Traditional sash and case windows do not automatically need to be replaced to improve comfort. In many buildings, the best approach is to repair defects first and then add proportionate thermal improvements that respect the original joinery.
1. Start with repair
Loose joints, decayed sills, cracked putty, missing seals, broken cords and distorted sashes can all affect weather-tightness and usability. Historic Environment Scotland recommends keeping sash windows in good repair and retaining existing fabric wherever practical. See our repair service.
2. Draught proof the moving sashes
Historic Environment Scotland describes draught proofing as the first goal when reducing heat loss from sash windows. Discreet brush or similar seals can reduce uncontrolled air leakage, rattles, dust and noise while retaining the original window. Ventilation still matters, particularly where there are combustion appliances or condensation problems.
Read about sash window draught proofing.
3. Use shutters and curtains
Original internal shutters are valuable building fabric and can also improve night-time thermal performance. Historic Environment Scotland reports a 51% reduction in heat loss from timber shutters in the research summarised in its guidance, while heavy lined curtains can also help. Shutters that have been painted shut or no longer operate may be candidates for restoration.
4. Consider secondary glazing
Secondary glazing adds an internal pane while retaining the existing sash window. Historic Environment Scotland reports that it can reduce heat loss by 63% in the research cited by its guidance. It can be particularly attractive where retaining historic glass and external appearance is important.
5. Consider appropriate glazing upgrades
In some windows, slim-profile or other specialist glazing may be technically and visually appropriate. The decision depends on rebate depth, sash strength, glazing-bar dimensions, historic significance, glass type and consent requirements. It should not be treated as a universal drop-in replacement.
6. Do not forget perimeter junctions
Gaps between the window case and surrounding masonry can allow water and air leakage. Traditional perimeter details such as sand mastic need to be maintained appropriately. See our sand mastic replacement information.
7. Manage ventilation and condensation
Making a building more airtight changes how moisture behaves. Kitchens, bathrooms, drying clothes and occupancy all add water vapour. Thermal upgrades should therefore be considered alongside safe ventilation. Persistent condensation should not automatically be blamed on the window itself.
Repair, upgrade or replace?
Replacement may be justified where a window is beyond economical repair or where another technical requirement cannot reasonably be met. But sound historic timber can often be repaired and upgraded. Our repair or replace guide explains the decision in more detail.
Frequently asked questions
What is the first energy-efficiency improvement for sash windows?
For a serviceable traditional window, dealing with defects and uncontrolled draughts is usually the logical first stage. Historic Environment Scotland specifically describes draught proofing as the first goal.
Is secondary glazing better than replacing the original window?
It depends on the building and objectives. Secondary glazing can provide a significant thermal improvement while retaining original fabric, which can be especially valuable in historic buildings.
Do thick curtains really help?
Yes. Closing well-fitted shutters or lined curtains at night can reduce heat loss, although they are part of a wider strategy rather than a substitute for necessary repairs.
Can old sash windows be double glazed?
Some can accept specialist glazing after assessment, while others are better suited to repair plus secondary glazing. Sash strength, glazing bars, rebate depth, historic glass and permissions all matter.
Authoritative guidance: Historic Environment Scotland – reducing heat loss from sash windows.