Winter 2004

Volume 18, Issue 2 Winter 2004

SEEING CLEAR
Designing for Transparent Facades

by Rick Voelker

The reference to transparent facades is somewhat of a misnomer since glass used in building construction today, as in the past, typically has some visual element. The architect will select the glass carefully on the basis of its transparent rendering for each specific building design.

In addition to the visual aspects of the building, the design professional must also consider the ability of the glass to meet the building occupant’s needs, as well as comply with local energy codes. The challenge for the designer is one of balancing the artistic element of the building design with the practical requirement for solar performance. Quite typically the glass in today’s building appears transparent at first glance, but in actuality may employ a number of enhancements that provide the balance required. These enhancements include energy-efficient coatings, silk-screen patterns and specific glass substrate selection. The seemingly transparent façade actually relies upon several visual features that are carefully selected to achieve this goal.

Trends
The World Trade Center 7 project in New York includes many current design trends. Designed by Skidmore Owings & Merrill of New York, the insulating glass units incorporate low-iron glass, warm-edge air spacers, hybrid low-emissivity (low-E) coatings, silk-screen patterns and gray silicone sealants. In this one project nearly all of the latest glass enhancements have been utilized to achieve the desired effect.

Glass Color
The most obvious aspect to consider when designing for a transparent façade will be the glass substrate color. Clear float glass continues to be the most popular substrate color. Based upon our data, clear glass accounts for approximately 63 percent of all projects we have sampled, but have not yet been constructed. This trend began several years ago and the percentage of clear glass specified has continued to grow each subsequent year. The second most popular glass color is green, representing approximately 16 percent of new projects. In this case, all green glass substrates have been included, even the high solar absorption products. Other glass colors such as gray, bronze and blue continue to be used, but at much lower levels than in previous years. These three color categories represent approximately 5 percent of the new projects sampled. One interesting trend worth noting is the increase in low-iron glass being specified.

Glass Coatings
Metal oxide coatings have been applied to glass for roughly 30 years in order to provide a variety of color and solar-performance characteristics. In the 1970s and early 1980s, reflective coatings were the workhorse of the industry. Metals such as stainless steel and titanium were used commonly. While very mirror-like in appearance, they offered several color and solar performance options. The trend for the 1980s and 1990s has most definitely been low-E coatings.

Low-E coatings have provided the design community with an excellent opportunity to achieve the transparent look. Typically lower in reflectance than even uncoated glass, they account for the vast majority of new project specifications. While generally capable of meeting energy code requirements, low-E coatings may be challenged in the future by more stringent code requirements. Another trend developing now is the popularity of hybrid low-E coatings. These coatings provide high visible light transmission and improved solar performance characteristic when compared to the standard low-E coatings. In addition to providing a new look, hybrid low-E coatings maintain the view through characteristics of the standard low-E products.

Silk-Screen Patterns
Silk-screen patterns applied to glass have also been a popular enhancement that not only provides a signature look to the glass, but actually increases the ability of the glass to keep heat out of the building. These products peaked in the late 1990s, but have suffered a bit in popularity at the turn of the century. They currently are back in good standing with designers due to their ability to provide a visual element to the building while increasing the solar performance. They still offer the most subtle method of achieving the transparent look. Offered in a variety of standard colors as well as custom color options, silk-screened patterns often allow one to custom balance the building for appearance and performance.

Energy Codes and Standards
Energy codes have become a significant consideration when selecting glass for a building project. The energy codes have been with us for many years, however the most current versions of the codes have raised the bar with respect to glass performance. ASHRAE Standard 90.1 Energy Standard for Buildings Except Low Rise is the standard individual states reference in their energy code. It was first published in 1975 and updated in 1980, 1989, 1999 and 2001. It is currently on a three-year cycle for review by ASHRAE and states can adopt revisions as their review cycles permit.

Currently, 15 states have adopted ASHRAE Standard 90.1, 1989, and list it as the reference standard. Another 18 states have adopted and list ASHRAE Standard 90.1, 1999, version as the reference standard. The 1999 version significantly increased the thermal performance requirements of a building wall construction and is responsible for a greater awareness of energy codes by those involved with glass selection.

Five states have already adopted ASHRAE Standard 90.1, 2001. This version had no significant changes in performance requirements, but rather organizational changes to the standard.

Another name to be aware of is the International Energy Conservation Code (IECC). This code is also adopted at the state level and includes provisions for residential building compliance as well as low-rise structures not covered by ASHRAE 90.1. IECC utilizes a simplified compliance method for low-rise commercial buildings based upon climate zones in each state. ASHRAE Standard 90.1, 1999, is the reference standard in chapter seven of the 2001 version of the IECC.

Both IECC and ASHRAE compliance are based upon U-factors and solar heat gain coefficients (SHGC) for the wall of a building. U-factor is the thermal transmittance of the entire wall. This would then include the assembly U-factor, which is comprised of the frame and the glass. The SHGC is the ratio of solar heat gain of the overall fenestration area.

Previously, the glass industry and most design professionals relied upon the shading coefficient (SC) value. To obtain the SHGC from the SC use this formula: SC X 0.86 = SHGC.

ASHRAE Standard 90.1 allows for three methods to determine compliance to the code. The first is the prescriptive method, which lists the maximum U-factor and SHGC for a specific climate zone. The second is the building envelope trade-off option, which allows trading of enhanced efficiency in one building component for decreased efficiency in another.

The third method is the performance approach. This allows for a comparison of the proposed design with a baseline or reference design.
You must demonstrate that the proposed design is at least as energy efficient as the baseline in annual energy usage.

Energy codes will have a significant impact on glass selection and future product development as it relates to coatings.

Green Building Design
It’s difficult to pick up a trade magazine today without reading about green buildings. The concept is easy to understand, but, until recently, a consistent definition of green buildings did not exist. The U.S. Green Building Council (USGBC) is a national, non-profit organization and the administrative authority of the LEED™ Green Building Rating System. The USGBC developed the green building definition and LEED rating system.

There are four levels of LEED certification where points are earned for compliance to the design methodology. These are:
        LEED Certified               26 - 32 points
        Silver Level                     33 - 38 points
        Gold Level                      39 - 51 points
        Platinum Level                 52+ points (69 possible)

The categories for LEED credits include: sustainable sites, water efficiency, energy and atmosphere, materials and resources, indoor environmental quality and a special consideration for the design process. The obvious category from a glass standpoint is the energy and atmosphere section. In this category a prerequisite is compliance to ASHRAE 90.1, 1999.
Additional points may be earned for new building construction when the building façade energy performance exceeds the minimum requirements of ASHRAE 90.1. This can best be accomplished by selecting a glass that has superior solar energy performance (SHGC and U-values).

Conclusions
When considering transparent facades and their design, the glass selection will be a critical element. The desire is for clear, colorless glass with very high light transmission. In addition, it must be capable of keeping the heat out. The challenge will be to comply with the energy codes and possibly obtain LEED certification. Today, the best solutions available are low-iron glass substrates, VRE coatings, silk-screen patterns or a combination of all of these. Certainly every project has the potential of incorporating multiple glass types. n

Rick Voelker is the technical director with Viracon of Owatonna, Minn.