Window to the World. What do we really know about Windows?

Types of Windows, Defects, and failures

Nemmadi was involved in an interesting Window pane audit of one of the major projects in Bangalore. The audit involved checking of the sliding windows sashes which were found to be falling off the track during operations which is potentially a safety hazard for people moving around the building. During the course of our audit, we learnt new things about the type & quality of windows, installation procedures, defects in glass due to manufacturing and various other issues. 

This article outlines the various types of windows and defects found in windows and our findings from window pane audit. 

Before we get into the actual findings from the audit, we would like to first introduce various types of windows that are used in construction along with the keywords.

Casement Windows Swing open, usually by rotating a hand-crank at the bottom of the window. Casement windows can provide great ventilation, because the entire window opens unlike many sliding window designs. The crank is generally easy to operate with one hand at arm’s length, which means they can be installed in locations that are high or otherwise difficult to reach.
Bay Windows Permanently extend outward (beyond house walls), and are composed of several angled window panels. Usually found in kitchens, bay windows can provide wide peripheral views that flat windows cannot.
Single Hung Windows Typically vertical sliders are often installed in smaller rooms such as bedrooms or bathrooms. “Single” hung refers to the fact that only the bottom sash opens, as opposed to double hung windows, where both windows can be opened
Double Hung Windows Similar in appearance to single hung windows when closed. However, “double” hung means that both sashes can be opened. For obvious reasons, a double hung window is able to provide better ventilation than a similar sized single hung window
Awning Windows


Essentially casement windows that are oriented to swing open vertically. Like casement windows, awning windows provide good ventilation and typically seal well
Horizontal Sliding Windows Probably more common than any other style, because they are versatile and relatively cost-effective. Horizontal sliding windows are often wider than vertically-opening single or double hung windows
Louvered Windows


Louvered windows are similar to louvered doors which are provided for ventilation without any outside vision. The louvers may be made of wood, glass or metal. Louvers can also be folded by provision of cord over pulleys. 

Types of Window Failures

With a brief introduction to various types of windows, I would now like to jump into the various types of failures in windows, during manufacturing and during/after installation.

  • Bernoulli’s Principle

In high rise buildings, the high wind blowing outside creates a low pressure in accordance with Bernoulli’s principle which states that “an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid’s potential energy”. The pressure inside the room is equal to the atmospheric pressure which is now larger than pressure outside. This difference of pressure causes an outward thrust and there is a chance of the window falling out or glass shattering especially in high rise buildings.

  • Frame-Related Breakage

Spontaneous failures of tempered glass can occur when the gaskets/setting block or edge block in the frame are missing. This is due to insufficient cushioning to glass against the metal contact in the frame.

  • Thermal stress

A tempered glass used in windows usually has higher temperature in the middle when exposed to sunlight compared to the edges resulting in high tensile stress at the edge of the glass. This continuous change in temperature can lead to cracked tempered glass.

  • Nickel Sulfide inclusions in tempered glass during manufacturing.

Another type of glass failure is due to formation of Nickel Sulfide (NiS) stones during the glass manufacturing process.

While nickel sulfide breaks are extremely rare, it’s still very helpful to understand how spontaneous breakage due to nickel sulfide can sometimes occur.

When glass goes through the tempering process, it becomes four to five times stronger than standard annealed glass, making it great from a safety perspective. However, nickel sulfide stones that can form during the production of tempered glass due to nickel contamination, can end up in the tension zone of tempered glass. When that piece of tempered glass is later exposed to varying temperatures in its final installed position, this tiny stone may expand due to heat, and cause the glass to shatter for no apparent reason.

Visually, you will not be able to see these tiny particles of metal but they can still be present in the glass, benign until they are aggravated. There is no known technology that completely eliminates the possible formation of nickel sulfide stones in tempered glass. And because nickel sulfide stones are so small, there is no practical way to inspect their presence in float glass.

How to know if a glass breakage is due to Nickel Sulfide Inclusion?

A very simple way to determine whether a glass breakage has been caused by Nickel Sulfide (NiS) Inclusion is to watch out for the ‘Butterfly Effect’ pattern on the glass. The implosion will center around a singular point on the glass and will fan out in a butterfly wing formation. See image below for better understanding.

Image Showing butterfly pattern shattering due to NiS Inclusion.

A possible solution to avoid NiS explosions is using a heat soaked glass. The Heat Soaking process includes thermally treating the glass further after completion, in large heat soaking ovens. The glass is heated to very high temperatures in an effort to force any panel with Nickel Sulfide impurities to shatter. The resulting glass that survives the testing therefore has a lower chance of containing a Nickel Sulfide Inclusion, reducing the risk of broken panels to the end user 

Window Pane Audit Findings

After a brief about the various types of windows and defects,  I would now like to highlight the defects identified during the window pane audit. The failure was attributed to the following reasons.

  1. Problems with sliding of Sash due to Worn out tracks, Collection of dirt, dust, and rust in the tracks and grooves, Worn out roller blades cause operational issues/dysfunctioning of sliding windows
  2. Alignment problems between window frames and sash.
  3. Mullion is not in proper alignment in some windows
  4. Window sash not properly aligned with the track, gap between track and sash resulting in falling of sash from track.
  5. Interlocking not provided to sash in some windows. Wherever interlock is provided, it is not secured with screws/not fastened. It can be easily removed to lift the sash out of track which can be a safety issue.
  6. Wool piles damaged in some of the windows causing operational issues. 

These problems were leading to Jammed/damaged rollers and worn tracks resulting in operational difficulties. Also alignment issues were leading to the sash coming off the sliding track. And during the process of opening a window sash, when excessive force is applied to slide the sash, the adjustment screws at the top and bottom of the track come out resulting in falling of the sash. Also, Alignment problems with sash and frame lead to sash coming out of the track and falling off. 

Lessons Learnt:

  1. The window must be properly designed by the manufacturer/designer according to the size of the opening in the building providing sturdy frames & mullions to prevent deflection and alignment problems.
  2. Regular maintenance of window tracks must be done by cleaning to prevent accumulation of dust/dirt causing operational issues.
  3. Damaged parts of the windows like wool pile and locks/hinges shall be replaced to prevent operation issues.


Blog Written By: Mr Shankar Narayanan, Sr PM,