PPG Industries, Inc.
Sweets Catalog Classification Code
08 81 00/PPG

PPG Industries, Inc.
PPG Glass Business and Discovery Center
400 Guys Run Road
Cheswick, PA 15024
Tel: 1-888-PPG IDEA (1-888-774-4332)
Fax: (412) 826-2299
Website: http://www.ppgideascapes.com
Email: ppg@ppg.com

GLASS SELECTION AND DESIGN GUIDELINES

Design Considerations:

Direct and reflected solar radiation, along with source heat from heating registers and radiant heaters, can create differences in temperature between the center and edges of glass. Such temperature differences can result in stresses and glass breakage.

Thermal stresses are typically lower for glass facing between N 60W and N 45E

Large lites with large edge areas are more susceptible to solar induced breakage than smaller lites with smaller edge areas. To reduce stress, avoid irregular or pattern-cut shapes.

Avoid unventilated indoor shading devices which trap a layer of solar heated air between the shading and the glass.

Locate registers on the room side of draperies.

Balconies, awnings, solar screens, louvers, trees and shrubbery may partially shade glass, introducing the stress of irregular heating.

Light-colored framing with low heat capacity minimizes edge stress. Care during installation is critical to avoid damage to edges and increased risk of thermal breakage.

The glass installer should become familiar with installation conditions and should complete a "thermal stress analysis" as outlined in PPG's Thermal Stress Guidelines.

The building designer is responsible for defining the duration and pressure of uniform loads, and the acceptable level of probable breakage at design load (in lites per thousand), and for assessing the applicability of local codes and safety glazing requirements.

Wind load charts for most glasses are also available in ASTM 1300-94. Software to perform this analysis is available to assist in design and product selection.

PPG has long been recognized as the industry leader for providing technical support and expertise to its customers.

• PPG Solutions Hotline
• http://www.ppgglazing.com on the Internet

The following topics are addressed in greater detail in PPG technical literature from these sources.

Glass with the right thermal performance is selected by its solar heat gain coefficient and U-value. Solar heat gain coefficients and U-Values for all PPG products can be found in this electronic presentation. Design considerations include:

• Summer solar heat gain through windows can be reduced by selecting a glass with a lower solar heat gain coefficient, or removed by air conditioning, nighttime conduction through walls and roof, or exfiltration and ventilation.
• Winter conducted heat loss through windows can often be offset completely by solar heat gain if glass with the right solar heat gain coefficient and U-value is selected.
• Summer, spring, fall conducted heat gain/loss.
• Infiltration, exfiltration, and ventilation vary with window frame seals and operability of the windows.
• Daylight, when exploited as a design element, can reduce lighting/cooling needs and costs.
• Building orientation, geometry and siting affect solar heat gain, daylight patterns, and the impact of wind on infiltration and exfiltration.
• Building Codes may dictate maximum allowable solar heat gain coefficients and U-values.

Selecting the Right Strength

Glass is available in four strengths:

1. Annealed glass
As produced glass is cooled in a very controlled manner, thereby minimizing residual internal stresses.

2. Tempered glass

Federal, state and local building codes require safety glass such as tempered glass wherever human impact is possible, e.g., doors and sidelites.

If broken, tempered glass falls out of its opening in interlocking clumps. For this reason, PPG recommends against its use for glazing above populated areas in commercial buildings and skylights in homes.

About twice as strong as annealed glass, heat-strengthened glass is made by cutting annealed glass to size, heating to near its softening point, then cooling faster than normal, but not as quickly as when producing fully tempered glass.

In general, heat-strengthened glass is specified where thermal stresses are high, e.g., spandrels, atriums, solariums, skylights and the like and building windows where shading or climates may be severe. It virtually eliminates the risk of spontaneous breakage (an inherent risk characteristic of fully tempered glass). Properly applied, the glass will resist most normal thermal and wind loads, and risk of in-service breakage is very low. Unlike tempered glass, if heat-strengthened glass should break, the pieces will be larger and tend to stay in the frame until removed.

Note: Heat-strengthened glass is not a safety glazing material and should not be used where human impact is a concern or where building codes require safety glazing. However, there are normally alternatives to the use of fully tempered glass in the spandrel and vision areas of high-rise commercial building, which should be strongly considered and evaluated whenever building codes require fully tempered glass for such applications.

