What U-Value can I expect from timber sliding sash windows ?


Answer. Recent advances in draft proofing and glazing systems mean todays Part L regulations are easily achievable when using wooden sliding sash windows. Long gone are the days of drafty, rattling sash windows.

If you wish to have traditional sliding sash windows with weights and pulleys, then you can achieve u-values of 1.4W/m2K for a softwood unit and 1.5W/m2K for a hardwood unit. This is achieved using an extensive range of seals and brushes in a strategic formation on frames, jambs and meeting rails. Double glazing units with warm edge spacers and Argon gas filling with a u-value of about 1.1W/m2K, combine to give the overall result.


  • Be sure that the U-value that you are being quoted by suppliers is an overall value and not just the glazing value.
  • Check that the quoted U-Value is certified by a suitably qualified and recognised simulator.
  • Ask to see credentials.

Kingsbridge Joinery’s hand wooden sliding sash window is certified by Build Check Ltd. All profile and PSI calculations are in accordance with BS EN ISO 10077-2:2003.