When thinking of some of the world’s most dramatic, visually breathtaking buildings, they most likely involve large expanses of glass. Before these architectural masterpieces can be created, the glass may need to be heat-treated for durability and/or safety reasons. The type of processing required—heat-strengthening or tempering—depends on the glass’ specific application.
There are two different types of heat-treated glasses, heat-strengthened and tempered. The similarities between the two include:
- Production using the same processing equipment
- Heating the glass to approximately 1,200 degrees Fahrenheit, then force-cooling it to create surface and edge compression
The differences between the two glasses are as follows:
- With tempered glass, the cooling process is accelerated to create higher surface compression (the dimension of force or energy per unit area) and/or edge compression in the glass. It is the air-quench temperature, volume and other variables that create a surface compression of at least 10,000 pounds per square inch (psi). This is the process that makes the glass four to five times stronger and safer than annealed or untreated glass. As a result, tempered glass is less likely to experience a thermal break.
- With heat-strengthened glass, the cooling process is slower, which means the compression strength is lower. In the end, heat-strengthened glass is approximately twice as strong as annealed, or untreated, glass.
The industry standard specification requirements for heat-treated or tempered glasses are ASTM C1048. For heat-strengthened glass, the requirement is a surface compression of 3,500 to 7,500 psi with no requirement for edge compression. Fully tempered glass will have either a minimum surface compression of 69 MPa (10,000 psi), or an edge compression of not less than 67 MPa (9,700 psi).
Safety glazings are typically specified to provide security or to keep occupants safe wherever there is the potential for broken glass to hurt people if it becomes a projectile, such as in a fire, explosion, tornado or hurricane.
Heat-strengthened glass is typically specified when additional strength is needed to resist wind pressure, thermal stress or both, and the additional strength or safety break pattern of fully tempered glass is not required. Another advantage of heat-strengthened glass is that when broken, the glass fragments are more similar in size and shape to annealed glass fragments and thus tend to stay in the opening longer than fully tempered glass particles. Although heat-strengthened glass is NOT a safety glazing by building code, this breakage pattern prevents the glass from falling and injuring someone.
Safety glazing is fully-tempered glass that meets the requirements of the safety glazing standard CPSC 16 CFR 1201 or ANSI Z97.1, or both. Safety glazing, when broken, fractures into relatively small pieces, thereby greatly reducing the likelihood of serious cutting or piercing injuries in comparison to ordinary annealed glass. For some applications, such as glass in doors, tub and shower enclosures and fixed glass in close proximity to a walking surface, fully-tempered safety glass is required by building codes and ordinances. Fully-tempered safety glass is often used for other applications where its strength and/or safety characteristic are desirable, such as table tops, counter tops, showcase enclosures, refrigeration and food service equipment, furniture and similar applications. Fully-tempered glass also is used in applications when significant additional strength is needed to resist wind pressure, thermal stress or both.
However, because of this break pattern, tempered glass is much more likely to fall from the glazing system immediately upon breakage. It’s essential for design professionals to consider this when selecting a safety glass. If it’s important for the glass to stay anchored in the glazing system, another form of safety glass, such as laminated glass, should be considered.
With both heat-strengthening and tempering, distortion is a possibility. Optical image distortion occurs in glass for many reasons, including glazing pressure, wind load, temperature and barometric pressure changes, or even changes in altitude between where a glass is made and where it is installed. Because of its fluidity at higher temperatures, glass also is inherently susceptible to roller wave, bow and warp while it is being heat-treated.
That’s why doing full-scale mock-ups under job-site conditions are recommended to evaluate the optical aesthetics of a specific heat-treating process. In addition, the following tips are suggested in order to minimize the potential impact of glass distortion that is inherent with heat-treating processes:
- Produce all heat-treated glass for a given project on the same equipment, using the same processing parameters
- Glass distortion may also occur due to strain patterns in heat-treated glass or interference fringe patterns in the fabrication of insulating glass units
- Use thicker glass, as it is less prone to distortion
- Orient heat-treated glass so that roller wave (the periodic wave imparted to glass during heat-treatment, measured by the peak-to-valley distance) is parallel to the window sill/header
While there is no industry standard to quantify permissible heat-treated glass roller wave, a tolerance of 0.005 inches is often specified. Vitro Architectural Glass (formerly PPG glass) recommends using a millidiopter specification instead, if it is available.
Safety is a prime consideration when determining the type of heat-treating process necessary for a project. Deciding whether to use heat-strengthened or tempered glass depends on the specific application. For example, heat-strengthened glass can be selected for applications that do not specifically require a safety glass product; and tempered glass should be used wherever safety glass is a requirement.
For complete technical information about using heat-treated vs. heat-strengthened glass, read Vitro Architecutral Glass Technical Document TD-138. For any other glass questions, please contact Vitro glass or call 1-855-VTRO-GLS (1-855-887-6457).
In addition to choosing the window style that’s best for your home, you may also want to consider the type of window glass you’re installing. There are six options, which all serve different purposes:
- Float glass: This is the most budget-friendly and has no natural tint. It is a single piece of glass that breaks into sharp shards when hit with force. Float glass can, however, undergo a lot of chemical processes to be strengthened. It can be used in single-pane and even double-pane windows once it’s been enhanced. Plain float glass that hasn’t been treated is typically low-cost and easier to break.
