Volume 6 Issue 7 August 2005
GLASS - BACK TO BASICS
As window and door manufacturers, you work with it every day. But maybe your company’s primary business is insulating glass (IG) and you’ve been thinking about incorporating laminated glass into your product lines. Regardless of what type of glass you work with, whether you’ve been in the business for a few months or a few years, maybe you just need a refresher course. So, with this in mind DWM magazine has teamed up with the Glass Association of North America (GANA) to offer glass basics 101. While the information presented focuses on coated-, insulating- and laminated glass, further information on other glass types can be found at www.glasswebsite.com. All of the information below is excerpted with permission from GANA’s Glazing Manual, 2004 Edition.
Flat glass products may be coated to enhance the thermal and optical performance characteristics of products used in residential and commercial glazing and transportation applications. There are two basic types of coated glass: solar control (reflective) and low-emissivity (low-E). The major differences are visible light transmission, ultraviolet (UV) light, visible and near infrared (IR) wavelengths of energy that are reflected and the directions in which these wavelengths are usually reflected.
The solar spectrum consists of UV with wavelengths ranging from 300-390 nm, visible light (390-770 nm) and infrared light (770-2100 nm). The distribution of energy within the solar spectrum is approximately 2 percent UV, 46 percent visible and 52 percent infrared.
Solar-control glass may have a variety of metal coating layers that are highly reflective of solar energy, i.e., those energy wavelengths from 300-2100 nm that constitute the solar spectrum.
The major benefits of reflective solar control glass include the following:
Aesthetic Appeal: Colors of silver, blue, copper, golden and earth-tone coatings, applied to the wide range of clear and tinted float glass, allows the architect considerable flexibility with exterior design.
Energy Savings: Through its ability to reflect, absorb and radiate solar energy, solar reflective glass reduces interior solar heat gain substantially. The added cost of the coating will generally be offset by the reduced size and operating cost of the heating and cooling systems.
Occupant Comfort: This is improved when heat gain is reduced and interior temperatures are easier to control.
This type of coated glass may have various combinations of metal, metal oxide and metal nitride layers of coatings that are nearly invisible to the eye. Some low-E coatings are highly reflective for the infrared part of the solar spectrum and all low-E coatings reflect long wave IR energy. Long wave IR can be described as the radiant heat given off by an electric coil-type heater, as well as the heat that comes from a hot air register. The re-radiated heat from room furnishings that have absorbed solar energy is still another form of radiant heat.
While some low-E coatings can be used in monolithic or laminated glass constructions, the coatings provide maximum performance when sealed within an insulating glass unit. The location of the low-E coating within a unit affects the product performance. A low-E coating on the second surface of an IG unit is more effective at reducing solar heat gain, especially when used in conjunction with tinted glass. The low-E coating will reflect IR, while the tinted glass reduces the solar radiation through the glass, resulting in less glare and heat gain.
Using low-E glass in commercial buildings and residential applications in warm climate regions, is generally the most practical way to maintain comfort levels.
In cold climate regions where occupants want to maximize solar heat gain from the sun while minimizing radiant heat loss, IG units commonly incorporate clear glass with low-E on the third surface. The low-E coating reduces heat loss through the glass in winter by reflecting interior long wave IR back into the home.
Center of glass U-values in the range of 0.25-0.36 can be achieved with low-E coatings on the second or third surface of IG units. Low-E coatings can be combined in an insulating unit with a solar-control/reflective coating and gas filling to create an insulating unit having lower U-values and a lower shading coefficient.
The major benefits of low-E coated glass include:
Aesthetic Appeal: The virtually invisible nature of low-E coatings provide a transparent appearance to the glazing material and building façade.
Energy Savings: Through its ability to reflect long-wave infrared energy low-E coated glass reduces winter heat loss and summer heat gain through the glass, and provides high levels of visible light transmittance into the building. The combination of thermal control and reduction in interior lighting requirements reduces energy consumption for residential and commercial buildings.
Occupant Comfort: This is improved when heat gain/loss is reduced by keeping the interior temperature stable regardless of the exterior environment and when natural daylight is introduced into the building.
