FAQs About Glass: All You Need to Know

Q: How is Glass Made?

A: The majority of glass in developed countries is produced by the float process. The composition materials of glass are mainly limestone, soda ash, and silica sand. These are melted in a furnace and then fed into a float chamber and a molten tin bath. The glass is then moved to a controlled cooling chamber called the annealing lehr, where it gradually cools. During these stages, the glass has the shape of a runway or ribbon. It is cut to the desired size and packaged later on. For more details about the making of glass, please check out this post.

Glass can come in various tints and colours

Q: What Are the Most Common Types of Glass?

A: Annealed glass, which is float glass made in the way described above is the most common type of glass. Besides the basic clear glass, it can also be made in a wide array of tints and colours to please all tastes and preferences. However, we can take it a step further and fabricate annealed glass in many different ways. For example, there are various types of coatings that can give glass interesting visual effects while it can also be heat-treated to acquire the properties safety glazing demands (increased strength – see below for more information on safety glazing). That being said, two or more pieces of glass can be separated by dry air space (see insulating glass units) to provide increased insulating properties.

It is also common to have two or three pieces of glass “bondedtogether. This is used in the making of auto glass. Inside each sheet of glass, there is a layer of plastic that is usually tinted to act as a UV filter. The bonded glass goes through an autoclave (furnace) that helps unify the plastic layer and the separate glass sheets. Once this is done, the glass bears high tearing resistance properties, meaning that even if the glass breaks, it will maintain its transparency and won’t scatter or hurt the driver. You can find out more about how is auto glass made, here.

Q: What is the Difference Between Solar Heat Gain Coefficient (SHGC) & Solar Shading Coefficient (SC)?

A: They are both terms that describe the amount of solar energy blocked from passing through a glass product; each in a different way. The SHGC is the portion (%) of solar energy that enters a space either directly or indirectly through a glass product *. Solar transmittance is the direct gain portion while the % of solar energy that is absorbed and then transmitted indoors through convection is the indirect gain portion. So, for clear glass, we have an SHGC of around 0.86, 0.84 of which is solar transmittance (direct gain) and the remaining 0.02 is convection (indirect gain). The lower the Solar Heat Gain Coefficient, the lower the solar gain.

The Shading Coefficient (SC), on the other hand, is an older term that is now been replaced by SHGC and represents the ratio between the solar heat gain for a specific type of glass and that of a double-strength clear glass. We can say that it is a measure of heat gain through glass from solar radiation. The lower the SC is, the lower the solar heat gain. For colder climates, it is recommended to prefer windows with higher SC while areas with a long air-conditioning season will benefit from low SC.

* The baseline is 1/8” clear glass.

Q: What Glass is Best for Sunbelt Areas?

A: The 3 most important factors to consider when choosing the glasses for windows are Solar Heat Gain Coefficient (SHGC), meaning the fraction of solar heat that can pass through the window glass, Visible Transmission (VT) – the amount of light that can enter a room through the window glass, and U-Value. The southern and southwestern regions of the US, are best protected against increased solar heat with glass products that have a low shading coefficient or low solar heat gain coefficient so that homeowners and businesspeople can reduce air-conditioning costs and not burden the environment. The insulating performance, as well as the u-value, are both of lesser importance.

At this point, it is important to know the difference between long- and short-wave infrared energy so you can make the best pick. Short wave infrared energy is the energy that is generated by the sun. It does not feel as heat until it strikes an object. Long-wave infrared is heat radiating from a surface (i.e. the roadway on a hot day).

Short wave infrared energy is the energy that is generated by the sun. It does not feel as heat until it strikes an object. Long-wave infrared is heat radiating from a surface (i.e. the roadway on a hot day).

Q: What Glass/Window is Best for Cold Climates?

A: The recommended SHGC value is 0.55 for cold climates. To eliminate the need to use energy consuming lights, it is recommended to use glasses with higher VT value. For cold climates, the value should best be less than 0.33. For countries/areas with extreme weather conditions and below zero temperatures, like Ontario, Canada, it is suggested to go for multi-pane (double- or triple-pane) windows, instead of single-pane ones, to increase their energy efficiency. This is because the vacuum created between the panes will act as a fine insulating material. It is also paramount to fill insulate gasses (i.e. argon, xenon or krypton) between the different panes to reduce energy losses even more.