- Laminated glass: Laminated glass is typically used for patio doors, balcony enclosures, and other places where glass damage would pose a safety concern. This glass will crack when it breaks but won’t leave its frame.
- Obscured glass: In rooms where privacy is important, consider this opaque (or partially opaque) glass option. Many people choose this for personal areas like bathrooms or quiet spaces.
- Tempered glass: Tempered glass is the most popular type for bathroom use, such as shower doors, which tends to be much stronger than regular glass. It's resistant to heat and has a very low risk of danger.
- Insulated glass: Insulated glass includes double and triple-pane windows. It is designed by layering glass. It creates a barrier to outside heat and cold when used in windows. It’s also nearly impossible to shatter.
- Low-E glass: Low-E glass keeps the heat out and lets in the sunlight by blocking UV rays. It’s best suited for those who love natural light.
Ultimately, you'll want to weigh the pros and cons of each to determine what’s important to you, but in general, one kind stands out as being the best glass window for a home.
Why Tempered Glass is the Best
Typically, homeowners choose tempered glass over all others because it is essentially a combination of the other glass varieties. Check out some detailed reasons you may want to consider tempered glass for your home:
1. It’s much safer.
Glass is known for being dangerous when it breaks because of the many jagged pieces that have the potential to cause injury. Tempered glass breaks off into small, edgeless pebbles, virtually eliminating the risk of bodily harm.
2. It’s extremely tough and durable.
It doesn’t take much to break “normal” glass. Tempered glass is a different story, as it is nearly four times stronger than typical glazing. It’s also scratch resistant, so it keeps its fresh appearance longer.
3. It can lower your energy bill.
In general, tempered glass tends to be energy-efficient due to its heat-resistant properties.
4. Its possibilities for design and style are endless.
The durability of tempered glass allows it to be suitable for unique window styles (e.g., frameless structures). Unlike opaque glass, tempered glass is transparent, making it an ideal choice for those who want a clean, sparkly look for the windows in their home.
5. It has amazing sound-proof capabilities.
Tired of hearing outside noise? Tempered glass blocks out sound well since it’s thick.
While other glass types offer some sound insulation, tempered glass has the versatility to encompass this and many attributes important to making a home comfortable, energy-efficient, and safe.
How to Choose the Best Glass for Windows
Choosing the best glass windows for your home doesn’t have to be discouraging if you know what factors matter most to you. Check out the chart below to determine which type of window glass best suits your needs:
Extreme Climate Tolerance
Energy Efficiency
Privacy
Breakability
Design Versatility
Float
X
Not insulated
X
Lets air in and out
✔
Has a tint option
X
Sharp when broken and not as durable
✔
Clear, patterned, or textured
Laminated
X
Insulated but not good for high humidity areas
✔
Keeps heat out
✔
Has a tint option
✔
Cracks when broken but glass stays in frame
✔
Many colors and tones
Obscured
✔
Not insulated but can withstand cold and heat
✔
Blocks the heat
✔
Is partially or fully opaque
X
Shatters when broken (pieces fall out of frame)
✔
Multiple design options
Tempered
✔
Insulated and absorbs heat
✔
Absorbs heat
✔
Has a frosted glass option
✔
Pebble-like when broken
✔
Multiple design options including patterned
Insulated
✔
Insulated with double and triple panes
✔
Prevents heat or cold air from leaving
✔
Has a tint option
✔
Almost impossible to break
X
Few design options
Low-E
✔
Insulated and designed for indoor climate control
✔
Uses glass components for controlling temperature
X
Limited options for privacy
X
Low-E coating does not increase glass durability
X
Few design options
Double Glazing vs. Triple Glazing
The terms “double-glazed” and “double pane” are usually used interchangeably to refer to the number of glass sheets (or panes) used in the window. Whereas a double uses two panes, a triple uses three. There's usually a type of gas or resin between each layer of glass to encourage bonding and durability.
Triple glazing is not always necessary but can be useful for extra insulation in harsh temperatures and noisy environments. Double glazing may be a better choice if you want to feel isolated, but with some sound and temperature from the outside world.
Both tempered and laminated glass may be used in the construction of double and triple-glazed windows.
FAQs about Residential Glass Windows
Why is tempered glass a good choice for home windows?
Tempered glass is often the best option for home windows due to its strength and energy efficiency. You save money on energy bills and get a sturdy glass that doesn't break into jagged pieces even when tremendous force is applied.
What are the benefits of having glass windows as opposed to another type of material?
Windows are for insulating your home, securing your things, and blocking out debris. However, the purpose of glass windows varies with the kind of glass you are using. For example, obscured glass is known for giving homeowners privacy, but it doesn’t necessarily allow much natural light to come through. Conversely, tempered windows let natural light in but offer fewer design options than obscured (which can come frosted, ribbed, etc.). Each glass category is designed a little differently for consumers who value specific features.
Generally, the benefits of using glass windows include the following:
- Brighter rooms
- Lower energy bills
- Easier to clean than most surfaces
- Tougher against the outdoor elements than other materials
- Quieter, insulated spaces
Glass Doctor recognizes the significance of your choice in glass windows for your home, which is why we’re determined to help you make the right one. Whether we’re installing tempered glass or another popular glass type, our goal is always to make your home a safe and comfortable space. To have the glass window of your choice installed, call your local Glass Doctor at (833) 974-0209 or set up an appointment online.