Optical and aesthetic quality requirements for coatings applied to glass are addressed in ASTM C 1376 Standard Specification for Pyrolytic and Vacuum Deposition Coatings on Flat Glass.
Laminated glass traditionally is defined as:
1. Two or more lites of glass and one or more interlayers of plasticized polyvinyl butyral (PVB) permanently bonded together under heat and pressure;
2. Two or more lites of glass and polycarbonate with an aliphatic urethane interlayer between glass and polycarbonate permanently bonded together under heat and pressure.
3. Two or more lites of glass bonded with one or more interlayers of a liquid resin cured and permanently bonded together by exposure to ultraviolet light, heat or chemicals.
4. Two or more lites of glass with an ionoplast rigid sheet interlayer (similar to PVB yet more rigid) permanently bonded together under heat and pressure.
5. Two or more lites (or sheets) of polycarbonate (or acrylic) with an aliphatic urethane interlayer between polycarbonate or acrylic bonded together under heat and pressure; and
6. Two or more lites and polyester (PET) film with a PVB interlayer between glass and PET permanently bonded together under heat and pressure.
Its most important characteristic is the ability of the interlayer to support and hold the glass when broken and/or plastic sheet when cracked. This provides for increased protection against fall-out and penetration of the opening. Most building codes require the use of laminated glass for overhead glazing as monolithic lites, or as the lower lite in multiple glazed units.
Laminated glass with PVB interlayers are generally 75- to 100- percent as strong as annealed glass of the same thickness depending on exposed temperatures, aspect ratio, plate size, stiffness and load duration. Laminated glass, however, can be made with heat-strengthened, fully tempered or chemically- strengthened glass for additional benefits, such as increased wind-load resistance, impact resistance or resistance to thermal stress. The ability of the interlayer to resist various kinds of penetration may also be dependent upon thickness, temperature and other variables.
Check with the fabricator for any additional limitations, such as roll distortion, that may result from this additional processing of laminated glass. There are several grades of PVB having different physical properties. Care should be taken to specify the correct grade for a given application. Consult the interlayer manufacturer/glass fabricator for full details.
Typical applications for laminated glass with PVB interlayers and cured resins include locations where safety glazing is required, such as doors and skylights. Laminated glass resists glass fall-out from windblown debris in hurricane/cyclic-windstorm prone areas and provides various levels of security protection in seismic, blast-resistant, bullet-resistant and burglary-resistant applications.
Laminated glass with ionoplast interlayers are similar to PVB laminates; however, the rigid interlayer provides additional performance in high design pressure and high security applications where lower deflections and higher penetration resistance is required after the glass lites have been broken.
Glass-clad polycarbonate contains glass layers to the exterior and one or more polycarbonate layers on the inside. This product combines the heat, chemical and abrasion resistance of glass with the impact resistance of polycarbonate. This laminated construction may also be unbalanced or asymmetrical, where a polycarbonate layer is exposed to the interior. Although not truly a glass-clad product, the industry recognizes it under the same category.
Organic coated glass-butyral consist of at least one lite of glass with its interior or protected surface laminated under heat and pressure to a composite sheet of PVB with a scratch-resistant polyester (PET) film. Optionally, the organic coated glass-butyral can be applied onto multiple-ply laminated glass.
The composite organic coating consists of an abrasion-resistant polyester film combined with a sheet of PVB for factory lamination to glass. The PVB is used to adhere the PET film to the glass surface. The composite must face towards the building’s interior.
PET films can also be laminated inside the laminated glass using PVB to bond the PET to the glass. This PET film can provide additional resistance to penetration and cyclic wind pressure.
Quality standards for laminated glass are defined in ASTM C 1172 Standard Specification for Laminated Architectural Glass and ASTM C 1349 Standard Specification for Architectural Flat Glass Clad Polycarbonate. Laminated glass for use as safety glazing is covered by ANSI Z97.1 and CPSC 16 CFR 1201 Category I and II.
In order to reduce heat gain or loss through glass, two or more lites may be sealed together to create an IG unit.
