This months newsletter is the second of a two part series on glass. The articles are designed to better educate you on glass itself; i.e. how it is made, types of glass, and glass safety.

Types of Glass:

Ordinary clear glass will allow approximately 85% of the light that falls upon it to pass through the glass. Glass is inherently strong but minor, microscopic flaws that are a portion of the glass itself allow the glass to crack and break. The thickness of the glass is dependent upon the size of the glass that is needed and the wind loads that the glass may come in contact with it.

Annealed, or “ordinary” glass, is glass that when it is manufactured is cooled slowly and under carefully controlled conditions. This “annealing” procedure removes most ordinary locked in stresses that might cause the glass to act unpredictably in its use or create conditions in its installation.

A heavier form of annealed glass is “plate glass.” Plate glass is thicker version of ordinary glass and is formed between high pressure rollers. The additional thickness increases the glasses strength.  The increase in strength, though, does not make it a safety glass.

Multi-Paned Windows
Single panes of glass, used in windows, are extremely poor insulators. A single pane glass window conducts, or moves, heat from one direction to another (remember heat always flows towards cold), from ten to twenty five times more than a properly insulated wall. A second sheet of glass applied to the same window, with airspace between the two sheets, cuts this rate of heat loss in half. The thickness of the airspace plays a relatively minor roll compared to the mere presence of the airspace. When a low conductivity gas is inserted into the airspace it further increases the thermal efficiency of the window.  Argon and sulfur hexafluoride are the gases most commonly used. The thermal efficiency of the window can be even further improved by the addition of a coating on the glass that blocks conductivity.  

Heat Tempered
Heat tempered glass is annealed glass which has been re-heated to a temperature of approximately 1200 degrees, near to its re-softening (liquid) point, and then forced to cool rapidly by cooling both sides of its surfaces with a blast of air, while its core is allowed to cool more slowly. This heat treating process produces additional strength and resistance to impact. The resulting glass is about four times as strong as annealed glass. When “tempered” glass does break it reduces the damaged surfaces to small square edged “granules or fragments” rather than long sharp edged shards like ordinary glass.  These qualities make tempered glass useful for windows exposed to potential impact, heavy wind pressures, and intense heat or cold. These qualities make tempered glass particularly useful for exterior doors, interior partitions, floor to ceiling sheets of glass, and glass doors that have no frame like shower doors.  Tempered glass is substantially more costly and does have some optical distortions.  

Heat Strengthened
For some applications, where additional strength is needed, but not to the degree of “tempered” glass, a “heat strengthened glass” might be used. Heat strengthening the glass doubles the strength of the glass.  Heat strengthened glass also falls between annealed and tempered in its distortions and its breakage qualities.   

Laminated glass is made by sandwiching a transparent vinyl film in between two sheets of glass. This vinyl sheet bonds the three layers together and holds the shards of glass, in a breakage situation, in place preventing a risk of injury. The types of glass used in laminated glass can be annealed, heat strengthened or tempered. In the United States, laminated glass is used in automobile windshields.  It also has applications in skylights where the vinyl lamination prevents glass from falling to the space/room below. Where the transmission of sound may be an issue, like hospital rooms, libraries or classrooms laminated glass also has applications. When two or more layers are installed it can be used as a safety glass for banks, and other facilities where security is an issue.  

Before the introduction of tempered and laminated glass “wired glass” was used as a safety glass to prevent human injury. Though wired glass had its advantages numerous injuries still occurred in conjunction with its use. Wired glass is produced by rolling a mesh of small wires into a sheet of hot glass.  Wired glass, though, has only half the strength of ordinary annealed glass due to the internal stresses from different rates of contraction of the wire and the glass during the cooling process. Its use as a safety glass has all but been abandoned since the early 1970’s. Wired glass is still commonly used in small door windows to allow someone to see persons on the other side of the door. Since the 2006 building codes wired glass is no longer allowed in locations subject to “impact.”

Tinted glass is made by adding amounts of certain chemicals to the molten glass mixture to produce a desired “hue” to the glass.    

Safety Legislation

As of the early 1960’s few building code guidelines addressed glass, and the estimated 320,000 annual injuries caused by people impacting glass. The three most common forms of accidents were caused by; failure to see the glass, slips and falls (even when it was previously known that the glass was present, like with a glass shower door or patio door), and accidents caused during the intentional breakage of the glass. 

In 1972, The U.S. Congress enacted the Consumer Product Safety Act, and one of the first acts of the Consumer Product Safety Commission was to address the hazards of glass. I will not address any specific “codes” of the act, but I will mention that when it comes to issues like safety glass there is no grand fathering of existing conditions.  The age of the property can not be a determining factor as regards safety glass.

Front Door 

Locations Subject to Human Impact (Hazardous locations where Safety Glass should be):

Doors (D)

  1.  Swinging doors with any glass pane larger than 3 square inches must be safety glass. Exceptions; jalousie and decorative glass panes.

  2. All storm doors.

  3. Glazing in doors and enclosures for showers, tubs saunas and whirlpools as well as windows.

  4.  Sidelites (windows at either side of a door) within 24 inches of the door frame and less than 60 inches above the floor or walking surface. Some exceptions do apply.

Large Windows (W)

  1.  If the square footage of the window is greater than 9 square feet.

  2. If the bottom edge of a window is less than 18 inches from the floor.

  3. The upper edge of the window is more than 36 inches from the floor.

  4.  A walking surface, Ex; a front sidewalk, is within 36 inches of the window.
    (There are exceptions).


  1.  Glass used in handrails. (These are usually laminated glass).

  2.  Glazing with any part less than 60 inches above a walking surface and within 60 inches horizontally of a pool or spa.

  3. Glass that is used in stairways or landings that is less than 60 inches above the
    walking surface and with in 3 feet horizontally of stairways, landings or with in 5
    feet of the bottom tread of a stairwayditions. 

View part I

To learn more about home construction, visit our website's Anatomy of a Home section

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