The hypothesis proposed during the schematic design of the Levine Hall, IAST Phase II project for the School of Engineering and Applied Science at the University of Pennsylvania was the following:
Use a highly energy efficient facade resulting in better indoor comfort with less consumption of energy and lower maintenance costs than standard, commercial curtainwalls.
KieranTimberlake Associates LLP
James Timberlake, FAIA
Richard Maimon, AIA
Yves Gauthier
The hypothesis proposed during the schematic design of the Levine Hall, IAST Phase II project for the School of Engineering and Applied Science at the University of Pennsylvania was the following:
Use a highly energy efficient facade resulting in better indoor comfort with less consumption of energy and lower maintenance costs than standard, commercial curtainwalls.
Approach
We began a comprehensive analysis of the wall, its components and available manufacturers. This was to ensure a proper and broad review of available construction methods, quality and types of curtainwalls. We contacted eighteen manufacturers of glass curtainwalls and researched both basic and specific criteria on each. Our original matrix of types analysis is appended. The analysis of the eighteen manufacturers is also appended.
Independent Review
We contacted and retained an independent consultant, Donald Prowler, FAIA. Mr. Prowler is a well known architect and an expert in energy, environmental and sustainable design issues. First and foremost our concerns had to do with comfort and meeting the energy codes related to a structure of this program and use. As nearly all of the design approval hurdles had been achieved related to the use of a glass curtainwall on the University main campus, we had some assurance that the general type of wall preferred - glass, curtainwall - had met with acceptance on a broad consensus basis. Mr. Prowler's assessment of our design was that opportunities could be had in employing a full glass wall, such as reduction in lighting and heating offsetting the general and common opinion of glass curtainwall inefficiencies and increased energy consumption. We also retained Ove Arup & Partners (USA), a preeminent engineering and design firm with substantial expertise in this field.
Components
The key components of an active, pressure-equalized, double-skin curtainwall are:
The external double glazed glass unit
The internal single glazed unit
An intermediate blind located in the cavity formed by the external and internal glass unit.
A continuous cavity air flow supplied by room air. The air has an inlet at the base and outlet at the head of the glazing frame.
Operation
An active, pressure-equalized, double-skin curtainwall works by employing the following characteristics:
The inner and outer glass units form an air plenum through which room return air is circulated and is extracted via the building's HVAC system.
The intermediate blind reflects some of the solar radiation incident on the facade back through the external glass unit and absorbs the remaining radiation and re-radiates it back as heat into the cavity. The heat is then extracted via the continuous air flow between the inner and outer glass units.
The inner glass unit is a barrier to heat and prevents it from entering the room to contribute to any increase in the cooling loads. (Glass is a barrier to low frequency radiation i.e heat, but is transparent to high frequency radiation i.e. visible). The blinds are fully adjustable allowing for full shading or visibility. They are unobtrusive to the external appearance of the building and require minimal maintenance.
Since the primary form of solar heat control is achieved by ventilating the absorbed heat in the cavity, the glass units can be clear (there is no need to use reflective or heat absorbing glass which alter perception of the exterior surroundings).
Since the room return air is at room temperature, the internal glass unit surface temperature is within 1- 2 degrees of room temperature. This eliminates surface radiation effects and improves occupant comfort.
The extracted solar heat gain can be used in various forms of heat exchange systems to offset heating and cooling loads.
Analysis
An active wall system employs a double (glass) wall with an interior mechanical vented cavity and internal blinds. Objective evaluation criteria might be summarized as follows:
Energy Code
Performance/Energy consumption
Availability
Constructability/Time
Maintenance
Cost
The subjective criteria might be summarized as follows:
Comfort
Value
Appearance
Summary of Findings and Recommendation
The summary of findings and our recommendations are organized by utilizing the objective and subjective criteria outlined in Analysis.
Energy Code
Based upon our analysis of the findings from Arup it is our professional opinion that the system designed will meet and exceed the ASHRAE 90.1 code, a more stringent energy code which the City of Philadelphia has adopted (from Act 222 and 90.a). A simple DGU (double glazed unit) wall, with or without interior blinds will not meet this code.
Performance/Energy consumption
The analysis of the performance of the wall and energy consumption is inconclusive at this time. However, based upon preliminary analysis of manufacturer's information, a vented 'active' wall will perform better and provide some energy reduction, particularly on the heating side cycle, from a standard DGU wall.
Comfort
Arup's analysis is conclusive that an 'active' system, with blinds, and without the added calculation of our interior manual shades, results in assuring greater comfort within perimeter rooms thereby addressing the concerns of the potential users.
Any glass wall has the potential of reducing the artificial light loads within the building allowing those in perimeter settings to work without turning on lights.
Availability
Two approaches are available to us: (1) Specify through performance standards the wall which may result in a custom, untested aggregation of individual parts, or (2) provide a manufacturer's 'standard' system which is readily available. It is our understanding that only one manufacturer has a tested and applied 'active' system available. All other manufacturers would have to provide us a 'built-up' system based upon a performance specification.
Constructability/Time
All curtainwalls have a custom component to them in that they must meet custom floor-to-floor heights, lengths, etc.. The tested and available 'active' system through Permasteelisa is as readily available as a comparable DGU wall. Going directly to a manufacturer (through proprietary specification) has advantages in terms of time relative to initiating the supply/ subcontract for this major system a the same time as the construction contract thereby ensuring delivery on time. The proprietary method also allows for the supplier/subcontractor to engineer the wall relative to the desired performance and building characteristics.
Maintenance
In comparison, an 'active' wall has marginally greater maintenance issues such as maintenance of the interior cavity and blinds (cleaning once a year).
Cost
There is no question that an 'active' system, due to the addition of the added layer, interior blinds and unitized system is at a premium to a comparable DGU wall. This premium has been identified to be in the range of 25- 35% over the comparable DGU wall.
Value
The increased premium may be seen as being offset by the increased comfort provided. More directly, the premium may be seen as being amortized over time by reduced energy costs - reflected in lower light levels, reduced heating cycle costs, and efficient cooling.
Appearance
From an appearance perspective there is little difference in a DGU versus an 'active' wall particularly if each employs shades or blinds. Qualitatively there are differences in curtainwall manufacturers, their products, shapes, quality of installation, follow-up and warranty.
Links
http://www.guardian.com/
http://www.permasteelisa.com/
http://www.ykkap.com/
Further Reading
'Using Multiple Glass Skins to Clad Buildings,' Werner Lang and Thomas Herzog, ARCHITECTURAL RECORD
© 1997– KieranTimberlake Associates LLP
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