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The Impact of Airtightness on System Design by Wagdy A.Y

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The Impact of Airtightness on System Design by Wagdy A.Y The Impact of Airtightness On System Design By Wagdy A.Y. Anis, AIA t is commonly believed that infiltration is due to poorly weather- Q = C(∆p)n (1) II stripped doors and windows. Although this is partly true, the fact is where 80% or more of infiltration is due to the many imperfections that are Q = volume flow rate, cfm C = Flow coefficient, cfm/(in. designed or built into exterior envelopes. “Will we have enough fresh w.g.)n air to breathe?” is a concern about air-tightening the envelope. ∆p = Pressure difference ANSI/ASHRAE Standard 62-1999, Ventilation for Acceptable Indoor Air n = an exponent ranging be- tween 0.5 and 1.0. An exponent of 0.65 Quality has taken care of that problem. Doesn’t air tightening cause represents many cases of wall and win- sick buildings? To quote, Joseph Lstiburek, Ph.D., P.Eng., Member dow leakage. ASHRAE,1 “in order to control air, you must first contain it.” Figure 1 illustrates the relationship between pressure difference and flow through building openings. Air pressure acting on building enve- that are connected to the building en- Three major sources of air pressure on lopes can wreak havoc with building per- velope. buildings are wind pressure, stack pres- formance if not properly understood and sure, and HVAC fan pressure. Two minor adequately designed for. Uncontrolled air Leakage Characteristics2 pressures are changing barometric air pressure across the building envelope and Air leakage can occur through pores pressure and temperature differentials within the building itself can cause in- in materials, cracks, holes, or other open- across the building envelope. They are filtration and exfiltration that overpower ings. Flow is produced by a pressure dif- inconsequential relative to the three HVAC systems. By disrupting the HVAC ference that provides the energy to over- major pressures. design, pressures can cause discomfort come friction and other losses. Air leak- and create infection control and indoor age transfers heat, water vapor, smoke, Air Leakage From Wind Pressure air quality problems. odors, dust, and other pollutants, either Although peak pressures are important In heating climates, exfiltrating air car- from outdoors into the building or from for structural calculations, mean values are ries with it water vapor and lost energy. sources within the building. more appropriate for computing infiltra- The water vapor condenses and causes Airflow characteristics vary according tion rates. Time-averaged surface pressures many problems, from wetting to bacte- to the size and shape of the opening. Long are proportional to the wind velocity given rial growth and deterioration of the build- paths with small cross-sections may ex- by Bernoulli’s equation, as follows: ing envelope. hibit laminar flow and have resistance 2 ρ U (2) In cooling climates, water vapor is proportional to velocity. Larger holes p = a H v 2 carried in with the infiltrating air, caus- may act like orifices with resistance to g where ing condensation, mold, and bacterial flow varying with the square of velocity. p = surface pressure, lb/ft2 growth. Moisture-laden air can travel Usually, different kinds of openings con- v within interstitial spaces due to nega- tribute to the total leakage. It is not prac- About the Author tive pressures inside those spaces. The tical to identify, measure, or calculate Wagdy Anis, AIA, is a principal and director of moisture can condense in strange each individually. Overall flow rates for Technical Resources at Shepley Bulfinch Richardson places like interior walls and ceilings the aggregate of openings take the form: and Abbott, Boston. ASHRAE Journal|December 2001 31 ∆ Pressure Figure 1: Airflow Difference rate is determined by the size of the hole and the pres- Flow Rate (cfm) sure difference. Area of Opening Figure 2: Stack effect. UH = approach wind speed at up- wind wall height H, ft/s ρ 3 a = outdoor air density, lb/ft g = gravitational acceleration, ft/s2 The difference between pressure on the envelope surface and local outdoor atmospheric pressure at the same level in an undisturbed wind approaching the building is given by Wind Pressure Heating Climate Stack Pressure = ⋅ ps C p pv (3) where 2 ps = difference in pressure, lb/ft Cp = local wind pressure coefficient (depends on ter- rain). Of the three major air pressures, wind is usually the greatest. If the wind hits the building broadside, air infiltrates on the windward side and exfiltrates on the other three sides and through the roof. If the wind hits at an angle, it positively pressurizes two sides, and air exfiltrates on the two leeward sides and through the roof. HVAC Fan Pressure Combined Effect Infiltrating air is