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Windows and Window Protection Systems

When assessing and addressing the vulnerability of windows, CPNI divides the problem and consequently our guidance, into two broad categories:

  • Punched window systems, which are ‘stand-alone’ windows incorporated into the façade of a building.  Typically, each individual window comes with its own frame, which is mounted into the façade.
  • Glass curtain walling systems, which consist of a ‘wall’ of glass supported by a purpose designed structure, often made from aluminium.  The glass curtain walling system typically spans between floors, giving the external appearance that the building is made entirely from glass.

The security properties of each of these types can be very different.  This page contains information relating only to punched windows and mitigation systems used in conjunction with punched windows.  Further information can also be found following the links on the page.


Features of Punched Windows

The elements impacting the security resistance of a punched window are:

  • The type, design and material of the window frame
  • The fixing of the window frame to the façade
  • The overall size of the window (this can impact on its blast resistance; bigger panes generally perform less well than smaller windows)
  • The rebate of the glass (the distance the glass protrudes into the frame)
  • The type and thickness of the glass used and whether or not it is a single pane, double or triple glazed (laminated glass is preferred for security applications)
  • How the glass is secured to the frame.  For punched windows, the glass is typically secured using ‘beading’, which for a variety of reasons, should be on the side of the building interior. 
  • Window locking systems and hinges.
  • A variation is Ribbon Glazing where a series of windows are fixed in a continuous row.

Blast Resistance

The majority of injuries arising from VBIEDS are caused by flying or falling glass and there are several options for reducing this risk.

Glazing measures are currently available which can either mitigate the effects from a blast threat by reducing the number of fragments entering the room, or protect the occupants from a specified blast threat and the fragments by remaining in the frame.

The following are considered to be mitigation measures:

  • The use of Anti-Shatter Film (ASF) and Bomb Blast Net Curtains (BBNC)
  • Laminated Glass in normal (non-blast enhanced) window frames where the glass is not bonded to the frame

The following are considered to be protective measures:

  • Laminated Glass in normal window frames where the glass is bonded to the frame
  • Blast Resistant Glazing in Blast Enhanced Frames or Fixed Point Glazing
  • Secondary glazing using laminated glass

A guidance note explaining the level of the protection provided by the measures listed above has been developed by CPNI.

It is important to understand the acceptable level of risk in order to determine the appropriate glazing enhancement. Where blast enhancement is required, laminated glass in normal window frames is generally recommended as a minimum.

If it is not possible to upgrade and install protective glazing, such as laminated glass, into existing buildings, the use of anti-shatter film (ASF) may be applied as a retrofit measure to mitigate the glazing hazard inside a building as a result of an external blast or impact. Applied directly to the glass, it is important that the correct film is specified for the glass type to ensure that the desired level of mitigation is achieved. CPNI has developed guidance notes on the use of ASF in buildings, key points for specifying ASF, how to assess the quality of its installation and how to assess existing ASF.

Guidance on how to install bomb blast net curtains has been produced for CPNI by Home Office Centre for Applied Science and Technology (CAST).


Resistance to Ballistic Attack

It is possible to manufacture window systems that are resistant to a range of ballistic threats.  Each manufacturer will have their own design, but mainly they are based on multiple layers of glass that are laminated together.  Some systems use polycarbonate or other types of material to work in conjunction with the glass to improve performance. 

When procuring bullet resistant glass, it is very important to specify against a known standard and you should ensure that the manufacturer can provide you a test certificate demonstrating their compliance to the specified standard.  Often, suppliers may buy in the glass and the window frames from different manufacturers, so you should ask to see certification for the three separate elements: the glass, the frame and the complete system.

Further guidance on the ballistic resistance of glass is available from this page and on the ‘Protection Against Ballistic Attack’ page.


Resistance to Forced Entry

Windows are inherently vulnerable to forced entry and will therefore need to be supplemented with an additional barrier – commonly referred to as Window Protection Systems.

Window protection systems are designed for external or internal use on buildings.  They provide protection against a range of forcible attack and a number of products have been tested and approved to CPNI levels using CPNI manual attack standards.  Protection systems are divided into the following categories:

  • Bar sets
  • Internal security blinds
  • Internal security grilles
  • External window void protection

It is important to also consider the use of intruder detection systems, which can detect either an attempted or actual breach of a window.  Further information is available from the links below.

If you are a client of CPNI and require assistance, please contact CPNI for advice. If you are not a client of CPNI please engage a reputable security specialist to help you.