Description
Most libraries are designed for public access and house primarily modern books and media collections that are not irreplaceable from a historical point of view. Hence, climate control is more concerned with human comfort aspects during hours of operation than maintaining an absolutely stable environment, as required for purely archival purposes. Nevertheless, many libraries are also guardians of rare documents and unique materials that demand a different level of safekeeping with respect to temperature, relative humidity, air quality, and control of light, often requiring the installation of separate systems for collection and non-collection spaces. In other words, climate control always needs to reconcile the requirements of user comfort on the one hand and conservation issues on the other hand.
Methods to achieve indoor thermal comfort and relevant conditions for artifacts in a public access library vary between climate zones. Human thermal comfort standards, prevailing microclimate, energy codes and standards, environmental issues, available energy sources, and sustainability concerns are all parameters shaping the system design with options ranging from mechanical, to passive and hybrid heating, ventilation and air conditioning concepts. Yet, the system approach is primarily determined by the type of library, its content and program, by operational aspects, the library’s size in terms of volumes, area and number of users. Interdisciplinary creativity, dialogue and sharing of information are crucial in achieving a synergetic interface between architecture, structure, and building systems.
The effectiveness and success of natural ventilation and cooling, and other alternative energy concepts, entails the owner’s commitment to understand not only the operative system aspects based on the seasonal, daily temperature differentials and hours of operation, but also the basic thermal and air quality parameters to provide occupant comfort and appropriate collection conditions. This is in particular true for systems that are not entirely automated, but are either semi-automatically, or even manually activated.
Cutaneous thermoreceptors are instrumental in regulating body temperature, and consequently are a critical determinant for human thermal comfort, a state dependent on variables reaching from non-quantitative individual differences to measurable non-environmental personal factors, such as activity levels expressed as metabolic rates and the insulating value of clothing to quantifiable environmental factors, including air temperature, thermal radiation, humidity and air speed. There is also a cultural factor at work: what may be perceived as comfortable in one part of the world, might be gauged as too hot or too cold elsewhere. To a modest degree, the boundaries of thermal comfort zones are elastic; modifications of the variables, for example, increasing the air speed, intensifies, in turn, evaporation coolness of the skin, thus expanding the acceptable temperature threshold, or contrary, a higher clothing insulation value is effective in countering a lower temperature setting.
Thermal comfort zones are defined in the context of energy codes and standards specific to varying energy strategies, reaching from entirely mechanically controlled buildings to fully free-running (i.e. not heated or cooled) buildings, taking advantage of beneficial climate conditions promoting the employment of passive thermodynamic processes for heating and cooling. In principle, there are two models for determining thermal comfort: 1. the static comfort model relevant for buildings equipped with mechanical heating ventilation and air conditioning (HVAC) systems that ensure constant indoor temperatures all year round and 2. an adaptive model for determining acceptable thermal conditions in occupant-controlled naturally conditioned spaces, primarily through the operation of windows only supported by mechanical ventilation using unconditioned outside air. Finally, there are mixed-mode buildings (often in use in Europe’s temperate climate zones) where the refrigeration equipment for cooling is only temporarily activated.[1]
Recommended inside design conditions for HVAC-controlled public, non-archival libraries are typically 20–22o Celsius, 40–55 % relative humidity with air movement at 0.13 m/s and 8–12 air changes per hour. However, certain climate zones will achieve appropriate comfort levels and conditions just by providing for heating and natural ventilation, while other zones will require only cooling and dehumidification, or the seasonal or year-round operation of full-scope heating, ventilation and air conditioning systems. In non-archival libraries, HVAC systems may operate only during opening hours, or cycle on/off with thermal satisfaction alone. Archival libraries and rare book collections, discussed later in this chapter, require strict climate control without relative humidity fluctuation and temperature set points relevant to the collection material. In general, uncontrolled high humidity levels and lack of ventilation facilitate mold growth (65 % relative humidity is considered borderline), a devastating threat to book collections. Systems operating within the above-listed boundaries provide not only thermal comfort for the occupants, but also reasonable protection for the contents stored and displayed in a non-archival library.
In the context of energy conservation and sustainability, the acceptance of climate parameters and meteorological variables, as a contributing form-giving force and the basis for a project’s energy program, are of critical importance. Building envelopes, conceptually reaching from sealed to semi-permeable and from transparent to opaque, as well as the form factor, the ratio of the envelope’s surface area to volume, are by far the most influential parameters impacting energy consumption. Solar and wind exposure, thermal transmittance and air leakage rates of envelope components, such as roofs, walls and fenestration, the ratio between opaque and transparent areas, the intensity of insulation and choice of construction materials, paired with the internal heat emission generated by occupants and equipment, determine the overall load profile of a building. In addition, changes in use patterns and occupancy numbers during operating hours, variations in the operating schedule as such, climatic shifts relative to time of day and season trigger significant load fluctuations to be considered and analyzed for the design, sizing and spatial organization of mechanical equipment.
