By Helmut Kientz, R.A. Hixson Design & Illustration

Transcription

1 By Helmut Kientz, R.A. Hixson Design & Illustration The process of selecting glazing for storefronts, office buildings, and other façades with large amounts of glass can be daunting. Glazing is chosen from spreadsheets that have endless varying data (Fig. 1), including: Tints Shading co-efficients U Values Light to Solar Heat Gain Co-efficients (SHGC) Light Transmittance Values Reflectance Light to Solar Heat Gain (LSG) All of these values are relative to each other and affect the efficiency of the glazing system in any number of ways. However, selection does not end here. Additional values can come into play. For instance, performance of insulated glass panels (one of the most common types of glass used today) can be affected by changing properties such as tinting and transparency, or by adding low-e coatings, reflective films, ceramic frits, shading devices, etc. All of this data makes the glass selection process difficult at best. To reduce the difficulties associated with the process, Hixson created a new graphical matrix method to assess manufacturers glazing data. This method takes the data from the spreadsheets and applies it to a threedimensional matrix that visually compares data and makes it easier to comprehend. Translating the Spreadsheet Let s look at how the spreadsheet works. As highlighted in yellow in Fig. 1 below, Monolithic (single pane) Performance Comparisons describes the properties of a ¼ thick sheet of clear float glass. Highlighted in blue, the second spreadsheet in Fig. 1 (with the heading Insulating Vision Unit ) brings additional challenges to Need Help? Some definitions of glass-related terms: Insulating Vision Unit: 1 units with ½ (25mm) air space and two ¼ (6mm) lites; interior lite is clear glass unless noted otherwise. Transmittance: Percentage values of Ultraviolet (UV) light Visible Light (VL) and Total Solar Energy passing through any particular glazing system. Exterior Reflectance: The percentage values of total incoming VL and Total Solar Energy that is reflected back to the outside of any particular glazing system. U Value: Overall co-efficient of heat transmittance measured in BTU/hr. The lower this number, the better. Shading Co-efficient: The ratio of the total amount of solar energy that passes through a glass relative to 1/8 (3mm) clear glass under the same design considerations. Again, lower values are better. Light to Solar Heat Gain Co-efficients (SHGC): Solar heat gain through the glass relative to the incident solar radiation + 86% of the shading co-efficient. Lower numbers are better. Light to Solar Heat Gain (LSG). The ratio of VL transmittance to solar heat gain coefficient. Note: The U.S. Department of Energy defines spectrally selective glass as any glass with an LSG of 1.25 or greater meeting energy efficient guidelines set forth by the Federal Energy Management Program. the review process. Differences that affect the transmission characteristics are created by modifying the coatings on any one of four sides of glass. The sides selected for coatings are designated by number. The outside surface of the exterior pane is designated as surface #1, the inside surface of the outside pane is surface #2, the outside surface of the inside pane is surface #3 and the inside surface of the inside pane is surface #4. These designations are shown on the graphic matrices. If triple glazing were to be used, the numbers continue in similar sequence so that the inside of the third pane is called surface #6.

2 Figure 1 In Figures 2 through 6, all of this numeric data has been converted to graphic matrices. You can see the differences of five selected types of glass. All pertinent values are shown to a relative scale. Figure 2 shows a ¼ thick, single pane of clear glass with all the characteristics of what goes through and what is reflected back out. In addition, the night-time coefficient of heat transmission is high, meaning that much of the heat inside is escaping to the outside at night: not a good situation in cold climates! Figure 3 shows a sample of an insulated glass unit represented as a graphic matrix. As you can see, the UV, VL and SHGC have changed greatly, showing that less of the undesired characteristics are transferring to the inside. Imagine trying to visualize the same information from a spreadsheet! Figures 4, 5 and 6 show additional comparisons of insulating glazing where factors such as the type of glass, low E coating, and tint are changed to produce desired properties. Note that the LSG at the bottom of each matrix changes. If the value is greater than 1.25, the glazing system meets the requirements of the Federal Energy Management Program for spectrally selective glazing and is considered to be green. Note also that the tint of the glass at the upper right of each graphic matrix. In general, the inside pane is usually a clear sheet of glass and the outside pane is the sheet of glass that modified by type of glass, coatings and tints. Spreadsheets Not Going Away Just Yet While spreadsheet data will remain as a standard to quantify glazing properties, Hixson has confirmed the accuracy of the glazing properties shown in the graphic matrices with several glass manufacturers. Therefore, Hixson s graphic representation of varying glazing properties can serve as a quick resource in narrowing down the glass selection process, simplifying what has traditionally been an all-too complex process.

3 Figure 2

4 Figure 3

5 Figure 4

6 Figure 5

7 Figure 6 Note: Solexia, Solarban and other glazing brand names shown within this document are registered trademarks of PPG Industries, and the sample pages of manufacturer spreadsheets shown are from PPG Industries. All rights reserved. Advocacy: Hixson is a "No-Bias" Company At Hixson, we seek to serve as extended team members of our clients organizations, not as representatives of companies with outside concerns. Therefore, we do not accept income or incentives or sell/represent products from equipment suppliers, contractors or any company that could improperly influence our opinion. Because of this, Hixson ensures that the information we present is unbiased and has been developed in the best interests of our clients.