Black Knight Secondary Glazing
Black Knight Secondary Glazing

Are Secondary Windows Effective?

Although the following is quite detailed it will give you a comprehensive   understanding of using secondary windows for thermal insulation and improving the energy usage in your property.


You lose a large part of your heating through your existing windows and doors and installing secondary windows is just about the most effective way you can reduce your heating bills.

Fitting replacement double glazing will only produce heat insulation of 55%;  fitting secondary windows incorporating energy efficient glass will achieve 63% - for truly energy efficient windows, click here.



We supply glazing manufactured to exacting standards for impressive energy saving results.  You can view their test results here.


Fitting discreet secondary windows will save you money compared to the cost of fitting full replacement windows and will continue to save money on your heating bills for years to come.

For thermal insulation, our energy efficient range of secondary windows is the recommended choice: 

  • System designed for maximum energy efficiency
  • Extremely effective at combating heat loss through your existing windows
  • Discreet design gives maximum glazing insulation without compromising on appearance
  • Better for the environment and saves energy
  • much cheaper and less disruptive to fit than replacement double glazing

When the system is correctly installed our secondary windows will achieve thermal insulation which is equal to or better than the requirements of Building Regulations.


Due to the discreet design of our secondary windows it can be fitted in most type of property including houses, flats, listed buildings and properties in conservation areas ensuring that appearance and character is protected.


Thermal Efficiency


The thermal efficiency achieved by insulation is the U value - the lower the U value figure, the better the window insulation will be.


There are a number of things taken into account to calculate the U-value of your windows and doors:


  • whether your existing window frames are single glazed or double glazed
  • whether you have wood, metal or PVCu currently
  • whether your existing glazing has low emissivity glass fitted
  • if you have sealed units whether they include argon filling
  • the thickness of your existing glass
  • the cavity between glass panes
  • the depth of the window frame
  • the quality and effectiveness of the perimeter seals


Performance data


For the purposes of this comparison we will assume that your existing window is made of timber and single glazed. By fitting secondary windows incorporating 6mm Low E Pilkington K thermal insulating glass we can achieve thermal efficiency (U values) for windows of 1.5W/m²K


This compares favourably with the insulation levels for residential replacement windows of  1.6W/m²K


If your existing windows are in good condition and do not need replacing, then Black Knight secondary glazing cuts their U value equally as well as replacing them and give you a cheap energy saving without having to send your existing windows to landfill.


Historic Scotland


In 2008 Historic Scotland commissioned a study from the Centre for Research on Indoor Climate & Health at Glasgow Caledonian University. The study tested traditional timber sash windows and casement windows investigating the thermal insulation properties when different treatments were applied. 


Although the report is comprehensive it was shown that the most important of the thermal efficiency results were as follows:

  • “Secondary glazing was the most effective overall option as it reduced heat loss through the window by 63%"
  • Replacement double glazing only achieved a 55% reduction.

If you would like to view the report in full, please click here



English Heritage


In 2009 English Heritage carried out a similar research project and they saw similar results. To view their report in full please click here.



Summary of the benefits of Black Knight secondary windows


Installing Black Knight secondary windows will: 

  • be cheaper than installing full replacements for insulating windows & doors
  • cut out draughts and cold spots
  • be energy efficient
  • offer significant reduction in noise (with the correct glass option selected)
  • extend the working life of your existing primary windows & keeps them out of land fill sites
  • install quickly and cleanly with no need to remove your existing windows
  • need only minimal redecorating (often none at all is needed)
  • give maximum heat insulation resulting in reduced fuel bills
  • give improved security
  • retain the original external appearance of your property and protects the value of your home
  • normally not need Conservation Area or Listed Building approval