Energy standards inform design requirements for building envelopes and HVAC equipment; paired with local meteorological records and site-specific air quality data, a comprehensive climate profile for a particular location evolves, offering fundamental information for the conceptual approach to indoor climate control, potential energy conservation, bioclimatic design and eligibility for green building certification.
Ventilation, the dynamic process of airflow within a building, is not only critical for achieving thermal comfort by controlling temperature and humidity levels, but also for assuring acceptable indoor air quality through the dilution of pollutants (grouped in odors and irritants) by both introducing outdoor supply air and exhausting stale air. In libraries, air pollutants are not only a health threat to the occupants but hazardous for rare book collections and archival material as well. Standards define acceptable indoor air quality, determining air change rates per hour (particularly critical for the protection of archival material), and quantity of ventilation air per occupant measured in liter per second, and prescribe compliance methods based on system design, reaching from mechanically to naturally conditioned buildings. A recommended value for libraries is 8.5 l/s per person, and the occupant rate for a reading room should not exceed 20 occupants per 100 m².
Reading rooms, closed and open-stack areas, archival storage, rare book collections, study spaces for individual and group work, multi-media labs, assembly spaces, internal data centers, administrative offices, food service and technical library support areas all require a specific system response to ventilation needs and thermal comfort based on project-specific parameters, such as occupant load, floor plate dimensions, space configuration, volume, location and adjacencies. For example, deep floor plates, often typical for libraries accommodating large stack areas, require a system approach capable of responding to skin and core loads. Stack lighting and user heat emission produce internal thermal loads that demand cooling, while the perimeter zones, depending on season and solar orientation, require either heating or cooling.
Construction systems, in-situ concrete, for example, offering thermal mass storage capacity, mostly control system design and the integration of ventilation distribution. Climate parameters, however, are the most influential force in the design of ventilation systems. Moderate outdoor temperatures, substantial temperature swings between day and night time or notable temperature differentials between sun-exposed and shaded building surfaces, prevailing wind patterns and climate conditions characterized by low relative humidity increase the potential for bioclimatic concepts, such as free cooling, night cooling (night ventilation of thermal mass), passive cross ventilation and evaporation cooling (assuming control of contaminants). Arriving at a system design that conserves energy, while satisfying ventilation needs and achieving thermal comfort, requires a holistic approach and integrated process.
In buildings equipped with complete HVAC systems or just mechanical ventilation, the rate of outdoor air ventilation and the degree to which the airstream is filtered prior to space infusion determines indoor air quality. Primary parameters are the outdoor air quality, the physical boundaries of the zone to be ventilated, the number of people occupying the zone, and the effectiveness of the air distribution system depending on type – both mixing or dilution ventilation systems and displacement ventilation systems are relevant for library installation.
Demand ventilation, based on carbon dioxide monitoring, allows substantial energy savings due to the reduction of conditioned supply air to zones that are not frequently used or fully occupied, such as assembly spaces, large conference rooms, semi-public stack areas or archival storage rooms. The sensors of the Seattle Central Library (OMA, 2004) are linked to the building’s energy management system, modulating air intake dampers and air distribution volume to curb indoor CO2 concentration to below 530 parts per million (by volume) or 0.053 %, and consequently increasing human comfort.[2] However, low CO2 concentrations are only one aspect in achieving high indoor air quality; eliminating volatile organic compounds (VOCs) from the building process, isolating ozone-producing printers and reprographic machines from the library environment, using green housekeeping products, and the commissioning, monitoring and maintaining of systems are of utmost importance as well.
Maintaining balanced temperature and humidity levels on open and often intercommunicating floors is of great complexity and requires automated operation. User control may be exerted in staff work areas, small meeting rooms, administrative offices and certain auxiliary spaces. There, operable windows for natural ventilation, individually controlled shading devices and thermostats may increase personal comfort.
