How-To

A Homeowner's Guide to Understanding Glass Types + Performance

You don’t need to completely understand the technology, but you should know that you have options.

When purchasing a window, most homeowners think about the type of operation they want—casement, double hung, glider, etc. They also consider the color and maybe even how a window’s aesthetics complement the architectural style of their home. But often, little consideration is given to the glass itself. Many people don’t even know they have an option of choosing one type of glass over another. Here is a helpful guide about the performance of glass, glass types, and the various glass options. Whether building a new home, remodeling, or considering replacement windows, this article will give you a better understanding about which window glass will be best for your home.

Two Panes are Better than One

While glass objects were being made as early as 2,500 BC, glass windows did not become a standard feature in the average home until the 17th century. For the next 300 years, windows were constructed with a single pane of glass, which worked fine for letting in sunlight, providing views, and preventing wind and rain from entering a home, but one pane was not effective at keeping homes warm in the winter or cool in the summer. In the 1950s, residential glass manufacturers started combining two panes of glass separated by a small air space. The motionless air between the panes acted as an insulator and reduced the buildup of condensation in the winter. This new technique decreased the dependence on storm windows but was just the beginning of the improvements to come.

Low E Coatings Block Radiation

Radiation from the sun, in the form of infrared light, can shine through a window and heat up the objects inside a room, which is not what you want on a hot summer day. Also, warm objects themselves can emit heat radiation, which means a warm room can lose heat by radiating it right through the window glass to the outdoors, and that’s less than desirable on a cold winter night. Glass alone will not reduce the effects of radiant heat transfer, but low emissivity (Low E) coatings can.

Low E coatings are microscopically thin metal coatings added to the surface of window glass. They are designed to reflect radiant heat, which reduces heat gain in the summer and heat loss in the winter. Low E coatings can also block ultraviolet light (UV) produced by the sun. UV radiation can cause flooring, furniture, and other indoor objects to fade and deteriorate. Ultraviolet light is also the form of radiation produced by the sun that causes damage to our skin.

A Low E coating is hundreds of times thinner than a human hair, so it’s only noticeable as a slight tint. They are often made of silver because of that metal’s ability to reflect radiation without significantly altering visibility. Adding additional layers and switching which surface they are applied to will change the performance of the glass. There are different proprietary names for these coatings, but one way to distinguish one type of Low E glass from another is by how many coatings have been applied. Low E glass has one coating, Low E2 has two, and Low E3 has three.

Insulating Glass with Gas

The first examples of dual pane glass were created by physically welding two panes together around the perimeter. While an improvement over single pane glass, the process only allowed the two panes to be separated by a very small space, less than 1/8 inch. Glass manufacturers eventually discovered that they could achieve greater insulating values by separating the two panes by about 1/2 inch and filling the space with argon, an inert gas found in the air we breathe. This type of assembly is referred to as an insulating glass unit. While insulating glass helps thermal performance, indoor comfort levels are still mostly controlled by Low E coatings.

How Glass is Rated

Here are the three primary ways window and door glass is rated.

Example of a window label that lists energy performance ratings.

1. U-Factor

Simply put, U-factor measures how well a window keeps heat inside your home. It’s a measure of total heat flow through a window from room air to outside air. Lower numbers indicate greater insulating capabilities. It’s a particularly important measurement for climates with colder winters.

2. Solar Heat Gain

If U-factor denotes how much heat leaves your home, the Solar Heat Gain Coefficient (SHGC) measures how much radiant heat enters your home. All you really need to remember is: The lower the number, the less heat a window lets in.

3. Visible Transmittance

As you may have guessed, this measurement describes how clear the glass is. While Low E coatings are thin, each layer does reduce the visibility just a little. The higher the visible transmittance number, the clearer the glass.

