What Is Safety Glass? (Impact Window FAQ)

When it comes to impact windows, we often talk about how they use safety glass instead of annealed glass to enhance their strength and durability in the face of severe weather events. But what is safety glass, exactly? 

Before you get new impact windows, you might like to know more about the parts that go into these wind mitigation measures. If so, this blog’s for you! Read on to get more information about what safety glass is, the different kinds of safety glass (and which one gets used for impact windows), how to tell safety glass apart from annealed (i.e., standard) glass, and more!

What Is Safety Glass? 

Safety glass is the name for a variety of specially-treated or reinforced types of glass that are designed to either resist breaking or be less dangerous when breaks occur. 

It is a standard for impact windows to have safety glass. If someone’s selling windows that don’t have impact-rated safety glass, they aren’t really impact windows!

The Different Types of Safety Glass

So, what are the different types of safety glass? There are three main types of safety glass that are used in different industries. Two of them are common for residential impact windows, one is mostly used in commercial applications where aesthetics are less of a concern.

Let’s go over each safety glass type:

Tempered Glass

Tempered glass is treated by being subjected to extreme temperature differentials, first heating the glass up, then rapidly cooling it. This increases the strength of the glass and makes it shatter into small, blunt pieces when the glass does break.

The dulled cubes are less likely to cause cuts and embed themselves as glass foreign bodies than annealed glass, which is why this is considered a type of safety glass. Often used in home fixtures like shower doors for its high durability and relative safety compared to annealed glass. Plus, it tends to be less costly than laminated or wired glass, as no extra material is needed.

Laminated Glass

Laminated glass takes two panes of tempered glass and joins them together with a laminate interlayer material like PVB or SGP. 

PVB, or polyvinyl butyral, is a semisoft, flexible laminate material that excels at absorbing and diffusing impacts, which allows it to easily flex with blows without breaking or tearing. This has the added benefit of making the material extremely good at soundproofing, deadening soundwaves as they pass through the laminate.

SGP, or SentryGuard Plus, is a proprietary material that is extremely rigid and strong. It repels force extremely efficiently so that glass panes using it are extremely resistant to breaking.

Both interlayer materials adhere to the glass strongly so that, when the glass pane breaks, the shards stick to the laminate instead of falling away. This helps reduce the risk of shards and glass spalling.

Residential impact windows typically have at least one layer of laminated glass, plus another layer of tempered glass.

Wired Glass

Wired glass has a wire mesh embedded into it to meet fire codes at the cost of visibility. When the glass over the wire breaks, the pieces of glass tend to stick to the wire mesh, holding the glass pane together. This can be helpful during fires, preventing the glass from completely falling apart due to heat stress cracks.

Often used for commercial applications like fire exit doors or security barriers. Not preferred for residential applications.

How to Tell if Window Glass Is Safety Glass

Impact windows in Florida typically use laminated safety glass rather than simple tempered glass or wired glass. So, the easiest way to identify if a window product genuinely has safety glass is to look at the thickness of the glass panes in the window.

A laminated glass pane will be noticeably thicker than an unlaminated, annealed glass pane. If you look at a cross-section of laminated glass, you’ll see three distinct layers of material: two panes of glass and a differently-colored film between them. 

Impact windows will have at least one laminated glass pane that should be more than twice as thick as normal glass. Here’s a cross-section view of a window to highlight the difference:

However, this can be more difficult to spot from the head-on view of the glass.

Another indicator of a window having safety glass is the sash’s weight. Because laminated safety glass has three layers of material compared to standard single-layer glass panes, laminated glass tends to weigh significantly more than non-impact glass.

So, another way to tell safety glass apart from basic annealed glass is to try to lift it. The heavier the pane is, the more likely it is to be laminated safety glass.

What’s the Difference Between Safety Glass and Annealed Glass?

We’ve already covered some of the major differences between safety glass and annealed (standard) glass, but how do they differ mechanically? How much stronger is the laminated safety glass than annealed glass? 

Tempered glass is already significantly tougher than annealed glass. How much tougher? Sources like Mannlee Building Products state that tempered glass is 4-5 times stronger than annealed.

When laminated, tempered glass becomes even more effective at preventing breakage and forced entry. A PVB interlayer has a tensile strength of 30 Mega Pascals (MPa) while SGP has a tensile strength of 60 MPa (Source: North Glass). Standard annealed glass has a specific yield strength of about 26.9 MPa while tempered glass has a specific yield strength of 80.8 MPa (Source: Glass Properties).

What the laminate interlayer does is hold the glass in place even if the glass is broken. This allows the glass to continue providing protection even after extreme impacts that cause the glass to crack. This makes laminated glass immensely more resistant to forced entry than annealed glass, going far above and beyond the flat 5x strength improvement of tempered glass.

Does Safety Glass Cost More than Standard Glass?

Yes. Safety glass, because it goes through extra processes and, in the case of laminated safety glass, has more layers of material, it tends to cost more than similarly-sized annealed glass of identical thickness.

However, that extra cost is more than justified by the sheer strength of safety glass compared to annealed glass.

With annealed glass, even something as weak as a typical golf ball hitting your window can break it. Meanwhile, an impact rated window with laminated, tempered safety glass can repel golf balls all day without chipping at all.

As a matter of fact, when testing a window’s impact resistance before approving it for use in residences in wind borne debris regions (WBDs), one of the primary testing methods an impact window goes through is firing a series of small steel balls at the window glass before subjecting it to a cyclic wind pressure test.

If the window glass breaks and lets air pass through it during the cyclic pressure half of the test, the window will fail and not be approved for residential use in WBD zones. This is known as the Small Missile Impact Test. 

Get Professionally-Installed Impact Windows with Laminated Safety Glass

Want to get windows that actually improve your home’s safety and security? Impact windows with laminated tempered safety glass are a great option! They can hold back the wind, rain, and flying debris endemic to hurricanes without breaking. 

Hurricanes aren’t the only threat that impact windows can thwart. With layers of laminated safety glass and reinforced frames, impact windows can stand up to forced intrusion attempts for much longer than plain glass windows, stopping many intruders from being able to access the inside of your home and buying time to get to safety or summon the authorities. 

But, to get the most out of your new impact windows with laminated glass, they need to be professionally installed! Expert installation ensures that your windows are properly anchored to your home so they can remain steadfast and prevent leaks or penetration when the window frame is exposed to heavy pressure or sudden impacts.

Karoly Windows & Doors works with the top impact window manufacturers to source the best windows and backs that up with expert installation from a team of dedicated professionals. We never outsource our installation work to a third party company. That way, you can rest easy knowing an impact window installation specialist is putting in your new windows with the right tools and training to do the job the right way.