unconditioned for temperature and mois- Figure 3: Combined wind, stack and fan pressure. ture content and can contain pollutants. It causes discomfort Absolute air pressure changes more rapidly with height out- and can cause imbalances in spaces such as patient isolation doors than indoors, causing the pressure differential. Mechani- rooms, protected environment rooms, or chemical storage ar- cal and elevator shafts, stairs, and atria connect the bottom of eas that are designed for controlled pressure, thus compromis- the building to the top and transfer the stack pressure at each ing pollutant control. floor to the envelope. Figure 2 illustrates stack effect. Chapters 16 and 26 of the 2001 ASHRAE Handbook—Fun- The taller the building, the higher the stack pressure. In damentals quantify wind pressures on buildings and their ef- heating climates, air exfiltrates at the top and is replaced by air fects on mechanical systems. infiltrating at the bottom of the building. In cooling climates, the opposite occurs. Cold air coming in can cause discomfort Stack Effect at the lower floors and overpower the heating system. In a 40- Have you ever approached the front door of a building in story building, with a 13 ft (4 m) story height, temperatures of winter and found the door really difficult to pull open, and 20°F (–7°C) outdoors and 70°F (21°C) indoors, and assuming then it opened with a “whoosh?” This condition is due to stack openings of equal area at the top and bottom of the building, effect. Stack effect or “chimney effect” in buildings is caused the infiltration stack pressure at street level (and exfiltration by the difference in weight of the column of conditioned air at the top of the building) is 0.4 in. w.g. (100 Pa). inside the building versus the air outside the building. This The design of garage exhaust systems needs to account for difference in weight creates a pressure difference across the stack pressure in the building above. Stack pressure can ex- building envelope. ceed exhaust fan static pressure. If it does, a garage under a 32 December 2001|ASHRAE Journal Mean Normalized Leakage Rate building can be a source of pollutants that are sucked into the 2 Building Type cfm/ft at 0.3 in. w.g. building. A good passive solution to the problem would be to 2 (No. in Sample) (L/s·m at 75 Pa) use vestibules and door weather stripping to isolate and com- Type 1 Data Type 2 Data Type 3 Data partmentalize the garage from the building above at stairs, Multi Unit Residence Buildings elevator shafts, and other physical connections that allow this Canada (12) 0.628 (3.19) stack pressure to connect the building to the garage. Compart- Canada (3) 0.787 (4.00) mentalizing the building horizontally into individual stories Canada (6) 0.636 (3.23) or small groups of a few stories and disconnecting them from Office Buildings the shafts fools the building into acting like a stack of single- Canada (8) 0.488 (2.48) story or low-rise buildings, thereby reducing stack effect. U.S. (7) 1.163 (5.91) Great Britain (12) 1.486 (7.55) Outside air and exhaust fans in multistory apartment build- Great Britain (13) 1.313 (6.67) ings often are not selected to account for stack pressure. Multi- Schools story systems with vertical ducts or shafts, such as bathroom Canada (11) 0.291 (1.48) exhaust systems in hotels, have been known to fail miserably, U.S. (14) 0.480 (2.44) with the bathroom exhaust from lower floors being sucked Commercial into the bathrooms at the upper stories of the building. If pas- Canada (8) 0.266 (1.35) sive compartmentalization is not part of the design, individual U.S. (68) 1.217 (6.18) systems that serve each floor might be necessary to overcome Canada (10) 2.746 (13.95) Industrial stack effect. Controls that include pressure sensors and vari- Great Britain (5) 1.368 (6.95) able speed drives then can adjust fan speed to account for Great Britain (2) 4.433 (22.52) changing pressures on the envelope. Supply systems with ver- Sweden (9) 0.285 (1.45) tical duct distribution shafts have to work against varying pres- Institutional sures during the year. There seems to be no easy solution that Canada (2) 0.169 (0.86) allows the original balancing to work reliably at all times. Type 1 Data: Test performed on whole building; total envelope area (includ- Have you ever noticed how some brick buildings will turn ing below grade) used to calculate NLR at 0.3 in. w.g. (75 Pa) whitish with efflorescence at the top parapet? That is due to air Type 2 Data: Test performed on whole building using different test pres- escaping at the juncture of the top floor’s walls and roof. Mois- sures; Data converted to NLR at 0.3 in. w.g. (75 Pa). ture in the exfiltrating air condenses on the back or within the Type 3 Data: Test performed on individual floors or suites; Exterior wall brick, dissolving salts from the masonry.
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