General Guidelines
Zone	Windows U-Factor	Windows SHGC
Northern	≤ 0.22 = 0.23* = 0.24* = 0.25* = 0.26* *Equivalent Energy Performance	≥ 0.17 ≥ 0.35* ≥ 0.35* ≥ 0.35* ≥ 0.40* ≥ 0.40*
North Central	≤ 0.25	≤ 0.40
South Central	≤ 0.28	≤ 0.23
Southern	≤ 0.32	≤ 0.23

Which Window Glass is Best for Your Area?

If you live in a colder climate, you want to minimize heat escaping your home and maximize radiant heat from the sun entering your home in the winter months, which means a lower U-Factor and higher SHGC.*

If you live in a warmer climate, heat escaping your home is not as important as preventing the sun’s radiant heat from entering, which means a higher U-Factor and lower SHGC.

There are additional factors when considering Low E coatings. If you want to protect grandma’s family heirloom or a valuable art collection, you may want to add specific coatings that will reduce the harmful effects of UV radiation, even if you live in a cold climate. In far northern climates the sun will not shine directly through windows located on the north side of the building, so it may be unnecessary to install windows with a high SHGC to take advantage of the warm sun rays. Large shade trees can reduce the need for windows with a low SHGC. If perfectly clear glass is important to you, remember that the more coatings you apply to a window, the more tint it will have.

The bottom line for glass performance is this: When building a new home or replacing windows in your existing home, make sure to have a conversation with the installation contractor or window dealer. Let them know which glass characteristics are most important to you.

*In northern climates it’s sort of a “pick your poison” when it comes to SHGC. If you choose a window with a low SHGC, you could benefit from the sun’s heat in the winter, but you also may have to manage the heat from the summer sun with shades, large overhangs, shade trees, etc.

Is Triple Pane Glass the Best Solution?

Windows with triple pane glass are more expensive. If you live anywhere but the coldest climates, the extra cost is unlikely to pay off in energy bill savings within the lifespan of the windows, especially if the rest of your home is poorly insulated. It’s not just the extra pane of glass that increases the cost. Adding a third pane increases the thickness and weight of an insulating glass unit, so window sashes, frames, and hardware need to be reconfigured to accommodate that extra size and weight.

All that said, an additional pane of glass creates more space that can be filled with argon and another surface in which to install another Low E coating. More insulation can provide an extra layer of comfort, especially in cold climates. Another advantage is that condensation is less likely to form on the inside of the glass when the temperatures dip far below freezing. Many people who live in a loud city or near an airport choose that extra layer of glass because windows with three panes will reduce more outdoor sounds that reach the inside of a home.

Cutaway of a window with three panes of glass.

Tempered Glass is Tough

Tempered glass is considered "safety glass." It’s manufactured to be up to four times stronger than standard glass. Also, if tempered glass does break, it shatters into tiny pieces that are much less likely to cause an injury than the large, sharp shards of falling glass that are generated when standard glass is broken.

Building codes often require windows to be made with safety glass if they’re installed in a location where there is an increased risk that a resident could fall and collide with a window. Examples include areas where a window is installed close to the floor, in bathrooms near a tub or shower, and stairways. All glass doors and extremely large windows also require safety glass like tempered glass.

Laminated Glass Holds Together

If you live on a golf course and someone drives a ball into your bedroom window, chances are, the glass will break and fall into the room in large dangerous shards. If your bedroom window is fitted with tempered glass, the odds are greater that it will withstand the impact, but there’s still a chance it will break and spray small fragments of glass into the room. If that same ball hits a pane of laminated glass, the glass could still break, but if it does, the sharp fragments will stay intact inside the window sash.

Like tempered glass, laminated glass is considered “safety glass.” It’s constructed by sandwiching a thin, near-invisible sheet of plastic between two panes of glass. When a laminated glass pane is broken, the plastic holds all of the pieces together. In addition to preventing airborne glass, laminated glass is strong and cannot be cut with a standard glass cutter, so it offers an additional layer of security against forced entries. Another benefit of laminated glass is that it’s better at dampening loud sounds than non-laminated glass.

Image of a Marvin Signature Ultimate Casement window with smashed glass.

