What are tempered glass windows?

What are tempered glass windows?

Chances are you’ve heard the term ‘tempered glass’ at some point. If you're considering replacement windows or building a new home, you probably have some questions concerning this topic. Is tempered glass the same as safety glass? When and what kind of windows need to have tempered glass? Do my current windows meet specifications?  In this post,  we'll examine what tempered glass is, how it's made and used, and answer some common questions including what the requirements are for its use in windows for North Texas homes.

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What is Safety Glass?

Before we specifically look at the characteristics of tempered glass, we need to address the broader topic of safety glass. Safety glass has two important characteristics that make it different from regular glass. It is specifically designed to be stronger than normal glass making it less likely to break. If it does break, then it is designed to stay together and be less prone to inflicting bodily injury by splitting into smaller shards. 

What Is Tempered Glass?

Heat-strengthened glass is cooled at a faster rate than regular annealed glass. The cooling rate affects the strength of the finished glass.  While it is stronger, it can break into large, sharp shards when shattered and not suitable for safety purposes such as shower doors. Tempered glass is cooled at an even faster rate than heat-strengthened glass.  Tempered glass is heated in a uniform manner and then cooled rapidly by blowing air on both sides simultaneously making it about four times stronger than normal glass. This process does not alter the properties of normal annealed glass and it retains its color, stiffness, and other characteristics. 

How is tempered glass made?

The glazing process for tempered glass begins with a preparation stage that includes cutting the glass to the desired size and examining the glass for imperfections. Next, the glass is put through a heat treatment in which the glass is heated to 600 degrees Celsius. 

The heating treatment is followed by “quenching”, a high-pressure cooling procedure in which the glass is cooled in seconds. The quenching cools the outer surface of the glass more quickly than the center leaving the center in tension and the outer surface in compression, this is what gives tempered glass its strength.

The added endurance caused by the differential heating means the glass can be stretched or pulled to a certain limit without breaking.

What Is Laminated Glass?

Laminated glass is made of multiple layers or lites of glass joined by a lair of plastic called PVB (Polyvinyl butyral). It is commonly used for safety glazing. Glazing, or safety glazing, refers to glass that has been processed to be less susceptible to breakage and more importantly to reduce the potential for serious injury when it comes to human contact.  Laminated glass stays in its frame because it remains attached to the plastic layer in the center.  It is most commonly used in automobile windshields and stays structurally intact in accidents, protecting motorists from getting cut on sharp shards of glass. 

Comparing the Two

Tempered glass is much stronger than laminated glass and better at resisting force up to certain levels. Laminated glass is favored for security purposes, particularly in commercial buildings. The layer of PVB makes it much harder to breach, despite it not being as strong. It is typically used as external glass. For buildings, tempered glass is often used in homes on the interior for purposes such as shower doors although it is good for tall commercial buildings for protection from the weather. 

Benefits for Home Use

In general, tempered glass is incredibly durable and reduces the risk of injury when it does break. Although it does not stay together the way that laminated glass sticks to the PVB, tempered glass breaks into small, round pieces as opposed to sharp shards with jagged edges. Broken tempered glass is easy to clean up with a vacuum or swept up using a broom and dustpan.

  • Safety: Tempered glass reduces the risk of injury due to its nature of breaking into small, circular pieces instead of sharp, jagged shards.
  • Easy clean up: Tempered glass isn’t indestructible. If the glass breaks it fractures into small fragments instead of large, jagged pieces.
  • Strength: Tempered glass is incredibly durable.

When is Tempered Glass Required in DFW?

According to the Residential Building Codes for Dallas, safety glass must be used in all doors (using either laminated or tempered glass). Additionally, safety glass is required in the following circumstances:

  • Glass that is within eighteen inches of the doors vertical edges when the door is in a closed position.
  • Windows that are within eighteen inches of a walking surface and in cases where a  piece of glass is over nine square feet of unobstructed glass.
  • Within 60 inches of a bathtub or shower drain or "in the vicinity of" a pool area including all shower doors and tub enclosures.

Outside DFW

Ordinances will vary based on where you live.  However, according to the National Glass Association, there are four criteria that require safety glass. These guidelines are the basis for local building codes.

  • The glazing (glass) is less than 18 inches above the floor
  • The top of the glass is less than 36 inches above the floor
  • The size of the glass exceeds 9 square feet
  • The glass is within 36 inches of where people walk

Any doors made of glass — sliding, French, or shower doors — are all always made of tempered glass.

Frequently Asked Questions About Tempered Glass

Is tempered glass bulletproof? 

No. Bulletproof glass is much more advanced, although glass tempering is one component of the process used to manufacture it.

What causes tempered glass to break?

Tempered glass can be broken in many of the same ways ordinary glass is broken, especially if the tempered glass is defective. Spontaneous or delayed breakage is also characteristic of tempered glass. This means that if the glass is impacted at a pressure point the glass may not release (break) immediately and there might be a delay before the damaged glass finally breaks. In some circumstances, the glass may appear to break randomly and for no reason.

Does tempered glass cost more? 

Tempered glass does cost more than standard annealed glass. Initial price estimates may not include the cost of tempering glass unless it has been specifically requested. Make sure to discuss the added cost with your window retailer if you are requesting tempered glass or it's required to meet the building code. Some windows require tempering but not all.

Homeowners often ask for our thoughts on tempering all of the windows on their order and we don't recommend it unless the window or door requires tempered glass by code then, yes, of course purchase the tempered glass. If it's not required by code then we generally say go with regular glass to save some money. Homeowners living near a golf course might consider additional tempered glass for some exterior windows if they are concerned their windows are at risk. Tempering glass can increase the cost of a window anywhere from 15% to 50%, depending on the size of the glass. Despite the higher cost of tempered glass, it is still cheaper than laminated glass. 

Is tempered glass considered safety glass?

Yes, absolutely. The manufacturing process for tempered glass makes it 4-5 times stronger than ordinary (annealed) glass. This process also makes the glass less dangerous in cases of breakage.

Organizations began working to improve safety standards around the glass in the 1960s because hospitals were treating thousands of glass related accidents. Safety glazing standards resulted in fewer broken windows and doors and less severe laceration injuries and deaths. 

Is tempered glass bulletproof?

No. Tempered glass is not bulletproof, however, it is a component of bulletproof glass.

Does tempered glass shatter?

Not exactly, although tempered glass can break it does not shatter. Unlike ordinary glass (annealed glass), which shatters into sharp shards when broken, tempered glass is designed to break into smaller pieces.

Sometimes they don't disengage and instead interlock with neighboring pieces. If the pieces interlock and don't fall this is OK because you can see that the glass needs to be taken down immediately and replaced. If the glass interlocks and falls in clumps it can be dangerous because the edges are jagged and abrasive.

Ask your window retailer about tempered glass

Tempered glass is the preferred choice for home applications where safety glass is required. Its design and characteristics make it resistant to damage and lessen the chances of injury if it does break and is easy to clean up. It is also the most cost-effective choice.  If you live outside DFW and have questions about your glass or are thinking about window replacement, contact a reputable professional in your area. If you are in DFW, contact us and one of our friendly and knowledgeable consultants will follow up with you in a timely manner. 

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     -Bobby

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