Look at comfort, air leakage and the whole installation before comparing headline figures.
Why the window feels cold
Single glazing can have a cold internal surface in winter. People sitting close to it may feel uncomfortable even when the room thermostat shows an acceptable temperature. Uncontrolled draughts can add to that discomfort.
A secondary layer can improve the conditions beside the window. The result depends on how the inner screen is detailed and sealed, the existing window and the temperature and ventilation of the room.
Understanding specifications
Ask whether a quoted thermal figure refers to the glass alone, the secondary frame, or the combined original and secondary window. These are different measures and should not be compared as though they describe the same thing.
Low-emissivity glass may be offered as part of a thermal specification. Ask which surface carries the coating, how it must be cleaned and what evidence supports the proposed configuration.
Savings need context
There is no single reliable saving for every Scottish home. Window area, exposure, heating pattern, energy prices and the rest of the building all matter. Do not turn a laboratory percentage for window heat loss into the same percentage saving on a whole-house bill.
When reviewing quotations, favour a clear description of the system and its supporting evidence over an unexplained annual saving or guaranteed payback period.
Keep the building in balance
Before ordering, check for draughts from other sources, failed external seals and defects in the original window. Consider the room as a whole, including ventilation and moisture.
Home Energy Scotland can provide wider energy advice. Secondary glazing is one possible improvement and should be considered alongside maintenance and other appropriate measures.