Window Energy Concepts & Glass Technology

From CWIs “Cool Windows for a Better California”

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Introduction

 

Inevitably, someone will soon ask you how low-E glass works. It's very likely that they already have a mistaken idea of the difference between energy-related terms like U-factor and Shading Coefficient or SHGC. Or they may think that low-E glass is just another kind of window tint. It's extremely doubtful that any of your clients have even heard of spectrally selective glass.

 

You may already know that the two key window energy performance numbers are the U-factor and the Solar Heat Gain Coefficient (formerly described as the Shading Coefficient). Another critical consideration is the visible light transmittance - the amount of light that is passed through the window into the building.

 

Energy is transferred through windows by means of one or more of these methods:

 

 

 

 

The U-factor (also known as the U-value) of a window accounts for conduction, radiation, and convective heat transfer from the warm side to the cold side of the window. A lower U-factor means a better-insulated window and is most useful in determining winter heating performance.

 

The Shading Coefficient and Solar Heat Gain Coefficient account for radiant heat transfer from the sun through the window. A lower SHGC means less solar heat passes through the window and is most useful in determining summer cooling performance.

 

Conduction

 

If your hands are cold, you might put them around a mug of hot tea or coffee to warm

them up. Heat from the hot liquid will then be conducted to the mug, and from the

 
mug to your cool hands. The amount of heat transferred depends on several linked factors: how hot the liquid is, whether the cup is insulated, how long you hold the cup, and how much of your hands are touching the cup. Of course, picking up a cold cup of coffee or tea doesn’t help warm your hands - there must be a temperature differential for heat transfer to occur.

 

 

 

The molecular makeup of the material (like the cup) determines how fast or slow heat moves through it. This value is the conductivity. The U-factor is the rate of heat transfer through a specific size and shape of that material or composite of materials (like a window).

 

Convection
 
 


When a gas such as air is heated, molecules become excited

and require more room to move. Heated air molecules expand

and rise because they are less dense than the cooler air

surrounding them. As the heated air rises, cooler, denser air

below circles in to take its place. After the warmer air rises,

it cools and begins to sink as yet warmer air moves in to

replace it. The transfer of heat by this cyclical movement of

air is called convection.Convective heat transfer can take

place in large areas like a room in a building or in small

areas like the cavity between two layers of glass. The amount of heat transferred is largely dependent on the difference in temperature of the surfaces. One way to reduce convective heat transfer within the glass cavity is by eliminating most of the air and replacing it with a gas (like argon) that is less conductive and more viscous, resulting in reduced convection currents. Cold convection currents within the glass cavity and within a room can also be lessened by use of a low emissivity insulating glass unit. This occurs because the surface temperature of the glass is warmer with low-E than without.

 

Radiation

 

Sifting in front of a campfire on a cold night your face and the front of your body warms up, while your back and the air around you remain cold. Invisible infrared energy emitted by the fire is being radiated to your face where it becomes energy you can feel - sensible heat.
 
 

 

 

 

 

 

 

 

 

 

 


Similar waves of radiant energy from the sun (a very, very hot fire) are emitted in a broad spectrum ranging from ultraviolet to visible to infrared. We are most aware of the narrow energy band that makes up the visible light spectrum; however, much of the energy received from the sun is not perceptible to our eyes. Electromagnetic energy emitted by the sun is measured by wavelength. The wavelength is the distance between the "crests" of each tiny undulating wave. The nanometer (.000000001 meter, 1 billionth of a meter) is commonly used to represent wavelengths of sunlight. For instance, green light is emitted between 500 and 530 nanometers. The shorter the wavelength, the more energy delivered; that is why ultraviolet (UV) light can burn our skin. Fortunately, only a very small percentage of the sun's energy is in the UV range - about half is visible and most of the rest is infrared. High Performance Glazing Technology

 

When sunlight strikes an object, the energy is reflected, absorbed or transmitted. When direct sun strikes a typical window, a small portion of UV and infrared energy is reflected, some is absorbed by the glass and warms it, but most of the energy passes right through. When the transmitted sunlight hits surfaces inside the room, the short-wave radiant energy is absorbed and the floors, walls, furniture and other surfaces heat up. The heat from these objects is transferred to the surrounding air by convection or emitted in the form of long-wave (far-infrared) radiation. Long-wave, room-temperature radiation is also generated by heating systems, lights, appliances and people.
 