Optical Distortion in Heat-Strengthened and Tempered Glass

The use of high-performance glazing, such as laminated heat-strengthened glass, enables windows and doors to withstand harsh coastal conditions. Engineering glass to perform under extreme conditions can create optical characteristics and distortions that occur as a result of the strengthening process required for these types of glazing.

What is Optical Distortion?

Optical distortion happens when an object viewed through the glazing may appear to have a "wave" or "bend" due to the lensing effect of the glass. Generally, this effect is more pronounced when viewed from an angle.

What Causes Optical Distortion?

Optical distortion can be caused by the heating and quenching process that glass undergoes to produce the increased strength characteristics required for many applications. Glass is passed through an oven and heated to the point of softening. Air blowers then rapidly cool the surface of the glass as it leaves the oven in a process called quenching. This rapid cooling places the surface of the glass under compression. The surface compression increases the strength of the glass to four to five times over the strength of standard annealed glass for tempered glass, and approximately twice as strong as annealed for heat strengthened glass. However, the surface compression can also induce a slight waviness to the glass as it is rapidly cooled. This waviness can result in optical distortion. At high temperatures the glass is subject to during heat treating, glass is also more susceptible to roller waves, bow, and warping. Optical distortion is not a defect, but rather a result of the strengthening process.

Impact-Resistant Glass can Protect People and Property

Homes in many coastal regions need extra protection from high winds and flying debris generated from tropical storms and hurricanes. Impact-resistant glass, sometimes referred to as impact glass or hurricane glass, is stronger than standard safety glass. Impact-resistant glass is laminated but made with a thicker plastic inner layer and may contain thicker panes as well. Some impact-resisting glass units are assembled with both laminated and tempered glass.

Impact-resistant glass is tested by hurling 2x4s at it at speeds near 35 mph and small steel balls at near 90 mph. It holds together amazingly well, especially when installed in a thick aluminum frame. And when the storm winds do blow, it’s important to have a window you can rely on, because if the window glass, sash, or frame should fail, a house in a hurricane can become pressurized, which could cause the walls to be pushed out and the entire roof to be blown off.

Decorative Glass

From the early days of glass making, it took a few thousand years to figure out how to make glass perfectly clear. Today, clear glass is not always what’s called for. Tinted, frosted, and obscured glass can be used on windows in bathrooms, in sidelights attached to a front door, or in any area in a home where both privacy and natural light are important. Textured glass can also add a touch of distinction on doors with glass panels.

Divided Lites

Up to the middle of the 19th century, large panes of glass were fragile and expensive to manufacture. In order to create large windows, muntin bars were used to join smaller individual panes. Today, expansive insulating glass units are stronger, less expensive to manufacture, and more energy efficient, so true muntin bars have largely been replaced with simulated divided lite (SDL) bars, which rest on the surface of the glass. Spacer bars can be added inside the glass between the SDL bars, which creates a more accurate replication of muntin bars. Grilles-between-the-glass (GBGs) are also available. As the name suggests, GBGs are decorative bars installed between the glass panes with no SDL bars on the surface. From the street, GBGs look like muntin bars, but because they live between the two layers of glass, the glass is easier to clean and maintain.

What About the Rest of the Window?

When it comes to performance, durability, and protection, glass is only one part of the equation. The type of material a window frame and sash are made from matters, as does the craftsmanship that goes into building a window. Materials like fiberglass do not expand and contract or become distorted like some vinyl products. Window distortion can subject an insulating glass unit to uneven pressure, which can lead to a seal failure that allows the argon gas to escape. Without the argon, a window’s insulating properties are diminished. Broken seals can also cause condensation to form between the two panes, which may result in cloudy stains that can diminish the view.

A premier manufacturer of made-to-order wood, clad, fiberglass, and aluminum windows and doors, Marvin builds its principle of human-centered design into every product. With more than 100 years of industry experience, Marvin has built a reputation for delivering the finest craftsmanship in windows and doors.

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