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Improving the Window U-Value

Heat loss through windows and openings represents 19% of the overall loss in a traditional flat and 11% in a detached house.

  • Draught-proof your existing timber frame sash & case windows with concealed brushes set into new grooves in the sashes and parting beads. Brushes are more effective and more attractive than surface-mounted rubber seals. You can reduce air leakage by up to 80%.
  • Use heavy full-length curtains, internal blinds or shutters to retain heat.
  • You can also insulate your shutters or add secondary glazing, although detailing should be carefully considered.

Original Glass

Historic windows with original glass should be retained, overhauled (timber may have distorted over a period of time), cleaned and fitted with concealed draught-proofing, instead of being replaced with modern double glazing. If the historic fabric is beyond repair or has been altered in the past, windows may be replaced subject to LBC.

Original crown glass is characterised by curved and bellied waves often visible in individual panes when viewed in sunlight. As it is no longer produced, historic crown glass should be maintained and preserved.

Well maintained timber windows can be repaired to last hundreds of years and be re-used at the end of their lifetime. Window frames should be made of sustainably sourced timber instead of U-PVC or other materials high in embodied energy. U-PVC frames spoil the building’s authentic character, are not able to be repaired and are difficult to recycle.


Reinstating wooden shutters in their original location is a surprisingly cost-effective solution, especially if you close them at night and when the room is not in use. Additionally, it enhances the historic character of your home.

On the upper floor, the windows with the shutters closed show up in the thermal image in a deep blue colour, demonstrating how little heat is escaping.

Secondary Glazing

With secondary glazing systems, a second window pane is added to the existing window.

Secondary glazing can reduce the heat loss to 50% and will also reduce noise. It is more heritage friendly than replacing windows with double-glazing and is as energy efficient as most types of slim-line double glazing.

In certain cases, it may require LBC and might also make regular maintenance and cleaning more difficult.

Best solution: mount a vertical sliding secondary glazing on the inner face of the sash & case window. The frame should be similar in size to the slim frame of the existing window to obtain a very discreet installation. It also allows shutters, where they exist or could be retrofitted, to operate.

Double Glazing

In Edinburgh, the use of double glazing is not permitted in listed buildings, although in certain cases the slim-profile system may be acceptable, but still requires LBC.

In terms of energy efficiency, slim-line systems are comparable to single glazed units with secondary glazing added. The gap between the two panes is very small and the profile should replicate the exact window details including the astragals and mouldings, in order to match the original historic features.

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