Medical Construction & Design

JAN-FEB 2016

Medical Construction & Design (MCD) is the industry's leading source for news and information and reaches all disciplines involved in the healthcare construction and design process.

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care areas with low relative humidity, it may become a condensation problem when converted to an intensive care unit or other occupancy spaces with more demanding ambient conditions. Thermal bridging Thermal bridging can also reduce temperatures be- low dew point. The average uninterrupted R-value of the insulation in the building enclosure is often considered when evaluating the thermal ef ciency of the assembly. However, conductive materi- als that bypass or penetrate the insulation plane, such as fl ashings, metal studs, attach- ments, etc., provide a path for heat loss and may result in temperatures below the dew point (Figure 3). Thermal bridging and the ef ect on temperatures should be carefully considered, especially since adverse ef- fects might be concealed from interior view, and the resulting damage may not be noticed until conditions are extreme. Similarly, components of the building enclosure assembly that are shielded or isolated from interior sources of heat may drop below the dew point. Changing equipment, and/ or applying certain fi nishes or furnishings on the interior face of exterior walls, may inadvertently alter moisture migration and accumulation in the building enclosure and adversely af ect performance. Air leakage Air leakage between spaces is also an important consid- eration for both interior and building enclosure perfor- mance. Air movement is ca- pable of transporting a much larger mass of moisture than dif usion through materials, resulting in condensation at temperatures below dew point. Pressure dif erentials, which move air from space to space, can result both from mechanical equipment or temperature dif erences known as a "stack ef ect." Air changes and pressure relationships for the various spaces in a healthcare facil- ity are specifi ed in ASHRAE Standard 170. Healthcare facilities need positive pres- surization to prevent contami- nants from entering (oper- ating rooms) and negative pressure to preclude exfi ltra- tion (laboratory). The ef ect of dif erential pressure on building performance should be considered as a potential contributor to energy loss and moisture damage. Air moves from high to low pressure to approach equilibrium. If interior spaces at varying air pressures are adjacent and communicate through ducts, gaps, interstitial spaces, cavi- ties or stud spaces, then un- intended air transfer between and around spaces can occur. Moving humidifi ed air can transport a large mass of mois- ture, potentially distributing moisture from humidifi ed areas to areas with surfaces below the dew point, resulting in condensation. To address this, an air barrier system can be installed. However, since air can move amid internal spaces and between the interior and exterior, buildings should be airtight between all areas of dif- fering environments, including exterior to interior and hori- zontally and vertically (Figure 4). The pressure dif erentials will also impact the location of the air barrier within the as- sembly (Figure 5). Improving compliance Monitoring pressure dif eren- tials can be tricky in health- care facilities. Recently, The Joint Commission initiated ef orts to improve compliance with pressurization standards since numerous hospitals fail to meet that recommended criterion. It is also vital that owners, designers and con- structors communicate and understand existing pressure relationships prior to expan- sion or retrofi t. This informa- tion can be used to identify how changes may af ect future performance or if additional air control is needed. Achieving proper enclosure performance requires a clear understanding of the exist- ing and proposed changes to interior environments. Control of pressure, temperature and humidity in healthcare facilities is critical for building envelope performance, patient comfort, infection control and equipment operations. Even minor changes should be evaluated for potential impact, including fi nish and equip- ment modifi cations. Andrea Baird, NCARB, P.E., LEED AP BD+C, is an architect with Raths, Raths & Johnson. She can be reached at albaird@rrj.com. Sarah Flock, NCARB, is associate and consult- ing architect with Raths, Raths & Johnson, Inc. She can be reached at skfl ock@rrj.com. Thermal Bridging Air Infiltration & Prevention Figure 4, above: Differential pressures within a building. Figure 5: Air barrier between interior spaces. Air barrier Metal studs 51 F 69 F Brick ties Thermal model of brick wall assembly without thermal bridges Thermal model of brick wall assembly with thermal bridges MCDM AG.COM | JA N UA RY/ F EBRUA RY 2016 | Medical Construction & Design 47 Figure 3: Conductive materials that bypass or penetrate the insulation plane, such as fl ashings, metal studs, attachments, etc., provide a path for heat loss and may result in temperatures below the dew point.

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