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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BY ANDREA BAIRD & SARAH FLOCK A healthcare facility must serve many functions. Its design must address patient care and provide a conditioned interior separated from the ex- terior environment, all while maintaining durability and aesthetics. To further compli- cate matters, the spaces must remain fl exible to accom- modate expansion, changing technology, remodeling or post-acquisition modifi ca- tions. However, what often is overlooked during these transitions is the impact of in- terior changes on the exterior building enclosure. Healthcare facilities are unique in that one building houses multiple microclimates with dif er- ent temperatures, relative humidity, air changes and/or pressure relationships. All of these varying conditions can impact the thermal and mois- ture behavior of the building enclosure. Interior temperature and relative humidity play important roles in occupant comfort and care. The tem- perature and relative humid- ity of healthcare facilities are maintained at higher levels than most other building types. Historically, operating temperatures and relative humidity were controlled to reduce electrostatic discharge, relieve respiratory issues and enhance patient comfort. Air temperature, relative humidity are interdependent When the temperature is reduced, the relative humid- ity increases with a constant amount of moisture in the air. If a communication path between two spaces exists, air and heat can move due to dif- ferential pressure, whereas va- por can migrate due to moving air or as a result of dif erential vapor pressure. Therefore, design and construction for adjacent rooms and spaces operating under dif erent en- vironmental conditions need special attention. If a mass of air with a given temperature and moisture content reaches a surface with a temperature below dew point, water will condense on the surface (Figure 1). With an elevated relative humidity comes a higher dew point and a greater risk of condensation. Since healthcare facilities operate at comparatively high humidity, a cold surface is more likely to be below the dew point. To illustrate, critical and intensive care conditions, as set forth in the ANSI/ASHRAE/ASHE Standard 170-2013 Ventilation of Healthcare Facilities (Standard 170), are between 70-75 F and 30-60 percent relative humidity. At 72 F and 30 percent, the dew point is approximately 39 F; however, at 72 F and 40 percent, the dew point is 46 F. Surfaces will fall below 46 F more frequently and under less severe conditions than below 39 F, which creates a greater potential for condensation and moisture accumulation. In an ef ort to reduce issues that may be associated with elevated relative humidity, ASHRAE has recently reduced recommended operating parameters to a 20 percent minimum in some short-term patient care spaces. However, this reduction cannot be used in all spaces, nor is lowering to 20 percent relative humidity enough to prevent condensa- tion in all instances. Dif erential interior ambi- ent air conditions create the potential for air and moisture migration, either between spaces or from interior to exte- rior, as dif erent environments attempt to reach equilibrium (Figure 2). Consider condensation potential during design If condensation potential is overlooked during planning, design or construction, mois- ture may reach unintended locations with temperatures below the dew point, resulting in potential condensation- related damage, contamina- tion and/or degradation. Condensation potential should be considered during design of new facilities, as well as altera- tions or renovations. While cooler temperatures might not have been a concern when a space was storage, of ces or short-term patient Window: Kuhar/Dreamstime.com INHIBITING CONDENSATION Vapor Migration Understanding communication between spaces, environment and people In an ef ort to reduce issues that may be associated with elevated relative humidity, ASHRAE has recently reduced recommended operating parameters to a 20 percent minimum in some short-term patient care spaces. 46 Medical Construction & Design | JA N UA RY/ F EBRUA RY 2016 | MCDM AG.COM Figure 1: Water will condense on a surface if a mass of air with a given temperature and moisture content reaches a surface with a temperature below dew point. Figure 2: Vapor migration based on temperature.

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