 


Some of the long-wave energy emitted inside the room finds

its way to the window surface. This long-wave radiation is

absorbed by ordinary window glass and then re-radiated or

emitted as heat, either to the outdoors or back inside, from

the glass surfaces. Window heat loss by this means can be

reduced if the glass is treated to absorb less and emit less

long-wave radiant energy.

 

The ability of a surface to reflect long-wave radiation is

Measured by its emissivity. Emissivity vanes from 1 (100%

of long-wave radiation emitted) to 0 (0% emitted). For glass,

the lower the emissivity, the lower the U-factor. Clear glass

has an emissivity of about .84 while bright aluminum foil has

an emissivity of .05.

 

Low emissivity glass coatings are designed to reflect long-wave radiation, thereby improving the thermal performance of the window as measured by the U-factor. The lower the emissivity, the greater the resistance to heat loss through the window which provides better winter performance.

 

The reflectivity and color properties of glass can also be altered to reject heat and reduce glare. However, tinted and reflective glass both have drawbacks, including appearance and a tendency to reduce the visible light allowed through the windows. While they may reduce solar heat gain, typical gray and bronze tints absorb and block a lot of visible light. Some special green and blue tints can reduce solar heat gain and transmit more visible light, but their obvious color has prevented widespread use in homes. All tinting and reflective treatments are designed to reduce the solar heat gain (SHGC) of the glass.

 

Low-E spectrally selective glass combines the best qualities of low-E and tinted and reflective glass. As with typical low emissivity coatings, the U-factor is improved - and because the emissivity is very low, the U-factor is better. In addition, the low emissivity coating has been engineered to selectively transmit visible light waves and reflect infrared heat waves. Thus the SHGC is low (which is good for reducing solar heat), the U-factor is low (which is good for reducing heat loss), and the visible light is high(which makes for bright, light interiors). Spectrally selective low-E glass improves both the U-factor and the SHGC without significantly reducing visible light transmittance.

 

Solar Heat Gain Coefficient (SHGC) and Shading Coefficient (SC) are related terms used to all describe and calculate window heat gain. Although Shading Coefficient has been used in the past, industry practice and the new California Energy Standards have switched to Solar Heat Gain Coefficient as the term for reporting solar heat gain in windows.

 

Here's the difference between SC and SHGC.-

 

Solar Heat Gain Coefficient (SHGC) is the fraction of incident solar radiation which enters a building as heat. It is based on the sum of the solar energy transmittance + the inwardly flowing fraction of absorbed solar energy on the entire window. It accounts for the glass and the frame of the window, and the SHGC value is determined for a whole window by means of an NFRC test. The SHGC value will appear on an NFRC label, just as the U-factor also appears there. If there is no SHGC value on the label, the CEC has a limited table of default SHGC values.

 

Shading Coefficient (SC) is defined as the ratio of solar heat gain through a particular glazing to the solar heat gain through a single lite of 1/8" clear glass. So if a tinted glass transmitted half as much solar gain as the reference 1/8" glass did, its SC would be 0.50. By extension, various shade treatments have also been given SC values when combined with specific glass types.

 

Throughout this document, you will see both SC and SHGC referenced. Although the specific value for each is different, they both measure the amount of solar heat gain - and for both, the lower the value the less solar heat gain is transmitted.

 

Glass Technology-The Choices

 

Window glass affects comfort more than any other component in your home.

 

Many homes just aren't very comfortable. Does winter cold or summer sun cause physical discomfort in the home? Or do they create uncomfortably high heating and cooling bills? Is there condensation on the windows, or fading carpets and furniture? Each of these headaches can be minimized by selecting the right glazing system. But faced with a wide array of glass technology, how does anyone make a choice?

 

You might already know that the U-factor of double-glazing is about twice as good as a single piece of glass. Do you know that a double glazed window with spectrally selective low-E glass can perform twice as well as typical double pane in winter and summer, even though they look about the same and are installed in the same window frame?

