Replacing timber windows in heritage buildings

Thu 10 Nov 2016

Amanda Chesson of George Barnsdale answers the key questions faced when replacing timber windows in listed buildings or Conservation areas.

Do I need planning permission to replace my timber windows?

If your home is Listed and the windows contribute to the character of the building, then listed building consent will be required. If you are in a Conservation area with an Article 4 direction, you will also need consent. Before considering any alterations, it is crucial to speak with your local Conservation Officer to help understand the potential impact of any change. Where a window is beyond repair, then the replacement must provide an accurate match to the original. The exception to this is where you are looking to replace an inappropriate window that has been fitted to replace the original.

When looking at replacement timber windows, what do I need to take into consideration?

Any replacement timber window should match the original window design as closely as possible. Factors to take into consideration include:

  • Replication of size, shape and the proportion of the window;
  • The accurate match of profiles;
  • Authenticity of glazing;
  • How the window opens;
  • Glass fixing methods;
  • The finish.

Can I incorporate double-glazing?

If your windows do need consent, then it's worth discussing the question of double-glazing with your conservation or planning officer.  Authentic period windows can be supplied with double-glazed units, but sometimes conseervation officers will insist on single-glazing. Where it can be demonstrated that the significance of the building will not be harmed, then narrow cavity insulated glazing units (also commonly referred to as ‘Slim IGUs’) may be acceptable in certain circumstances.

Incorporating a cavity of 8mm and less with reduced spacer-bar sightlines, narrow cavity glazing is unobtrusive and provides a greater degree of comfort than single glazing. 

What should I look for when sourcing narrow double-glazing?

Both the British Woodworking Federation (BWF) and the Glass and Glazing Federation (GGF) have issued technical guidance to their members on the specification of narrow cavity insulated glazing units. A conscientious timber window manufacturer who offers an historic / conservation range will adhere to this guidance to ensure the premature failure of glazing units.

When comparing manufacturers who supply narrow cavity glazing, ask for evidence of the following:

  • CE marks on the glazing, supported with a Declaration of Performance (DoP);
  • Evidence of testing from the glazing supplier, and that the system description for the products tested are consistent with the units ordered;
  • Conformance with BS EN 1279: 2005, the standard for Glass in Buildings – Insulating Glass Units.

Can my windows be fixed using putty?

Yes, but linseed oil putty, the traditional material used to fix glass is not suitable for modern glazing as it reacts to a component used within the seal of a sealed unit, in addition to requiring on-going maintenance. However, there are some timber window manufacturers who do offer a putty glazing system that incorporates modern materials that match the aesthetics of linseed oil putty.

Is it difficult to match the slender glazing bars of the original?

Any replacement window must be in keeping with the architectural style and period of the original, so the accurate matching of glazing bars will be pivotal to sympathetic replacement. If the window uses standard double-glazing, correct slim glazing baars can be achieved using 'plant-on' bars, which are indistiguishable from structural bars taht physically separate the glass panes. However, some Conservation officers prefer structural glazing bars. Plant-on bars provide better thermal performance, longer life for the glazing unit and reduced maintenance but structural bars can be used with slim double-glazing units.

For further reading, see the WWA's Heritage Windows Guidelines.