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Some Planning Authorities, when granting Planning Consent for works to be carried out on Listed Buildings, buildings in Conservation Areas in Scotland and Conservation Areas in England where an Article 4 Direction has been placed on a property, place conditions on the consent that indirectly limits the type of glazing that can be used.

The conditions usually relate to the dimensions of the window components in place when the listing or conservation area was created. These restrictions on window component design will often restrict both the overall thickness of the Insulating Glass Units and restrict the glazing rebate upstand and therefore the edge-seal depth (edge of glass to top of spacer-bar), usually referred to as the IGU edge-seal sightline.

The conditions imposed by Planning Officers may result in:

  • Overall thickness of the IGU of 12mm, 14mm or 16mm meaning a cavity width of 4mm, 6mm, or 8mm respectively when using 4mm glass.
  • Where window sashes/casements are sub-divided into small panes, all panes must be individually glazed, internal/external astragals may not be permitted.
  • Limits on window components usually result in limits on the visual width of glazing bars which sub-divide sashes/casements.
  • The limits imposed on glazing bar widths means that IGUs of standard construction with edge-seal sightlines of 10mm – 12mm cannot be used as the spacer-bar would be visible after glazing.
  • Wide cavities even 6mm enable multiple reflections to be seen in the glazing this may result in the use of Double Glazing being rejected

Some IGU manufacturers supply IGUs with a significantly reduced edge-seal sightline, these IGUs may not have the necessary EN 1279-2, EN 1279-3 and EN 1279-4 test evidence to demonstrate they are durable and therefore may not have a reasonable service life.

The GGFs position in relation to these IGU being placed on the market without the necessary test evidence is given in our publication – Narrow Cavity Insulating Glass Units (IGUs) with Reduced Spacer-bar Sightlines which can be downloaded from the GGF Members Area on

It is GGF Best Practice that its Members do not manufacture and place Low Sightline IGUs on the market unless they have the necessary test evidence for that specific specification to demonstrate durability and enable them to comply with the requirements of the Construction Products Regulations. GGF Members should also refer to the GGF Industry Guidance document – Insulating Glass Units (IGUs) – Conforming to the Construction Products Regulation (CPR) which can be downloaded from the GGF Members Area on

Steve Rice
GGF Director of Technical Affairs

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