A typical building loses 10%of its heat through the windows.

Solar Radiation and Visible light penetrates through fenestrations (windows, doors, and other openings),which leads to heat gain in the interior of the space. Glazing in windows traps the heat inside. Window glazing should retard the transmission of heat and control solar radiation and glare.

The heating and cooling equipment to maintain comfort levels depends on proper performance of windows.

High performance and energy efficient windows greatly reduce the energy consumption and pollution levels. They have

• Lower heat loss
• Warmer heat surface
• Less air leakage

Factors affecting heat gain

• Orientation of windows – East and West facing windows experience maximum heat gain in afternoon, especially during summer months.
• Shading devices.
• Landscape features surrounding the windows such as plants, trees, etc. that reduce heat gain.
• Glazing.

Factors affecting window performance

• Frame Material
• Number of panes
• Space material
• Gas between panes
• Glazing emissivity

Window Glazing

Glazing is a part of the window which is made of glass. Common types of glass used in the building industry are float glass, tempered glass, laminated glass, etc., all of which can be glazed singly or double or triple glazing units.

Characteristics of Glazing

• Visible Transmittance
• Visible Reflectance
• Solar Heat Gain coefficient (SHGC) or Shading Coefficient (SC)
• U-Value
• Ultra violet Transmittance
• Spectral Selectivity
• Glazing color
• Sound transmission

Types of Glazing

• Single glazing
• Double glazing
• Triple glazing

Single Glazing

Describes the performance of a window by a single pane of glass.

Double Glazing

• Describes the performance of a window by two panes of glass.
• The space between them is filled with air or gas fill (usually krypton or argon) which is sealed tightly.
• Gaps vary from 6mm to 12mm,12mm being better optimal performance.
• Prevents heat and energy loss from inside.
• More energy-efficient than single-pane.

Triple Glazing

• Consists of three panes of glass separated by air or gas fill.
• The space between them is filled by gas (usually krypton or argon) which is sealed tightly.
• Prevents heat and energy loss from inside.
• More energy efficient than double pane.

Glazing by Function

1.Thermal Insulation-Insulating Glass

Low Emissivity Glass (LOW-E GLASS)

• Glass with a special invisible film of metal coating that reflects and blocks out heat.
• Thin coating of metal bounces the sun rays back and controls interior temperature.
• Two types of low e glass passive and solar control.

It provides protection against harmful sun rays ,and is energy efficient as it achieves a balance between capturing and reflecting heat.

Reflective Glass

• Glass is treated with a metallic coating that cuts off solar heat.
• This coating also gives a one-way mirror effect, preventing visibility from outside thus maintaining privacy.

2.Sound Insulation- Sound Insulating Glass

Acoustic Glass

• Consists of two or more sheets of glass bonded together with one or more acoustic interlayers.

3.Safety and Security – Safety Glass

Toughened Glass

• This type of glass is tempered and is stronger than ordinary glass.
• An annealed glass is heated in a furnace up to 600 c or above and then quickly cooled down.
• Clear and maintain visibility.
• Used in structural glazing windows, fire-resistant doors etc.

Laminated Glass

• Consists of two sheets of glass that are pressed and bonded under heat.
• A thin layer of plastic or resin is sandwiched between them finishing lamination.
• It absorbs ultraviolet light and heat.
• Mostly fitted on the exterior face of window.

4.Decorative-Structured Glass

Translucent/Etched Glass

• Decorative glass put through a process of etching to produce a frosted pattern.
• Artistic carving on the surface leave a white frosted finish.
• Used to create designs on glass for glass doors, windows, furniture, etc.

Self cleaning Glass

• Glass with a special chemical coating (titanium dioxide) on the outer surface that reacts with daylight to break down dirt.
• Cleanses itself by daylight and rain.

The thermal efficiency of glass depends on

• Thermal transmittance of glass
• Thermal transmittance of frame
• Solar heat gain through glass
• Air-tightness of glass frame assembly.

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