The majority of IG units consist of two lites of glass enclosing a hermetically sealed air space. The lites are held apart by a spacer around the entire perimeter. The spacer contains a moisture-adsorbent material called desiccant that serves to keep the enclosed air free of visible moisture. The entire perimeter of the assembly is sealed.
The most commonly-used edge construction contains a metallic spacer of roll-formed aluminum, stainless steel, coated steel or galvanized steel. It is sealed with a single seal of polysulfide, polyurethane or hot-melt butyl, or with a dual seal consisting of a primary seal of polyisobutylene and a secondary seal of silicone, polysulfide or polyurethane. The corners of the metallic spacer may be square-cut and joined with a metal, plastic or nylon corner key, may be miter-cut and brazed, welded or soldered, or may be bent.
Recent years have seen the introduction of warm-edge technology products as spacer materials. These products include extruded butyl materials, foam rubber-based materials, formed plastics and metal strip based products, many with desiccant included as a component.
Improvements in edge of insulating glass U-values as a result of warm-edge technologies play a vital role in meeting overall window performance requirements for state adopted residential fenestration codes.
The thermal performance of IG units is enhanced by using solar control substrates and coated glass (low-E or reflective), coated polyester suspended films, insulating gases (such as argon, krypton or xenon) and warm-edge technology products. Initial heating and cooling equipment costs and ongoing operating costs are reduced.
Industry product classification, performance requirements and testing procedures for insulating glass units are defined in the following ASTM International documents:
• E 773 Standard Test Method Accelerated Weathering of Sealed Insulating Glass Units;
• E 774 Standard Specification for Sealed Insulating Glass Units;
• E 2188 Standard Test Method for Insulating Glass Unit Performance;
• E 2189 Standard Test Method for Testing Resistance to Fogging in Insulating Glass Units; and
• E 2190 Standard Specification for Insulating Glass Unit Performance and Evaluation.
Most insulating glass fabricators voluntarily participate in IG certification programs. The purpose of these are to assure the user that the purchased product is a faithful replica of one that has passed certain prescribed tests. Therefore, participants in a certification program must complete the following requirements: 1) submit specimens of their production product to independent testing laboratories for the prescribed tests; and 2) agree to periodic, unannounced inspections of their regular production by an independent agency to ensure that actual production employs the same materials and techniques as the tested specimen.
Be Clean: What to Do (and Not to Do) When Cleaning Glass
GANA offers the following guidelines for the cleaning of glass
The following are things to do:
• DO clean glass when dirt and residue appear;
• DO determine if coated glass surfaces are exposed;
• DO exercise special care when cleaning coated glass surfaces;
• DO avoid cleaning tinted and coated glass surfaces in direct sunlight;
• DO start cleaning at the top of the building and continue to lower levels;
• DO soak the glass surface with a clean water and soap solution to loosen dirt and debris;
• DO use a mild, non-abrasive commercial window cleaning solution;
• DO use a squeegee to remove all of the cleaning solution;
• DO dry all cleaning solution from window gaskets, sealants and frames;
• DO clean one small window and check to see if procedures have caused any damage;
• DO be aware of and follow the glass supplier’s specific cleaning recommendations;
• DO caution other trades against allowing other materials to contact the glass;
• DO watch for and prevent conditions that can damage the glass; and
The following are things NOT to do:
• DO NOT use scrapers of any size or type for cleaning glass;
• DO NOT allow dirt and residue to remain on glass for an extended period of time;
• DO NOT begin cleaning glass without knowing if a coated surface is exposed;
• DO NOT clean tinted or coated glass in direct sunlight;
• DO NOT allow water or cleaning residue to remain on the glass or adjacent materials;
• DO NOT begin cleaning without rinsing excessive dirt and debris;
• DO NOT use abrasive cleaning solutions or materials;
• DO NOT allow metal parts of cleaning equipment to contact the glass;
• DO NOT trap abrasive particles between the cleaning materials and the glass surface;
• DO NOT allow other trades to lean tools or materials against the glass surface;
• DO NOT allow splashed materials to dry on the glass surface; and
• DO NOT start cleaning without reading the entire GANA bulletin on glass cleaning.
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