Reflective, Low Emissivity, and Insulating Glass

Modern glass is called upon to perform many tasks. An important one is to control the amount of heat and light that passes through the glass. Three types of glass designed for this purpose are:
· Reflective Glass
· Low Emissivity Glass
· Insulating Glass

Reflective glass is clear or tinted glass that has a very thin layer of metal or metallic oxide on the surface. The reflective coating reduces heat gain and glare from the outside while allowing visible light to enter. Characteristics include:
· Appearance. Reflective glass gives a building a mirror-like appearance. The coatings are available in silver, copper, gold and earthtone. They can be combined with tinted glass to give a building a beautiful exterior.
· Energy savings. Because it reflects and absorbs the sun’s rays, reflective glass reduces the amount of solar radiation that enters the building. This can save money in heating and air-conditioning costs.
· Comfort. Reflective glass reduces variations in the interior temperature of a building.

Three different processes are used to deposit the coating on the glass:
· Wet Chemical Deposition
· Vacuum Deposition
· Pyrolitic Deposition

Wet Chemical Deposition-In this process the glass is submerged in a tank containing a chemical solution. The metallic oxide is transferred to the surface of the glass by a chemical reaction.
The coating is very fragile and must be protected immediately. The usual method of protection is to use the glass in an insulating glass unit or in a laminated glass product.

Vacuum Deposition-In this process the glass is placed in a vacuum chamber containing a special atmosphere. When electrical energy is added to the chamber, a complex magnetic reaction takes place that causes the metal atoms to strike the surface of the glass at high speed. The atoms coat the surface of the glass uniformly. This process is commonly called sputter coating. Sputter coated reflective glass has a few disadvantages:
· It cannot be heat strengthened or tempered because the heat would destroy the coating. And, since the coating increases the amount of the sun’s rays the glass absorbs, it may be necessary to heat treat the glass before coating.
· The soft coating can be damaged easily before installation.
· The glass has limited compatibility with sealants.

Pyrolytic Deposition-The word pyrolytic is used to describe a change brought about by heat. In pyrolytic deposition the metallic oxide is added to the glass while the glass is hot. This can be done in an oven or during the process of making the float glass. Generally, pyrolytic coated glass is installed with the coated side facing outdoors. The coating itself reflects most of the sun’s rays before they reach the glass. In many cases, this eliminates need for heat strengthening or tempering that might be required with other types of reflective glass. Characteristics include:
· Pyrolytic coatings are more durable than wet chemical or sputter coatings.
· Annealed glass with a pyrolitic coating can be heat treated or tempered without affecting the coating.

Low emissivity glass, commonly called low E glass, is a type of reflective glass that is gaining in popularity, especially in residential and office applications. Low E coatings are very thin metallic coatings that reduce visible light transmission by about 10 percent compared to uncoated glass. They are applied using either the vacuum (sputter) or pyrolytic process. Characteristics include:
· Low E glass:
· Reduces heat loss through windows.
· Re-radiates the heat absorbed from sunlight back inside the room.
· Allows sunlight into a room without letting heat escape outdoors.
· Resists ultraviolet light, which results in less damage to carpets, draperies and other furnishings.
· Reduces glare.

The main reason low E glass has these advantages is that it reflects sensible heat. The heat generated by hot water or steam radiators or the heat from hot air ducts are examples of sensible heat. Low E glass retains more of this heat indoors than other types of reflective glass. In northern areas, low E coatings let in the heat from the winter sun while retaining the heat generated from inside the building. In southern areas, low E coatings are usually applied to bronze, green or gray tinted glass. They reduce glare and reflect the sun’s heat away from the structure.

Insulating glass units, commonly called ig units, are made from two or more lites of glass separated by a sealed air space. The metal tube around the perimeter of the insulated unit which seperates the two lites of glass is called the spacer. This spacer comes in thicknesses of 3/16″ and larger. It is filled with a special moisture absorbing material called a dessicant. The perimeter of the entire unit is sealed with a high grade sealant. Characteristics include:
· IG Units:
· Reduce the tendency of condensation to form on the room side of the glass.
· Reduce cold transmittance at windows and helps maintain a uniform temperature. In the winter, ig units reduce heat loss and in the summer they reduce heat gain.
· Reduce the level of noise from the outside.

There are two types of ig units commonly manufactured:
· Single Seal Units
· Double Seal Units

The difference between the two, as their names suggest, is the presence of a single or double seal between the spacer and the glass.

Every ig unit is fabricated according to a set process:
1. The glass is cleaned.
2. The spacers are cut to size.
3. A corner key is inserted into one end of the spacer.
4. The spacers are filled with dessicant.
5. The spacer frames are assembled.
6. If the unit is to be double sealed, a ribbon of polyisobutylene (PIB) tape is applied to one side of the spacer frame. This tape forms the primary seal.
7. The frame is set on the first lite.
8. If the unit is to be double sealed, a ribbon of PIB tape is applied to the facing side to the spacer frame.
9. The second lite is set. The lites are usually clamped together or secured by a weight.
10. The perimeter is sealed.

Single-sealed units can use several types of sealants:
· Hot melt butyl
· Polysulfide
· Silicone
· Urethane

Double-sealed units can use PIB tape for the primary seal and hot melt butyl (one part silicone, or two-part polysulfide) for the secondary seal.

IG units need not use the same type of glass. Tempered and annealed glass can be used in the same unit. Patterned glass can be used but the pattern should face the outside. If one of the lites is reflective or tinted glass, it must face the exterior. If reflective glass is to face the interior, it may be necessary to temper one or both lites to guard against thermal breaking. A sandblasted finish is not recommended for an insulating glass unit because sandblasting reduces the strength of the glass.

Each surface of an insulating glass unit is designated by number:
Number 1 surface – faces the exterior.
Number 2 surface – inside of the first lite.
Number 3 surface – faces the number 2 surface.
Number 4 surface – faces the interior.

When reflective glass is used in an insulating glass unit, the surface on which the reflective coating is placed makes a great deal of difference. For example, a bronze coating placed on the number 1 surface creates a mirror effect. The same coating placed on the number 2 or 3 surface creates a bronze tinted effect.

How Low E, Reflective & IG Contributes to Energy Efficiency
Low E, reflective and IG glass contribute to energy efficiency by increasing the effectiveness of the insulating system. Energy efficiency is measured in two ways:
· U value
· R value

The U value is a measure of the heat gain or loss through glass due to the difference between the indoor and outdoor temperatures.
· The lower the U-value, the less heat is transmitted through the glass.

The R value measures the overall resistance to heat transfer. The R-Value is the reciprocal of the U-Value.
· The higher the R-Value, the less heat is transmitted through the glass.

For example, a material with an R value of 19 is a much better insulator than one with an R value of 6.