The tendency in modern ventilation and air conditioning engineering for libraries leans towards the use of dedicated outdoor air systems, DOAS, that handle all latent loads (and part of the sensible loads due to preconditioning of the outside air with energy recovered from the return air stream ) and parallel systems comprised of optional all-air variable air volume systems, fan coil units, unitary water-sourced heat pumps or chilled beams satisfying the sensible load requirements specifically demanded by a particular use and space. First costs for DOAS tend to be higher in comparison to conventional all-air HVAC systems. However, the long-term energy savings, assuming total energy recovery, are substantial, and full compliance with indoor air quality and thermal comfort standards is achieved. With the elimination of condensation formation on parallel equipment, the risk for microbial contamination is greatly reduced, making the concept ideal for archival libraries. A prerequisite is the design of a sealed building envelope, minimizing uncontrolled outdoor air infiltration.
Libraries located in moderate climates, described by low average temperatures, have the potential of operating in reduced or free cooling mode during most of the year, or at least during the swing seasons, by supplying the building directly with filtered, but unconditioned, outdoor air, as is the case for the Seattle Central Library designed by Rem Koolhaas. In Seattle, maximum average outdoor temperatures below 21o Celsius for most of the year allow the library to operate for extended periods in free or reduced cooling mode.
Utilizing the thermal storage capacity of concrete slabs for cooling or heating is known as concrete core activation, a concept employed inside the extensive stack areas of the Jacob-und-Wilhelm-Grimm-Zentrum in Berlin (Max Dudler, 2009). There, 14o Celsius outdoor air circulates through a grid of 10 cm diameter aluminum tubes embedded into the neutral zone of the slabs, cooling the concrete floor. In absorbing the excess internal heat, the temperature in the stack area is lowered, while the temperature of the circulating air is raised to 21o Celsius, a suitable temperature for reintroduction into the space, satisfying ventilation needs.
Concrete core activation utilizes the thermal storage capacity of concrete slabs for cooling or heating. Aluminum tubes are embedded into the neutral zone of the concrete slab.
For concrete core activation 10 cm diameter aluminum tubes with internal radial fins for optimized heat absorption are used.
A double-skin facade, as developed for the Sendai Mediatheque in Japan (Toyo Ito, 2001), creates a ventilation-controlled buffer zone between the outside environment and the building’s interior climate. In winter mode, with vents closed, the static air volume detained between the single-glazed outer skin and the insulated inner glazing system heats up and creates an effective thermal barrier between the exterior and the interior environment, thus enhancing the comfort zone along the inner curtain wall; in summer mode, openings at the bottom and the top of the cavity (the space between the two glass skins) initiate stack ventilation, consequently reducing the thermal load inside the cavity and the thermal load of the adjacent library floor, respectively. Under low humidity conditions, the stack effect can support passive cooling by forcing cooler air from the sun-opposed building side across the floor plate. A dot screen etched into the exterior glass panels reduces sun infiltration and glare.
Sendai Mediatheque, Toyo Ito, 2001. In winter mode, the very narrow, south-facing double-skin facade creates a thermal buffer zone between the exterior and interior glass curtain walls.
Computerized adjustable louvers, expandable and retractable shading devices, fixed canopies and lightshelves, treatment of glass surfaces, and the installation of specialty solar control glass are available measures in managing interior thermal loads and ventilation needs generated by expansive glass surfaces. Finding a balance between desirable visual indoor-outdoor connections, daylight optimization, and solar gain, while preventing overheating, excessive glare, and damage to the collection from overexposure to ultraviolet light requires an interdisciplinary approach. To minimize solar gains inside the glass enclosure of the Seattle Central Library, a triple-layered, krypton-filled glass pane, with a diamond-shaped expanded aluminum metal mesh suspended between glass layers, covers half of the envelope.
The Philological Library at the Freie Universität in Berlin (Foster + Partners, 2005), offers a unique approach to ventilation. A tubular space frame in the shape of a prolate spheroid, clad on the exterior with alternating rectangular aluminum and glass panels, and on the inside with a translucent white fiberglass membrane, generates a double shell with an interstitial space utilized for ventilation. Similar to the concept of a vertical double-skin facade, computer-operated ventilation panels at the base and at the crest initiate or block airflow, thus regulating the temperature between the two membranes and the indoor climate respectively. Inside the dome, a structurally independent, multi-story concrete structure accommodates stacks and workspaces. Concrete core activated slabs support space heating and cooling.
Philological Library, Freie Universität, Berlin, Foster + Partners, 2005. The envelope section shows the interstitial space between interior and exterior membrane that is utilized for ventilation.
A conventional mixing ventilation system supplies air at high velocity to rooms through overhead diffusers, inducing the existing air to mix and to achieve temperature equalization and uniform contaminant concentration. Exhausted air is returned through ceiling grilles or ceiling plena. Displacement ventilation systems, on the other hand, deliver air at low velocity, typically near floor level, into the space. Superior indoor air quality, containing high levels of outside air, is achieved by the unidirectional transport of conditioned supply air through buoyancy forces generated by heat sources – people, computers, lighting – right to the breathing zone of occupants. Contaminants are not diluted, but carried above the breathing zone by the rising air.
