The Science of Secondary Glazing: How a 150mm Air Gap Cuts Heat Loss by 70%

Published 28 July 2026 · Sash Window Draught Proofing

For owners of period properties, improving window performance involves more than choosing the lowest U-value. Original sash windows contribute to a building’s character, proportions and historic value. Replacing them with modern sealed units may improve thermal performance, but it can also remove original timber, alter the façade and create planning or conservation concerns.

Secondary glazing provides a different approach. It adds a separate internal glazing system behind the existing sash window. The original window remains in place, while the new pane creates a sealed thermal and acoustic barrier.

When correctly designed, secondary glazing can reduce heat loss through single-glazed windows by up to 70% in favourable conditions. The result is improved comfort, lower heating demand and better thermal efficiency for sash windows without replacing the historic fabric.

What causes heat loss through sash windows?

Heat moves through a window in several ways:

• Conduction: Heat passes through the glass and timber frame from the warm interior towards the colder exterior.
• Convection: Air next to the cold glass cools, sinks and circulates, carrying heat towards the outside.
• Radiation: Warm surfaces inside the room emit infrared energy towards the colder pane.
• Air leakage: Gaps around the sashes allow warm air to escape and cold air to enter.

Older sash windows are particularly vulnerable to air leakage. Movement over time can create gaps between the sashes, staff beads, parting beads and frame. Worn cords, loose hardware and historic settlement can also affect how tightly the window closes.

Professional sash window draught proofing addresses these gaps by installing discreet seals and adjusting the sashes. However, the original glass remains a single layer. Secondary glazing adds a second, independently sealed barrier.

How secondary glazing creates a thermal barrier

Secondary glazing is fitted to the room side of the existing window. Depending on the property and reveal depth, the new pane may be fixed, hinged, sliding or lift-out.

The key thermal feature is the air cavity between the two glazing systems.

Why still air slows heat transfer

Air is a relatively poor conductor of heat when it remains still. A secondary glazing unit traps air between the original sash window and the new internal pane. This creates resistance to heat flow.

The cavity also interrupts convection. Without a sealed gap, warm room air can move directly across the face of the cold original glass. With secondary glazing, the air within the cavity becomes a separate, slower-moving layer.

A typical high-performance installation may use an air gap of approximately 100–150mm. A 150mm cavity is particularly useful where acoustic performance is also important, because the distance between the panes helps reduce the transfer of airborne sound.

The system works through several combined effects:

1. The original single-glazed pane remains the first barrier.
2. The wide cavity slows conduction and limits air movement.
3. The secondary pane provides a second physical barrier.
4. Perimeter seals reduce draughts and uncontrolled air leakage.
5. Low-emissivity glass can reduce radiant heat transfer further.

The air gap is important, but it is not the only factor. Poor seals, an unsuitable glass specification or a deteriorating primary window can reduce the result.

Why a 150mm air gap can approach 70% heat-loss reduction

The performance of a window is commonly expressed through its U-value. This measures the rate of heat transfer through the window in watts per square metre per degree of temperature difference. Lower values indicate better insulation.

Typical indicative values are:

For example, reducing a window’s U-value from 5.8 to 1.9 W/m²K represents approximately a 67% reduction in heat transfer through the glazed area.

This is the basis for the “up to 70%” figure. It describes potential heat-loss reduction through the treated window, not a guaranteed 70% reduction in the property’s total heating bill. Whole-house savings also depend on roof insulation, walls, floors, ventilation, heating controls and the number of windows treated.

Historic England’s guidance on secondary glazing also highlights the value of low-emissivity glass, particularly for reducing radiant and conductive losses in historic buildings. The best specification should be based on the property, window condition and available reveal depth rather than a headline percentage alone.

Does secondary glazing eliminate condensation and mould?

Condensation occurs when warm, moisture-laden air reaches a surface cold enough for water vapour to condense. Single-glazed windows are often the coldest internal surface in a room, so they can collect moisture during winter.

Secondary glazing raises the temperature of the internal-facing surface. This reduces the likelihood of condensation forming on the room-side glass and can help prevent the damp conditions that contribute to mould around window reveals.

A properly sealed system can therefore eliminate recurring condensation on the original window in many situations. However, it does not remove moisture from the building. If indoor humidity is high, ventilation is inadequate or water is entering through the structure, condensation and mould may still occur elsewhere.

For the best outcome:

• Maintain reasonable indoor humidity.
• Use kitchen and bathroom extraction.
• Avoid blocking background ventilation.
• Inspect external pointing, sills and drainage.
• Ensure the cavity and seals are correctly detailed.
• Allow access for cleaning and periodic inspection.

Secondary glazing should be viewed as part of a moisture-management strategy, not a substitute for ventilation or repairs.

Secondary glazing compared with double glazing

Both systems use two panes to improve insulation, but they are installed differently.

Double glazing

Double glazing replaces the original single-glazed sash with a factory-sealed unit containing two panes separated by a narrow spacer. It can deliver strong thermal performance, particularly with low-emissivity coatings and inert gas filling.

