Cold air around an old window is uncomfortable, but the right remedy depends on where it enters. Secondary glazing adds an internal framed layer that may reduce air movement across the opening while also improving the temperature of the room-facing glass. It cannot repair rotten timber, a leaking wall junction or every gap in the original sash.
This is a draught-proofing decision guide, distinct from our drafty sashes versus secondary glazing comparison. All performance and cost figures below are indicative or illustrative, not forecasts or guarantees.
Find the draught before choosing the fix
On a cold, windy day, notice whether air enters at the meeting rail, sash cords, staff beads, pulley box, sill, frame-to-wall junction, trickle vent or adjacent door. A smoke pencil or tissue can help show movement, but do not obstruct a required vent or use a flame near curtains and timber.
The original window should be checked for rot, loose putty, movement, failed paint, gaps and water ingress. Secondary glazing can create a useful inner seal, but it is not a substitute for repairing a primary window that is structurally unsound.
The practical air-gap question
For many period windows, a 100–200 mm gap between the original pane and the secondary unit is a practical design range when the reveal allows it. This is guidance rather than a rule. The cavity must leave space for handles, shutters, curtains, cleaning, ventilation, sightlines and safe operation. A wider gap is not automatically better if it makes the room difficult to use.
The frame and seals matter as much as the distance. A poorly measured or badly installed unit can leave a perimeter leak, while a well-maintained primary sash can still have its own designed or accidental air paths. Ask for the complete window arrangement rather than a gap figure alone.
What improvement might be expected?
In some suitable windows, adding a well-sealed secondary layer may reduce heat loss through the window area by around 50–65% in an indicative comparison. The result is specification-dependent: glass, low-emissivity coatings, cavity, frame, seals, original window condition, wall junction and installation quality all change the calculation.
That is not a promise about the room or energy bill. Heating demand also depends on walls, roof, floors, doors, controls, exposure, occupancy and ventilation. Use the thermal performance guide for the wider context and the secondary-glazing U-value guide for how window calculations should be read.
Draught-proofing options compared
| Option | What it may help with | Limits and questions |
|---|---|---|
| Brush seals or sash draught strips | Gaps at meeting rails and moving joints in a sound primary window | Profile, movement, paint condition and installation quality matter; do not block intended ventilation. |
| Removable film | A temporary barrier over some simple openings | It can be visually intrusive, seasonal and fragile; it does not create a robust framed cavity or acoustic specification. |
| Secondary glazing | A separate inner seal, warmer room-facing surface and possible acoustic benefit | Requires a suitable reveal, careful measurement, access planning and case-specific heritage checks. |
| Primary-window repair | Rot, putty, frame and water-ingress problems | It may be essential before any inner layer, but it does not by itself add a second acoustic or thermal pane. |
These options are not interchangeable. A sound sash may benefit from careful draught strips; a noise-exposed listed window may need a surveyed secondary system; a leaking frame needs repair. The best project can involve more than one intervention, sequenced so that moisture and ventilation are not trapped.
An illustrative energy example
Suppose a household spends £2,000 a year on heating. If a window improvement were associated with a 10–15% reduction in whole-home heating use, the arithmetic would be roughly £200–£300 a year. This is an illustrative calculation, not a forecast or guaranteed saving. It assumes the percentage applies to the whole bill, which may not be true.
Actual energy outcomes depend on the starting window, the number of openings treated, thermostat settings, heating system, weather, occupancy, fabric elsewhere and energy prices. A quotation should not promise a bill reduction without explaining the basis for the estimate.
Installation quality matters
Even a good specification can underperform if the frame is out of square, the seal is interrupted or the original reveal is not prepared properly. Ask how the installer will:
- measure uneven or moving period openings;
- protect paint, timber, shutters and original glass;
- position the frame without compromising the primary window;
- maintain designed ventilation and access;
- seal corners and transitions around bays; and
- check operation, cleaning and handover instructions.
For a bay window, geometry, corners, sill depth, linked units and curtains are property-dependent. Do not assume a standard panel will stop every draught at a curved or faceted corner. A survey should precede a quotation.
Heritage and Historic England guidance
Historic England’s guidance on improving the energy efficiency of historic buildings supports an approach based on significance, minimum intervention and understanding moisture. Sealing an original window should not be treated as an isolated action: the building fabric and ventilation need to remain healthy.
Listed Building Consent, planning permission, an Article 4 Direction or landlord and leaseholder approval may apply. “Internal” and “reversible” do not automatically decide the matter. Check the property’s status and the proposed fixings with the relevant authority before work starts. The period-property efficiency guide provides useful background.
Survey before you buy
Bring a simple record of:
- the locations and times of noticeable draughts;
- photographs of the inner and outer window, sill and reveal;
- the condition of paint, putty, timber, beads and catches;
- the depth available behind the original window;
- curtains, shutters, furniture, radiators and cleaning access; and
- the property’s listed, conservation, leasehold or landlord position.
Ask for the proposed gap, glass, frame, seals, opening style, fixing, ventilation and aftercare in writing. If condensation is also present, read the condensation and mould-prevention guide before the installation so draught reduction does not accidentally become moisture concealment.
Draught-proofing FAQ
Does secondary glazing stop all cold air?
No. It may reduce air movement across the window opening when well surveyed and sealed, but draughts can remain at the original sash, wall junction, doors, vents or other openings. Complete elimination is not a responsible universal claim.
Is a 100–200 mm air gap necessary?
It is a practical range for some period-window designs, not a requirement for every project. Available depth, access, ventilation, curtains, shutters and heritage details determine what is appropriate.
Should I use brush seals instead?
They may help a sound sash at specific moving joints and can be part of a wider plan. They will not provide the separate pane, cavity or acoustic options of secondary glazing, and the seal profile must suit the window’s movement.
Can secondary glazing reduce my energy bill by 10–15%?
That percentage can be used as an indicative planning scenario in some discussions, but it is not a forecast. The whole-home result depends on the complete building, heating system, behaviour and energy prices.
Can I fit it to a bay window?
Possibly, but bays are property-dependent. Corners, faceted geometry, sills, linked frames, curtains and access need to be measured and designed together.
What should I do first?
Arrange a survey before buying materials or approving a quotation. Contact SecondaryDoubleGlazing to discuss the draught locations, original window condition and the practical gap available.
Secondary glazing can be a considered way to reduce cold air and preserve original windows, provided the design responds to the actual opening. Repair the primary window where needed, keep ventilation visible and judge claims against a written specification rather than a headline percentage.