In cooling mode, supply air temperature in displacement ventilation systems is about 5.5o Celsius higher than required for conventional ceiling-supplied mixing systems, that is roughly 18o versus 13o Celsius. Energy efficiency is achieved by reduced conditioning needs and by being able to use outside air directly for more hours of the day. In employing a variable air volume system, the supply airflow can be modulated to allow for demand ventilation. Manipulating supply airflow and supply air temperature, or both, controls the temperature of the ventilated space.
The described buoyancy effect causes the stratification of air masses, permitting the creation of clean thermal comfort zones within 3 m above occupied floor levels, while air layers above this datum operate at temperatures and contamination levels outside the human comfort range, as explicitly reflected in the temperature profile of the University of Zurich Law Faculty Library (Santiago Calatrava, 2004). Stratification avoids the need to condition the entire air volume of large open spaces, such as the atrium in the Law Faculty Library, the central reading room in the Jacob-und-Wilhelm-Grimm-Zentrum, Berlin, or the large glass envelope of the Seattle Central Library, for human comfort, thus aligning architectural concept with energy conservation goals.
During spring and fall, based on temperature differentials between inside and outside air and actual weather data, computer-automated vents open at the bottom and top of the atrium, initiating a flow of unconditioned air through the core of the multi-story Law Faculty library. In winter mode, with all envelope vents closed, the building operates in mechanical heating mode, supplying conditioned air to the multiple levels of the atrium. In summer mode, with outside temperatures above 25o Celsius, linear automated vents along the base and ridge of the cupola, with a combined opening area of 14 m², provide cross ventilation, reducing heat build-up caused by solar gain, thermal load of occupants, lights and equipment.
During the summer months, conditioned air is supplied to the balconies through diffusers located in the base of the wall-encompassing shelving system to achieve acceptable thermal comfort. Vertical ground-source heat pumps are located around the perimeter of the building; with the earth acting as a heat sink in cooling mode and a heat source for preheating supply air in winter mode, the need for a cooling tower and chiller is eliminated.
Law Library, Universität Zürich, Santiago Calatrava, 2004. Inside the elliptical atrium, the fluid motion analysis shows stratified air masses, with temperatures of above 30o inside the glass cupola and 26o Celsius at the entry level. This stratification avoids the need to condition the entire air volume of large open spaces.
Unlike ceiling-supplied mixing systems, displacement ventilation systems are less suited for projects requiring extended periods of heating. Consequently, in the case of the Jacob-und-Wilhelm-Grimm-Zentrum, the displacement system is limited to the central reading room. Both the 92 horizontal skylight units, glazed with highly insulated solar control glass and equipped with automated horizontal perforated shades, and the deep circumferential stack wings define the thermal envelope of the introverted space. Insulated vertically from exterior temperature conditions and protected horizontally against extreme passive solar gain, the central reading room operates predominately in free cooling mode. However, under heating demand, the location of the supply air grilles above the reading tables generates an airflow pattern comparable to a mixing system. For indoor air quality, the stacks are equipped with a conventional mixing ventilation system, with diffusers integrated into the canopy of the shelving units.
Jacob-und-Wilhelm-Grimm-Zentrum, Berlin, Max Dudler, 2009. Partial cross section of skylight units depicting the automated horizontal perforated shading devices.
Jacob-und-Wilhelm-Grimm-Zentrum. The roof of the central reading room consists of 92 horizontal skylight units. Highly insulated solar control glazing protects the space against passive solar gain while reducing glare on the interior.
Library-integrated data centers, in particular, but also multi-media and computer labs, and reprographic centers, often located in the core of the building, generate isolated, sizeable thermal loads that require cooling. To this end, ceiling-integrated, active, chilled beams tend to be a very effective application. Dehumidified, preconditioned supply air from a dedicated outdoor air system is forced through nozzles in the beam, inducing existing air from the space to pass through the cooling coil, delivering cool air to the space. Through grilles at ceiling levels, return air is ducted back to the air handler. By modulating the chilled water flow rate, the system reacts to sensible load changes in the space. Advanced design solutions for chilled beams combine cooling with other building systems, such as fire suppression systems, fire and smoke detectors, lighting and public address systems.