Its limitations for period properties may include:

• Loss of original glass and timber.
• Heavier sashes and altered operating characteristics.
• Changes to glazing bars and frame proportions.
• Greater visual impact from thicker sealed units.
• Potential planning or conservation objections.
• More difficult repair if the sealed unit fails.

Secondary glazing

Secondary glazing adds an independent internal pane while retaining the original window.

Its main advantages include:

• Preservation of original timber and period glass.
• Reversible installation.
• Strong thermal improvement without external alteration.
• Significant acoustic performance with the correct glass and cavity.
• Suitability for many listed buildings and Conservation Areas.
• Flexibility to use discreet fixed, sliding or hinged units.

Its limitations include:

• More visible internal framing than a single pane.
• Reduced access to the original window when the unit is closed.
• The need for careful cleaning and maintenance.
• A higher initial cost than basic draught proofing.
• Potential restrictions on opening the primary window, depending on design.

For many heritage properties, secondary glazing offers the best balance between conservation and performance. It does not necessarily outperform every modern double-glazed unit on a laboratory U-value basis. Its advantage is that it improves performance while preserving the original building fabric.

Secondary glazing and sash window draught proofing: which comes first?

Draught proofing and secondary glazing solve related but different problems.

Professional draught proofing seals gaps around the original sashes. It is usually the more cost-effective first step where the main issue is cold air infiltration. Typical prices are approximately £150–£300 per window, depending on size, condition and access.

Secondary glazing addresses heat transfer through the glass as well as residual air leakage. Typical costs are approximately £400–£800 per window, depending on the frame system, glass specification, configuration and installation requirements.

Choose draught proofing if:

• The primary problem is a noticeable cold draught.
• You want minimal visual change.
• The windows remain structurally sound.
• Your budget is limited.
• Noise reduction is not the main objective.

See our cost of draught proofing guide for a breakdown of typical pricing and cost factors.

Choose secondary glazing if:

• Maximum thermal improvement is the priority.
• The property faces a busy road, railway or flight path.
• You want to retain original sash windows.
• The building is listed or within a Conservation Area.
• You need a substantial improvement in comfort and acoustic performance.

Combine both if:

The original windows are valuable but draughty, the property is exposed to noise, or you want to maximise the return on investment. Sealing the sash window first helps the secondary glazing perform as intended.

The acoustic benefit of a wide cavity

The same 100–150mm gap that helps thermal performance can improve sound insulation. Different pane thicknesses, laminated acoustic glass and airtight perimeter seals can interrupt sound transmission more effectively than standard single glazing.

The final result depends on frequency, glass type, frame design and installation quality. Draught proofing alone may provide modest noise improvement by closing gaps, while acoustic secondary glazing is more appropriate for persistent traffic or aircraft noise.

For location-specific considerations, homeowners can review this Westminster secondary glazing guide, the Islington area guide, or the Camden area guide. Properties affected by local traffic or aircraft noise may also benefit from the Richmond secondary glazing guide and this Islington and Canonbury acoustic guide.

You can also compare local installation approaches through the Westminster secondary glazing installers page, Camden installation guide, Westminster glazing resource and Marylebone secondary glazing guide.

Is secondary glazing worth the investment?

For a period property, the value is not limited to reduced heating demand. Secondary glazing can provide:

• Warmer internal window surfaces.
• Fewer cold spots and draughts.
• Lower risk of condensation on the inner face.
• Reduced external noise.
• Retention of original architectural details.
• A reversible alternative to window replacement.
• Improved comfort without changing the external façade.

The payback period depends on the number and size of windows, energy prices, occupancy and the condition of the existing frames. Draught proofing generally has a shorter payback because its initial cost is lower. Secondary glazing offers a larger performance gain where heat loss and noise are both significant.

Frequently asked questions

Is a 150mm air gap always the best option?

Not necessarily. A 150mm cavity is a high-performing option, particularly where acoustic insulation and deep window reveals are available. The ideal gap depends on the system, glass type, ventilation detail and the geometry of the existing window.

Can secondary glazing be installed in a listed building?

It is often suitable because the work is internal, reversible and leaves the external façade unchanged. However, listed building consent, estate approval or other permissions may still apply. A property-specific assessment is advisable.

What is the typical sash window soundproofing cost?

Costs vary according to window size, access, the required acoustic glass and the number of windows. Secondary glazing typically costs more than basic draught proofing, but it provides a wider improvement in thermal and acoustic performance. A survey is needed for an accurate quotation.

Will secondary glazing stop all mould?

No. It can reduce condensation on cold window surfaces, but mould may continue if humidity, ventilation or water ingress is not addressed.

Improve sash window performance without losing character

Secondary glazing works because it combines a second pane, a wide still-air cavity, improved airtightness and, where appropriate, low-emissivity or acoustic glass. A 150mm air gap can substantially slow heat transfer and may bring window heat-loss reductions close to 70% in favourable installations.

For many Victorian and Georgian properties, it is a practical alternative to replacing original windows. Our secondary glazing for sash windows is designed around the existing frame, period details and conservation requirements.

If you are comparing options, start with a survey of the window condition, reveal depth, ventilation and noise exposure. We can then advise whether draught proofing, secondary glazing or a combined approach provides the most appropriate long-term return. Contact Sash Window Draught Proofing for a professional assessment.