 

What makes one insulating glass unit better than another?

 

A visual inspection won't tell you much about the energy saving potential of the insulated glass unit in your window. Even when you know what to look for, it's almost impossible to identify a glass coating or gas fill by sight. Yet these new coatings, along with innovative spacer designs and exotic gasses are essential to a modern high performance window.

 

In this section we spotlight the technical advances that make spectrally selective low-E high performance glass so good. Insight into these glazing system concepts will show you what you and your clients should look for in your next windows.

 

With the right window glass, a home will be warmer in winter and cooler in summer. Sound good? Read on to find out how spectrally selective low-E glass systems can work in your home.

 
 
Multi-glazed windows incorporate two or more layers of glass

into a single window sash. Sealed insulating glass (IG) units are

typical in the modern window, although some early double pane

windows were made of two panes of glass welded together at the

edges. An IG unit consists of two or more layers of glass, with a

spacer around the perimeter to separate the glass layers, seal the

unit, and bond it together. As a convention, the glass surface

names are numbered, with surface one always being the outside

face of the outer piece of glass. For example, a double glazed IG

unit has four glass surfaces with surface # 1 facing outside air and

facing #4 into the room. The surfaces are identified by number so

that the location of surface coatings can be easily described and

understood. Correct surface identification is important because

the location of coatings can be important.

 

High Performance Glazing Technology

 

Low Emissivity Coatings - what are they?
 
 


Low emissivity (low-E) glass

coatings reflect invisible long

wave radiation from radiant

heat sources. Clear glass has

an emissivity of around 0.84,

meaning that it absorbs and

emits about 84% of the long-

wave radiation that strikes the

glass surface. Glass with a low-E

coating on one surface will have

an emissivity from about 0.35 to as low as 0.04, thus reflecting back 65% to 96% of long-wave radiation. Low-E coatings improve the window U-factor by reflecting long wave radiation, rather than absorbing and conducting the heat out through the glass. Since a lower emissivity results in a lower U-factor, there is a real difference between a mid-E" emissivity of 0.20 or higher vs. a truly low emissivity value below 0.10.

 

You can easily demonstrate the effect of a low emissivity material yourself, by taking a piece of aluminum foil and placing the shiny side about an inch above the back of your hand. You'll instantly feel long-wave radiation reflected back to your hand as it warms. Low-E glass coatings work in the same manner, only they're transparent!

 

The difference between low-E glass and spectrally selective low-E glass

 

In general, low emissivity coatings are designed to reduce the heat transfer caused by long wave radiation. As you may know, the sun's rays contain energy in various spectrums or wavelengths. Once solar radiation passes through glass it strikes the interior surfaces in the room and is absorbed and then emitted as long wave radiation. Standard low-E glass reflects that long wave radiation and reduces heat loss from the room. Spectrally selective low-E glass (like Cardinal LoE 2 and PPG Sungate 1000) also block long wave radiation, but they have another important function. The multiple layers of silver in the coating allow the glass to selectively transmit and reject certain wavelengths of solar radiation. Spectrally selective low-E coatings are designed to maximize the transmission of visible light and to reduce transmission of longer wavelength heat in the near-infrared spectrum. In summary, low-E glass reduces heat loss, and spectrally selective low-E glass reduces heat loss and heat gain.

 

The chart below shows the transmittance of several types of glass in the portions of the solar spectrum. An ideal glass for cooling climates might transmit zero UV and zero infrared, while transmitting 100% of the visible light. This would eliminate most UV degradation and solar heat gain without blocking any visible light you can see, spectral low-E comes closest Wavelength (manometers) to that ideal. Hard-coat low-E transmits much more heat in the infrared than spectral low-E. Ordinary tints also allow more infrared than spectral low-E and additionally block
 
 

 

 

 

 

 

 

 

 

 

 

 


Publication Information

 

© Copyright 1998 California Window Initiative

 

For more information contact:

 

California Window Initiative

604 Bancroft Way

Berkeley, CA 94710

(800) 600-9050

(510) 649-9593

Fax (510) 649-9593

 

Fact Sheet Source

 

The information in this fact sheet was excerpted with permission from the California Window Initiative’s “Cool Windows for a Better California.”