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Glazing properties are the properties of the materials, coatings, and constructions that make up windows, skylights, translucent panels, or other products used to let sunlight into a building.  

Good glazing properties are important because they optimize the transfer of heat and light into and out of the building. Appropriate values vary by climate, size, and placement of the aperture.

There is no one best kind of glazing to use.  It's not unusual for a single building to have three, four, or even five different kinds of glazing for apertures in different sides and at different heights on a building.  

The most important properties to balance and optimize when designing energy efficient buildings are the visible light transmittance, U-value, and solar heat gain coefficient.  

Visible Light Transmittance

The percentage of visible light that passes through a window or other glazing unit is called the Visible Light Transmittance (T_vis or VLT).  An opaque wall would have a T_vis of 0%, while an empty opening would have 100%; many un-tinted glass and plastic materials have a T_vis of 90% or more.  

More light is often not better, as it can cause glare and overheating.  Tints, frits, and coatings can be chosen to produce any T_vis; common values are often 30 - 80%.

You can also control which wavelengths of light that you transmit into the space. "Spectrally selective" windows allow visible light in while reflecting most other wavelengths, such as infrared and/or ultraviolet.  Ultraviolet light can fade and otherwise deteriorate interior finishes and furnishings.  Infrared light is heat, and is often undesirable in warm climates.

Spectrally selective windows can block certain wavelengths of light

U-value

U-value measures how well glazing insulates--or rather, how poorly glazing insulates.  U-values measure <a href="http://en.wikipedia.org/wiki/U-value#U-value">thermal conductivity</a>, the rate of heat transfer per unit area, per unit temperature difference from the hotter side to the colder side.  This is W/(m²K) in SI units, BTU/(h°F ft²) in Imperial units.  U-values are either measured for the glazing only (“center of glass”) or for the entire window assembly (including framing and spacers).  The entire window assembly is the value most-often referenced.

In cold climates, a low U-value is usually the most important window property and a rule of thumb is to look for windows with a U-value of 0.35 or less (imperial units). In warmer climates, low U-values are often less important than the Solar Heat Gain coefficient. 

R-values are 1 / U-values.  A table of some typical and cutting-edge constructions are below.  

U-values for various glazing constructions

U-Values and Heat Transfer

Glazing units transfer heat through conduction, convection, and radiation. Each of these affects the U-Value.

U-values can be decreased by reducing the radiation of heat from the glazing.  Low-emissivity ("low-E") coatings achieve this by reducing its emissivity, the ability of a material to emit radiation.  In a multi-pane window, these coatings may be applied to the interior or exterior pane, depending on whether heat is meant to be kept inside the building or kept out.  They may also be applied to multiple panes.

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Heat transmission and radiation from a window

U-values can also be decreased by reducing the convection within the glazing unit.  The simplest way to do this is to subdivide the air space by adding more panes.  Interior panes are often merely thin films, since only the outer panes need to be structural materials like glass.  Air between panes can be replaced by denser gases like Argon or Krypton, which both reduce conduction and convection.  Some constructions fill the space between panes with cellular materials or batting to reduce convection, though these units are no longer transparent, so they can no longer be used for views.

Window constructions to reduce convection between window panes.

Finally, U-values can be reduced by reducing the conductivity of the materials in the glazing unit.  As mentioned above, denser gases like Argon conduct less heat than air.  More significantly, the window frame should not conduct heat around the glass. Metal framing should be "thermally broken" to separate interior metal elements from exterior elements.  Wood or fiberglass also conduct less heat than many metal window frames..  The framing of windows and other glazing can cause so much heat loss that a unit's overall U-value may be double or triple the "center of glass" U-value.  Thus it is important to use U-values that include framing.

Solar Heat Gain Coefficient

Solar Heat Gain Coefficient (SHGC) measures how much of the incoming heat from sunlight gets transmitted into the building, versus how much is reflected away. It is a number between zero and one. 

An SHGC of 0 means none of the sun's heat reaches the inside, while an SHGC of 1 means all of the heat from the incoming sunlight comes through. Choosing the right SHGC depends on the size and placement of the apertures as well as climate and other design factors.

The SHGC is especially important in hot sunny climates, and you should generally use glazing with lower SHGC (below ~0.4). Buildings in cold climates should generally have higher SHGC to enable passive solar heating and to reduce heating loads.

The SHGC for ordinary uncoated, un-tinted glass can be .9, while values can be as low as .25 or even .15 for some specialized glazing units.  In spectrally selective glazing, the SHGC can be independent of visible light transmission.  The Light to Solar Gain ratio is used to measure the effectiveness of spectrally selective glazing and is visible light transmittance divided by solar heat gain coefficient.

Adaptive Properties

Some advanced glazing systems can change their visible light transmittance, solar heat gain coefficient, and other properties. 

• Liquid crystal windows change from clear to frosted or dark when a voltage is applied by a control system, improving their privacy but not changing their solar heat gain.  
• Thermochromic coatings turn from clear to dark at high temperatures (generally when struck by direct sunlight), reducing their Tvis and SHGC.  
• Photochromic coatings turn from clear to dark when struck by light; many sunglasses use this feature.  
• Electrochromic windows change from clear to dark when a voltage is applied by a control system, also reducing their Tvis and SHGC.  
• For additional information on adaptive properties, see Building Science Fundamentals.

Other Considerations

Some other significant variables to consider with windows or other apertures are infiltration rates, light distribution angles, condensation, and acoustics.

Infiltration is air leakage through the framing of a glazing unit. Tightening a unit can improve effective U-values by 10% or more.  Standard leakage rates are .3 CFM/ft² while tight units can be as low as .02 or even .01 CFM/ft².

Light distribution angles are the direction that light is transmitted into the building.  Ordinary windows let light travel straight through, while advanced glazing units may bounce the light to different angles, or spread it diffusely through the room.  This is usually especially important for skylights.

Condensation can occur in glazing units when there is a large temperature difference from inside to outside.  In addition to being unsightly, this can cause mold and mildew, which is detrimental to indoor air quality.  Good glazing units control condensation.

Glazing generally transmits more sound than walls; this can be problematic for buildings in noisy locations.  Some glazing units have better acoustic damping than others, particularly multi-pane constructions that use layers of different material in their framing.