The interface between the building and its mechanical systems is not solely critical for reasons of functionality; rather, the selection and application of systems is often deeply intertwined with the architectural concept and expressed aesthetics. The vertical air distribution system requires strategically located shafts within the plan configuration, and horizontal distribution branches require ceiling plenums, or the construction of raised access floors, impacting the achievable clear floor-to-ceiling height within stories, and consequently, the overall height of the building – a critical cost factor. The placement of supply air intake and return air exhaust is directly related to the building envelope, the building’s height, form and plan, as well as the environmental conditions, orientation and site context. Decentralized air handling systems offer flexibility in terms of building integration, allow reduced duct sizes and shorter runs, and the possibility of addressing specific thermal and air quality demands. Centralized systems, due to their size, often require mechanical penthouses. Cooling equipment usually is centralized or part of self-contained interior and exterior systems with external cooling towers.
In the Jacob-and-Wilhelm-Grimm-Zentrum, the structural column grid is divided into a much narrower grid with vertical, non-structural piers that allow the integration of air distribution risers and other building infrastructural components. Vertical duct runs feed into the pipe manifolds embedded in the concrete slabs for core activation, into the supply diffusers in the reading room, the air outlets along the perimeter of the building, providing thermal comfort at the window worktables, and into the diffusers integrated into the canopies of the stacks. The latter concept, paired with the collection of return air at selected points, eliminates horizontal duct runs or plenum space inside the collection area. Avoiding the need for suspended ceilings optimizes the efficiency of the concrete core activated floor slabs and reduces the overall floor-to-floor height in the stacks.
Jacob-und-Wilhelm-Grimm-Zentrum. Removable panels allow access to the building’s vertical infrastructure located in the non-structural piers. Duct risers inside the piers feed into the concrete core activation system and into supply diffusers. The perforation of the panels contributes to the acoustical quality of the space.
Preservation of special collections, comprised of rare books, unique and often irreplaceable archival documents of different media, and an unlimited variety of historical artifacts and artwork, demands the design of environmental control systems tailored to very particular needs. The obligation to provide public and scholarly access to collections, and the desire to display valuable pieces within the premises of a library, are to a certain degree counterintuitive to the concept of safekeeping and further complicate the organizational and building systems approach to housing and preserving collections. Finding a compromise between the ideal environmental conditions and the realistically achievable technical, operational and economical measures, paired with a collection-specific risk analysis, should be part of the preschematic design phase involving archival librarians, conservators, programmers and building consultants.
A number of threats can affect collections, among them (in decreasing order of seriousness): light, relative humidity, temperature, air pollution, pest infestation. Control of a library’s indoor relative humidity, temperature and air quality is the quintessential prerequisite for the protection of the collection. It minimizes the risk of humidity-related decay processes, the deterioration of collections due to poor air quality, ventilation and the infestation by insects. Vibration-free installation of equipment and ductwork, the routing of pipes outside critical areas and the provision of emergency power supply for mechanical systems will eliminate mechanical risks to the collection caused by shock, vibration and leaking pipes and maintain climate control and system monitoring during power interruptions.
Mold damage in historical file; Book showing damage from insect infestation (“bookworms”)
Air quality standards are achieved by complex filtration systems. The contamination of a system’s internal components, such as cooling coils and supply lines, as well as dust build-up on fans and inside ductwork, is prevented by prefiltration, while fine-particulate filtration and gas-phase filtration provide protection to the collection material itself. It is recommended that archival and specialty collection libraries store chemically stable permanent collections at 50 % relative humidity and a temperature between 15 and 25° Celsius, maintaining the above-mentioned air movement speed of 0.13 m/s and hourly air change rates between 8 and 12. Chemically unstable library and archive collections should be housed in cold store, or cool store, with temperatures and humidity tailored to the specific objects.
Selected threats, at-risk materials, damages and control measures
Footnotes
Buildings in the U.S. are regulated by ANSI/ASHRAE standard 55-2010, “Thermal Environmental Conditions for Human Occupancy”, while European buildings follow standard EN 7730-2005, “Ergonomics of the Thermal Environment”, and EN 1525, “Indoor Environmental Input Parameters for Design and Assessment of Energy Performance of Buildings”.
CO2 levels in outdoor supply air typically range from 300–500 ppm. An outdoor air ventilation rate of 7.5 l/s per person will dilute bioeffluents in a building with sedentary activity patterns to an acceptable degree. Indoor CO2 concentration should not exceed the outdoor concentration by more than 650 ppm and concentrations greater than 5,000 ppm may constitute a health risk.
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Originally published in: Nolan Lushington, Wolfgang Rudorf, Liliane Wong, Libraries: A Design Manual, Birkhäuser, 2016.