Consists of two or more layers of glass sandwiching a vinyl interlayer to form a single, unified construction. Typical applications include overhead glazing, e.g., skylights, solariums, atriums, as well as safety glazing when approved by building codes. Virtually all types of glass products, including heat treated glass, can be fabricated into laminates.

SELECTING THE RIGHT LIGHT CHARACTERISTICS

Selecting glass with the right visible light transmittance and reflectance can greatly improve the appearance of the building and comfort of occupants. Coordinating vision glass with interior space design can lower the costs of artificial lighting. The key is to consider glass as one contributing component of the entire building design, including the following design criteria:

• Daylighting requirements: Selecting a glass with too high a transmittance of visible light can create overlit interiors, causing contrast glare and occupant discomfort. Contrast glare can be reduced by coordinating selection of light transmittance, colors of interior surfaces and furniture, window treatments, and levels of artificial lighting.
• The ratio of window to floor areas: Designers tend to select glass with higher transmittance when windows are small. However, because of the potential for contrast glare, lower light transmittance could be the better choice.
• Blind/drape clutter: Lower transmittances mask the exterior view of random positioning of blinds and drapes, and other interior clutter.
• Occupants’ views to outside: Even on sunny days, very low light transmittances can create the feeling of overcast weather. Bronze glass and larger windows with lower transmittances tend to soften this effect, while grays may intensify it.
• Glare/reflectance on CRT screens: Lower transmittances can reduce glare significantly.
• Local climate: Fewer sunny days – and northern locations – generally suggest selection of a glass with higher transmittance.
• Building orientation: Glass with different light transmittances may be installed on different exposures to compensate for different sky lighting.
• Matching/harmonizing vision and spandrel glass:

NOTE: PPG does not manufacture spandrel glass products. PPG is providing the following information to assist in the selection of suitable spandrel glass. PPG recommends that the glass fabricator be contacted for samples and additional technical guidance

PPG recommends insulating glass spandrels when the light transmittance of the vision glass is 30 percent or less and a close match is integral to the design. The spandrel and vision insulating glass units should be identical with one exception: a ceramic frit is applied to the indoor lite of the spandrel unit.

The approach offers the advantages of a warranty (if offered by the fabricator) on the coating and seal, long-term aesthetic integrity, and lower maintenance. Other approaches include:

1. Opacified coated glass is comprised of a single lite of coated glass identical to the exterior lite of the vision glass backed by an opaque plastic film. The film protects the glass from damage during construction and from the dirt and moisture build-up inherent with shadow boxes. Vision-to-spandrel color match is generally acceptable, but is less accurate than is possible with either an insulating glass unit or a shadow box application.

2. Shadow boxes are comprised of a single lite of the same glass as vision lite #1, with dark insulation set back one to two inches and protected by a vapor barrier. Although shadow boxes can provide a reasonably good match, the potential is high for deterioration of aesthetics. Dirt and moisture tend to collect between insulation and glass, causing staining and spotting. These and other risks of shadow boxes are detailed in "Spandrel Recommendations," a PPG Technical Services Memo dated February 14, 1986.

Reflectance + Absorption + Transmittance = 100% of the sun’s energy to which the glass is exposed. Glass can be designed to reflect, absorb or transmit more or less energy, meeting a wide range of design criteria by increasing or decreasing one or both of the other characteristics. For example, to reduce transmittance (and solar heat gain), either reflectance or absorption – or both – must be increased. The formula dictates that low-reflectance glasses with low solar heat gain must absorb a high percentage of the sun’s energy, and therefore must usually be heat-strengthened to resist the additional thermal stresses.

Simulating lighting conditions in a mockup, even if the mockup is small scale, is strongly suggested. For example, install a sample of the proposed glass in a double-hung window and view it from the interior and exterior. Or construct a model to simulate indoor and outdoor lighting. For a major project, arrange for a large-scale evaluation at PPG’s Design Verification Facility. Ask your PPG sales representative to suggest the right procedure for your project.

The Special Effects of Metallic Coatings on Surface #3:

1. Lower indoor reflectance. Select a glass coated on surface #3 when indoor light levels are high and outdoor levels are low or night time viewing is desired.

2. Higher measured outdoor reflectance, but a less mirror-like appearance. Third surface coated glass will appear softer and less reflective, since the images reflected from surfaces #1 and #3 are out of phase.