Black Knight Secondary Glazing
Black Knight Secondary Glazing

Secondary Glazing Sash Windows


If you need to fit secondary glazing sash windows over any existing timber or PVC boxsash windows then the Black Knight designs are flexible enough to meet any configuration that you may need.


Secondary glazing for sash windows

Secondary glazing can be easily fitted to both single sash windows and double sash windows (adjacent windows that share a common weight box).


Most sash windows are found in Victorian and Georgian properties and these properties are often Listed Buildings or are in a Conservation Area and the existing windows are subject to restrictions that prevent the primary window from being changed and very often you will find that secondary glazing will be the only acceptable solution.


And even if your property does not fall into this restriction the existing sash windows are an integral design element and fitting modern replacement windows will detract from the style and character of the property and will possibly reduce the value of your home as a result.

Problems with a traditional sash window

The problems with sash windows are the same whatever sort of property you live in - whether it is a rural idyll, a Victorian terrace in the suburbs or an urban mansion block flat.


  • poorly insulated


  • major source of heat loss


  • draughty and tend to to rattle


  • major security risk


  • noise is often a major problem


  • full replacements are very costly


  • suffer badly from condensation


In 2009 English Heritage carried out a tests on a sash window from the late 19th century to determine how thermally efficient it was.


During the test they also fitted bespoke secondary glazing and the results were significant:


  • the secondary glazed sash window achieved a U Value of 1.8 W/m²K and a reduction in heat loss of 58%


The full test results can be seen here


In 2008 the Centre for Research on Indoor Climate & Health at Glasgow Caledonian University carried out a similar thermal insulation test:


  • “secondary double glazing was the most effective overall option, as it reduced heat loss through the window by 63%.”
  • "double glazing only achieved a 55% reduction"
  • “the test window was draught proofed ... although the U value of the window was not improved ...”


To see the full report please click here

It is possible to fit secondary glazing sash windows to all types and styles os box sash windows - please see our gallery for some examples

The benefits of secondary glazing to sash windows

  • stops heat loss & reduces heating bills
  • stops draughts & rattles
  • the most cost-effective solution for for noise reduction & soundproofing
  • when fitted with low emissivity glass performs better than repalcement double glazing
  • a fraction of the cost of completely replacing your existing sash windows
  • retains the features and character of your property

More benefits of secondary glazing to sash windows

  • improved security - can be fitted with additional security locks
  • can be glazed with toughened or laminated safety glass
  • control any condensation problems
  • protect against damage from wet rot & black mould infestation
  • reduced redecorating & maintenance requirements
  • a quick, clean installation
  • no need to redecorate afterwards

We offer a range of vertically sliding secondary double glazing units specifically designed for use with traditional sash windows.

Secondary glazing sash windows incorporate slim sections and smooth operating mechanisms:

  • follow the lines of your existing sash windows
  • no obstruction of existing views
  • easy to open for cleaning and ventilation
  • finger-tip operation
  • additonal security locks available
  • safety glass options
  • fully reversible sashes for easy cleaning
  • concealed fixings and perfect trims ensure an elegant finish
  • arched and curved heads available
  • hundred's of RAL colours