Black Knight Secondary Glazing
Black Knight Secondary Glazing

Secondary Glazing Thermal Insulation

This information is designed to give you a comprehensive explanation of how secondary glazing performs for thermal insulation, energy efficiency & energy saving and how Black Knight can assist you to successfully stop heat loss through windows and doors.


  • official test data shows that replacement double glazing only produced heat insulation of 55% whilst secondary glazing with energy efficient glass achieved 63%

For thermal insulation, our energy efficient secondary glazing is often the best choice:


  • designed for maximum energy efficiency
  • totally effective against heat loss through the primary windows
  • elegant and discreet designs
  • the best-performing single glazing insulation available
  • environmentally friendly materials
  • much more cost-effective than replacement double glazing


When correctly specified and installed our secondary glazing will achieve a thermal insulation value with an equal or better rating than the Building Regulations requirements.


Fitted in houses, flats, Listed Buildings, properties in Conservation Areas across the UK  ensuring that the character and integrity of the original design and appearance is maintained.

Thermal insulation efficiency

The thermal efficiency of a window is expressed as it's U value - for comparison the smaller the U value is, the better the window is insulated.


The following factors will affect the thermal efficiency of your windows and doors:

  • single glazed or double glazed
  • metal frames or wood or PVCu frames
  • standard glass or low emissivity glass
  • air or gas filled sealed unit double glazing
  • glass thickness
  • glass cavity on double or secondary glazing
  • frame depth
  • quality of the seals

Optimum thermal insulation performance

Using secondary glazing incorporating 6mm Low E Pilkington K thermal insulating glass:


  • Will achieve thermal efficiency U values for windows of 1.5W/m²K
  • the standard requirement for residential replacement windows is 1.6W/m²K


If you currently have single glazed windows that are in good condition and do not require replacing but are poorly insulated - then Black Knight secondary glazing with thermal glass will:



  • improve their U value just as well as replacing them entirely
  • effectively insulate the windows and give you real energy savings

Secondary glazing thermal insulation benefits

Black Knight secondary glazing windows are:


  • cheaper than replacement windows & doors
  • will eliminate draughts and cold spots
  • truly energy-efficient improved window insulation
  • will give significant noise reduction
  • are quick to install with no mess
  • will only need minimal redecorating after installation
  • will give maximum heat insulation and reduced heating bills
  • improved security