Types Of Glass

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There are many types of glazing available for windows, especially since many glazing technologies can be combined. These window glazing technologies include the following and click on the words for more details:

SINGLE GLAZE:

A single-glazed window or door is exactly what the name implies. It consists of one pane of glass that is available in thicknesses of 3/32” to 9/16”. Nearly all solar energy—light and heat—passes through this glass, and energy loss is considerable. While you may like the price of single glaze glass it may cost extra money to heat and cool your household with only single panes of glass, or glaze in place. It provides little energy efficiency with only a R-1 insulating value. There are Heat-Absorbing Tints that can be added to enhance the energy performance of this glass.
Single-glazes glass is offered in three basic types:

● Annealed Glass
● Tempered Glass
● Heat Strengthened Glass

Annealed Glass is glass without internal stresses caused by heat treatment, i.e., rapid cooling, or by toughening or heat strengthening. Glass becomes annealed if it is heated above a transition point then allowed to cool slowly, without being quenched. Annealed glass breaks into large, jagged shards that can cause serious injury, and thus, the reason it is considered a hazard in architectural applications. Building Codes in many parts of the world restrict the use of annealed glass in areas where there is a high risk of breakage and injury, for example inbathrooms, in door panels, fire exits and at low heights in schools or domestic houses.

Tempered glass (toughened glass) is a type of safety glass that has increased strength and will usually shatter in small, square pieces when broken. It is used when strength, thermal resistance and safety are important considerations. Tempered glass will shatter into small squares. In commercial structures it is used in unframed assemblies such as frameless doors, structurally loaded applications and door lites and vision lites adjacent to doors. Toughened glass is typically four to six times the strength of annealed glass.

Heat-strengthened glass is glass that has been heat treated to induce surface compression, but not to the extent of causing it to “dice” on breaking in the manner of tempered glass. On breaking, heat-strengthened glass breaks into sharp pieces that are typically somewhat smaller than those found on breaking annealed glass, and is intermediate in strength between annealed and tempered glasses.

Impact (laminated) Glass:

Impact (laminated) glass is a type of single-glazed safety glass that holds together when shattered. In the event of breaking, it is held in place by an inter layer, typically of polyvinyl butyral (PVB), between its two or more layers of glass. The inter layer keeps the layers of glass bonded even when broken, and its high strength prevents the glass from breaking up into large sharp pieces. This produces a characteristic “spider web” cracking pattern when the impact is not enough to completely pierce the glass.
Laminated glass is normally used when there is a possibility of human impact or where the glass could fall if shattered. Skylight glazing and automobile windshields typically use laminated glass. In geographical areas requiring hurricane-resistant construction, laminated glass is often used in exterior storefronts, doors and windows. The PVB inter layer also gives the glass a much higher sound insulation rating, due to the damping effect, and also blocks 99% of incoming UV light. Impact glass provides a strong barrier against forced entry and cannot be cut from one side only, which renders glass cutters useless.

GAS FILLS:

To improve the thermal performance of windows with insulated glazing, some manufacturers fill the space between the glass panes with gas.
For these gas fills, window manufacturers use inert gases—ones that do not react readily with other substances. Because these gases have a higher resistance to heat flow than air, they (rather than air) are sealed between the window panes to decrease a window’s U-factor.
The most common types of gas used by window manufacturers include argon and krypton. Argon is inexpensive, nontoxic, nonreactive, clear, and odorless. Krypton is more expensive but has a better thermal performance.

HEAT-ABSORBING TINTS:

Heat-absorbing window glazing contains special tints that change the color of the glass. Tinted glass absorbs a large fraction of the incoming solar radiation through a window. This reduces the solar heat gain coefficient, visible transmittance, and glare.
Some heat, however, continues to pass through tinted windows by conduction and re-radiation. Therefore, the tint doesn’t lower a window’s U-factor. However, inner layers of clear glass or spectrally selective coatings can be applied on insulated glazing to help reduce these types of heat transfer.
Gray- and bronze-tinted windows—the most common—reduce the penetration of both light and heat into buildings in equal amounts (i.e., not spectrally selective). Blue- and green-tinted windows offer greater penetration of visible light and slightly reduced heat transfer compared with other colors of tinted glass. In hot climates, black-tinted glass should be avoided because it absorbs more light than heat.
Tinted, heat-absorbing glass reflects only a small percentage of light, so it does not have the mirror-like appearance of reflective glass.
Note: when windows transmit less than 70% of visible light, indoor plants can die or grow more slowly.

Low-emissivity (Low-E) coatings on glazing or glass control heat transfer through windows with insulated glazing. Windows manufactured with Low-E coatings typically cost about 10%–15% more than regular windows, but they reduce energy loss by as much as 30%–50%.
A Low-E coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The Low-E coating reduces the infrared radiation from a warm pane of glass to a cooler pane, thereby lowering the U-factor of the window. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. A Low-E coating can also reduce a window’s visible transmittance unless you use one that’s spectrally selective.
To keep the sun’s heat out of the house (for hot climates, east and west-facing windows, and unshaded south-facing windows), the Low-E coating should be applied to the outside pane of glass. If the windows are designed to provide heat energy in the winter and keep heat inside the house (typical of cold climates), the Low-E coating should be applied to the inside pane of glass.
Window manufacturers apply Low-E coatings in either soft or hard coats. Soft Low-E coatings degrade when exposed to air and moisture, are easily damaged, and have a limited shelf life. Therefore, manufacturers carefully apply them in insulated multiple-pane windows. Hard Low-E coatings, on the other hand, are more durable and can be used in add-on (retrofit) applications. The energy performance of hard-coat, Low-E films is slightly poorer than that of soft-coat films.

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