You may also consider low-emissive (or Low-E – see below) glass panes that bear a metallic oxide coating (low-e coating) on the outer surface of the innermost glass, which prevents heat from the inside of the house or office space to escape.

Finally, storm windows (they are mounted on the outer or inner side of the main window) can help increase energy efficiency in climates with severe weather conditions. To reduce condensation, you can use warm-edge spacers.

Q: What is Low-E Glass Coating?

A: Glass absorbs light or heat energy. When that happens, the energy is either re-radiated by the glass surface or transferred by moving air. Emissivity is a material’s ability to radiate energy. Darker coloured materials have a high emissivity while highly reflective ones have a low emissivity. What all materials have in common is that they radiate heat in the form of infrared energy (long-wave), which is relevant to the temperature and emissivity of their surfaces. Reducing the emissivity of your window glass surfaces will improve its insulating properties. Low-E glass coatings (microscopically thin coating – layers of low emissivity materials – that reflects long-wave heat or infrared heat) help minimise the amount of UV and infrared light that can pass through glass without reducing the amount of visible light transmitted. You can also find Low-E coatings that reflect short-wave solar infrared energy too. In other words, Low-E coatings help maintain temperature within the home at optimal levels, preventing heat loss during the winter months and heat from the outside to enter your home in the summertime.


Q: Should I Apply the Coating on the 2nd or 3rd surface of an Insulating Glass Unit? What is Best?

A: Know that the U-value and light transmittance of the glass unit will not change whether coating is on the 2nd or 3rd surface. What will change, though, is the Solar Heat Gain Coefficient, which will be higher if the coating is on the 3rd surface. However, it all depends on the design conditions of your specific application. If, for example, you live in a cooling climate and want to reduce solar heat gain, it will be best to use the coating on the 2nd surface. In heating climates, a coating on the 3rd surface will allow you to utilise passive solar heat gain.

Q: Can I Use Tinted Glass with Low E-Glass?

A: Absolutely. You may use tinted glass in an insulating glass unit (as the outdoor lite) to reduce the amount of solar energy entering your home through the unit and opt for Low-E glass as the indoor lite to improve the insulating properties of the unit.

Q: What is Safety Glazing?

A: In most residential doors and windows, glazing means glass. The type of glass we usually see in doors and windows is flat glass (various types). However, it can also be glass blocks, as well as plexiglass, which is why we tend to use the term glazing more often than glass.

Safety glazing should not break into large pieces that can cause injuries to the residents of a house. Tempered glass is the most common safety glazing in residential applications and is made by heating glass and then instantly cooling it. This gives the glass a greater resistance to direct impact, compared to ordinary glass. You may also come across heat-strengthened glass that is also referred to as tempered glass. However, it is not as strong as real tempered glass; hence does not qualify as safety glazing.

If, for example, you live in a cooling climate and want to reduce solar heat gain, it will be best to use the coating on the 2nd surface

Car windshields usually come with laminated glass, which might qualify as safety glazing. It has 2+ sheets of glass bonded together with a plastic sheet. Another type of laminated glass is bullet-resistant glass.

Older homes and skylights bear wired glass, which is glass with a wire mesh inside. This is not considered safety glazing.

In short, safety glazing is glass made in a specific way so that it reduces the risk of injury.

Note: You may also hear of the term security glazing. Glass products with security glazing usually have multiple layers of (usually laminated) glass to achieve max impact resistance from anything from forced entry to ballistic assaults and explosions.

Q: What are the Most Common Applications of Safety Glazing?

A: According to federal law, safety glazing should be used in areas/locations within a building or a home that could potentially cause an accident or risk of injury to the people in it, such as shower and bath enclosures, doors and immediately adjacent sidelites, and passages with walking surfaces adjacent to the bottom edge of the glass and the glass (within 18 inches of the floor).

For residential overhead glazing, there are building codes that require the use of tempered glass. For commercial building applications, the glass should be laminated. For the instances where tempered glass is allowed in commercial applications, there is usually some kind of restraining screening right below the glass. You can identify safety glazing products by their marking usually located in a corner of the glass piece.

Category: Glass EssentialsWindow Glass