracks/gaps/holes where air and moisture can enter, paying particular attention to wall penetrations. In hot, humid climates, design to minimize infiltration by maintaining indoor spaces at a slightly higher static pressure than outdoor atmospheric pressure; operate with positive building pressure. Ventilation HVAC equipment draws in outdoor air for ventilation and to replace indoor air removed by exhaust fans and combustion processes. Ventilation air can contain considerable moisture, especially in hot, humid climates. Water vapor in the ventilation air often represents the single largest source of moisture in a building. We can use ventilation airflow (Qv = A Va) in Equation 4 to find ventilation moisture load Wv. Given ventilation airflow (Qv = 2000 cfm), ventilation moisture load on a hot, humid day in Jacksonville can be calculated (Wv = (156 58) = 820,000 gr/h). 14 SYS-AM-15

20 Moisture Sources Contrast the moisture load in Jacksonville to that in Denver. On a typical hot day in Denver, each pound of ambient air at 93 F, 10% RH contains only 24 grains of water vapor. Again, given ventilation airflow (Qv = 2000 cfm), ventilation moisture load can be calculated using Equation 4 (Wv = (24 58) = -290,000 gr/h). Note that a negative ventilation moisture load results. In Denver, ventilation air can actually remove moisture from the building rather than add it. Most designers use ambient weather data (see side-bar) to estimate ventilation air conditions and ventilation moisture load. Ambient weather data describes historical conditions in a general geographical region. However, local ventilationair moisture content may be even higher than indicated by ambient conditions. For instance, a roof-mounted intake in Jacksonville may introduce very warm air (110 F) with very high moisture content (170 gr/lb or more), especially when the sun reappears after a rain shower. Since outdoor airflow cannot be lower than the minimum required for proper ventilation, the air introduced for ventilation must be dehumidified. Select and operate HVAC equipment and systems to dehumidify the ventilation air at all load conditions. Condensation Condensate forms whenever moist air contacts a surface at a temperature below the dew point (Figure 9). Figure 9 Surface Condensation SYS-AM-15 15

21 Moisture Sources Inside the exterior walls, water vapor can enter by vapor-pressure diffusion, by exfiltration or infiltration, or by evaporation from liquid-water leaks raising internal dew point. In occupied spaces during the cooling season, a supply-air duct or chilled-water pipe behind an interior wall may cool the wall surface below dew point. The concrete beneath carpeted floors may be very cool compared with average space temperature. During the heating season, indoor water vapor can easily condense on cold windows, cold wall surfaces within the space, and inside the exterior wall structure. The mechanical equipment room offers many cold surfaces for the formation of unplanned condensate, especially during the cooling season. The outside surfaces of air handlers and supply-air ducts, water chillers, chilled-water and return-water pipes, and condensate drain pipes all operate at low temperatures. If the equipment room is not conditioned, equipment-room dew point may be very, very high, especially in hot, humid climates. Inside the air handler, surface temperature approaches supply air temperature. If design philosophy results in a high equipment-room dew point, any air leaks into the air handler downstream of the cooling coil may cause significant condensation and flooding inside the air handler. To avoid unplanned condensation in any location, either raise the surface temperature or lower the air dew point or both. No other alternative exists. In summary, many sources of moisture in buildings must be considered. Some can be eliminated, others can be minimized, but none can be ignored during building design and operation. l 16 SYS-AM-15

22 Moisture and Building Envelope Four major building elements must be designed and operated properly to minimize the impact of moisture in the building w Envelope w Occupied spaces w Equipment room w HVAC equipment Let s start with the building envelope. Unwanted, unplanned condensate forms inside walls and ceilings if the dew point of the internal air exceeds the coldest surface temperature. Since water vapor can enter the wall cavity by evaporation, vapor-pressure diffusion or infiltration, all three of these potential water-vapor sources must be managed to prevent unplanned condensation. Prevent Liquid-Water Intrusion Water vapor inside the building envelope structure increases if evaporation from liquid water occurs. Design and construct exterior walls to keep liquid water out. Liquid water includes not only rainwater and melted snow, but also ground water, water from leaky pipes and unplanned condensation. Use a water barrier near the outside surface of exterior walls to keep the rain and snow out. Design the roof to drain freely and to be watertight. Be sure to drain ground water away from the building. Seal all envelope penetrations. Seal underground wall-floor systems and wall-floor joints. Minimize Vapor-Pressure Diffusion Since the indoor-to-outdoor temperature difference cannot be controlled, condensation prevention relies on low moisture content (low dew point) within the walls. In addition to sealing against liquid water, design to limit water-vapor diffusion into the wall condensation occurs if moist air can penetrate to a cold surface. Use a vapor retarder on the warm side. Warm side (as mentioned above) in cooling and mixed climates means the outdoor side of the ceiling or the wall structure; in heating climates, it means the indoor side. Do not use two vapor retarders one on the warm side and one on the cold side. Moisture trapped between the two vapor retarders cannot escape and condenses inside the ceiling or the wall at the cold surface. Also, include a sealed vapor retarder in the ceiling of the top floor: in heating climates use a vapor retarder on the space-side, and in cooling climates use one on the attic side. SYS-AM-15 17

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