A difference in temperature between two points within any material will result in heat being transferred from the hot point to the cold point. This is evident with the transmission of heat through a glazing.
Heat may be transferred in various ways:
- By conduction within the material itself—the heat is transferred from one molecule to the next when heated. For example, a metal rod with one end heated up will transfer this heat throughout the rod
- By convection in liquids and gases—temperature variations within a liquid prompt differences in density, which cause the molecules to move around. This occurs because the hot parts have a smaller mass and rise, while the opposite is true for the cold parts. These movements balance out temperatures. For example, when heating a saucepan of water, the temperature eventually becomes constant
- By radiation—any heated body gives off energy in the form of electromagnetic radiation. This radiation crosses physical spaces more easily and effectively than light waves. By contrast, when light waves meet an obstacle, they release part of their energy to the obstacle, which, in turn, emits heat. This method of heat transmission requires no area and can also take place in a vacuum, as is the case with solar radiation or an electric light bulb
Double glazing is designed to limit heat lost through conduction within the glass by inserting an insulating space of air or gas between the two sheets of glass.
Sound Insulation with Laminated Glass
Glazing with normal PVB (polyvinyl butyral) interlayer
The main function of this type of glazing is to provide safety and limited security. Such glazing also offer enhanced sound insulation due to change in material properties through which sound has to travel (i.e. elasticity and density).
Glazings with acoustic PVB
Most acoustic interlayer’s are multilayer products that offer enhanced acoustical benefits. Acoustic interlayer’s, when properly selected, laminated, and installed, are capable of meeting architectural safety glazing codes for all applications, including EN 12543-4 requirements.
The performance levels of symmetrical double glazings are often lower than those of a single-pane glazing with the same total glass thickness. To eliminate the mass-spring-mass effect, the air space between the panes of glass must be widened in order to make the spring created by the air space more flexible. However, this could result in glazings which are too thick and which require equally wide frames, which would add significant weight to the unit. This would also increase convection within the air or gas space, which would be detrimental in terms of thermal insulation. For these reasons, insulating glazing is not widely used in practice for sound reduction.
Double glazing units with laminated glass
Laminate glass can also be used in double glazing. The figure below shows the improvement in performance when laminated glass is used. The gain can be seen primarily in the high-frequency zone, since it flattens out the critical-frequency trough.
The factors that affect the levels of sound insulation provided by various glazings can be summarized as follows:
- Increased thickness provides slight improvement
- Using laminated glass and acoustic laminated glass provides significant improvement in performance levels
- Always use dissymmetrical glazing
- Use a substantial air space
- Use thick glass in most instances
- Use a laminated glass (conventional PVB or safety) in place of one of the two monolithic glasses
- Use a laminated glass with acoustic PVB for high levels of sound disturbance
Solar control needs in North America are generally determined by geographic region.
In the southern region of the United States, a low SHGC is desirable. This type of high-performance window will greatly reduce the amount of solar heat energy entering the home. In the northern region of the United States and throughout Canada, a high SHGC and a low U-Factor is the best combination to allow passive solar heat inside the home while still ensuring excellent thermal insulation to keep the heat in.
In contrast to residential, geographic region is less important than the internal heat levels generated by people and machinery. In light of this fact, throughout North America, low-SHGC glazing is generally used to reduce the amount of solar heat energy entering the building.
Few building materials offer a more compelling range of design possibilities than glass. The ability coat or tint glass enables you to create virtually any aesthetic, while its reflective properties add another unique dimension to your design palette. AGC offers a broad spectrum of exterior glazing products that enable you to add these characteristics to distinguish your building projects. Whatever you envision, let AGC bring it to life in glass.T
Safety Glass Products
In designing safety glass, two primary concerns must be addressed. The first consideration is protecting individuals from harm caused by sharp, broken glass. The second is protecting against the threat of falling glass (defenestration).
Only a small number of glass products meet the breakage pattern, defenestration, and resistance criteria to address these issues: tempered and laminated glass. Tempered glass is designed to break in such a manner as to eliminate large, sharp edges from being formed, thus greatly reducing the risk of injury. Meanwhile, laminated glass is designed to reduce the risk of glass falling from the opening after it is broken. A combination of tempered and laminated glass will effectively address both of these issues.
Another key function of safety glass is to protect people and property against the burglary and vandalism of private homes, shops, and offices. In this context, the glazing should remain in place and should prevent anyone or anything from penetrating it. Additionally, safety glass can be fabricated to provide protection against firearms and explosions.
Daylighting and Visible Transmittance
The emphasis on daylighting (the practice of designing structures to maximize the use of natural light) continues to increase in step with the movement toward greater sustainability. The benefits of daylighting include reduced energy consumption and increased indoor environmental quality. As such, the merits of daylighting are recognized by the USGBC and its LEED® certification program. Studies have shown that daylighting can also increase the productivity of building occupants and improve the test scores of students.
Bullet Resistance—ASTM F-1233 and UL-752
There are many standards used in North America for the testing and classification of bullet-resistant glasses. These standards make a distinction between the resistance that glazings demonstrate against various types of weapons and ammunition. For each category of weapons that are tested, glass products are considered bullet-resistant if they stop all the bullets on a set of different rounds fired from a specified distance. The various classes and levels of bullet-resistant glass may include products that offer a number of levels of protection. A glass meeting the requirements stipulated for a given class of weapons also meets those of the classes below it. However, there is no correlation between classes/levels for different weapon types.
Burglar-Resistant Glass—ANSI/UL 972
Laminated glasses provide an element of security against “smash and grab” thefts. Whether protecting merchandise in a store display, or guarding a homeowner’s porch door or window against intruders, laminated glass provides the needed security. Laminated glass protects against forced entry by resisting repeated blows from hammers, bricks, or other weapons—and deterring burglars from perpetrating the crime.
ANSI/UL 972 Testing
ANSI/UL 972 test standards define the specific methods that are used to classify glasses in terms of their resistance to burglary. This testing uses the impact of a steel ball as a surrogate for a variety of burglary tools such as hammers, bricks, or crow bars. Testing consists of dropping a 3.25-inch (82 mm), 5-pound (2.26 kg) steel ball across a designated vertical distance at glazing specimens conditioned at different temperatures. The test specimens should measure 24 inches x 24 inches (610 mm x 610 mm) in size. There are five impacts per specimen.
In order for glazings to qualify as burglary-resistant under these standards, the steel ball must not penetrate the laminate during all five impacts.
Hurricane Impact Resistance Glazings
Building codes in the coastal counties of the United States require that, in windborne debris regions, glazing in buildings shall be impact-resistant or protected with an impact-resistant covering meeting the requirements of SSTD 12, ASTM E 1886 and ASTM E 1996, Florida Building Code TAS 201 and 203, or AAMA 506.
In accordance with the windborne debris provisions of these building codes, glazed openings located within 30 feet (9.144 m) of grade must meet the requirements of the ASTM E 1996 Large Missile Test. This test simulates the effects of large, wind-driven debris that can impact the glazing during a hurricane—such as broken roof tiles, branches, patio furniture, etc.
Window systems are certified if three similar specimens pass in accordance with the following criteria, after completion of the impact and cycling portions of the ASTM E 1996 testing. (a) All test specimens must resist the large or small missile impacts, or both, without penetrating the pane of glass (b) Test specimens must resist the large or small missile impacts, or both, with no tear formed longer than 5 inches (130 mm) or no opening formed through which a 3-inch (76 mm) diameter solid sphere can pass freely.