Thermal Efficiency for Sash Windows: 10 Ways Your Period Property Isn't Holding Heat

Published 15 July 2026 · Sash Window Draught Proofing

Victorian and Georgian terraces are valued for their proportions, timber joinery and original architectural details. However, many were built before modern insulation standards existed. Their single-glazed sash windows, ageing timber and numerous moving joints can make it difficult to retain heat during a London winter.

The good news is that improving thermal efficiency for sash windows does not necessarily mean replacing the originals. With a carefully planned combination of repairs, sealing, glazing improvements and wider draught reduction, many homeowners can improve comfort and potentially raise their EPC rating by one or two bands.

The exact result depends on the whole property, including roof insulation, walls, floors, heating systems and existing EPC score. No window upgrade can guarantee a particular rating. However, addressing the windows is often one of the most practical ways to improve performance without altering the exterior period appearance.

Why Victorian and Georgian terraces lose heat

Original sash windows lose heat in two main ways:

1. Air leakage: Cold air enters through gaps around the sliding sashes, meeting rails, parting beads, staff beads and frame-to-wall junctions.
2. Radiant and conductive heat loss: Single-pane glass provides very little resistance to heat escaping from the room.

A draughty sash window can also make a room feel colder than the thermostat suggests. Occupants may increase the heating to compensate, increasing energy use without resolving the underlying problem.

Typical original single glazing has a much higher U-value than modern insulated glazing. The higher the U-value, the more heat passes through the window. Adding effective seals and a second internal pane can significantly improve the overall performance while keeping the original timber and glass in place.

10 ways your period property may not be holding heat

1. Single-pane glass is allowing rapid heat loss

Most original sash windows in London terraces contain single glazing. A single pane has limited insulating value and becomes cold on winter evenings. Heat from radiators and room occupants is transferred towards the glass and lost to the outside.

Replacing the entire window is not always the best first step, particularly in a listed building or Conservation Area.

Practical options include:

• Adding secondary glazing on the room side.
• Using low-emissivity glass where re-glazing is appropriate.
• Installing slimline heritage double glazing only where the existing frame and planning context allow it.
• Retaining original or historic glass where it contributes to the character of the property.

Secondary glazing for sash windows can reduce window heat loss by up to 70%, depending on the system, glass specification, cavity and installation quality.

2. Gaps around the parting beads are creating cold-air paths

Parting beads separate the upper and lower sashes within the box frame. They are narrow but important components. Over time, timber movement, paint build-up and general wear can create gaps between the beads and the sliding sashes.

These gaps allow cold air to enter continuously, especially on exposed elevations or upper floors.

A specialist can install discreet brush seals or replace worn beads with draught-proofed alternatives. The work is designed to maintain sash movement while reducing uncontrolled air infiltration.

3. Worn staff beads are failing to seal the window

Staff beads hold the sashes in position at the front of the frame. They are frequently affected by cracked paint, shrinkage, impact damage and repeated removal during previous repairs.

When staff beads no longer sit tightly against the sash, air can bypass the window perimeter. This is a common reason for rattling, whistling and cold spots around the opening.

Replacing or upgrading staff beads is usually less disruptive than replacing the entire window. It also allows the installer to incorporate a modern seal without creating a visible change to the external elevation.

4. The meeting rail has an open or ineffective seal

The meeting rail is where the upper and lower sashes meet when the window is closed. It is one of the most vulnerable areas for draughts because it combines a moving joint with a relatively large exposed surface.

Signs of a problem include:

• A visible line of daylight.
• A cold stream of air at hand level.
• Sashes that rattle in windy conditions.
• Difficulty locking the window tightly.
• Crumbling paint or damaged timber at the rail.

A properly fitted meeting-rail seal can substantially reduce air leakage. It should be installed without preventing the sashes from opening for ventilation or emergency escape.

5. The frame-to-wall junction is not airtight

Heat loss does not occur only through the moving sashes. Gaps can also develop between the timber window box and the surrounding masonry. Previous building work, settlement and failed sealants can leave cracks around the reveals and sill.

During a survey, the installer should inspect:

• The internal window reveals.
• The sill and lower corners.
• The junction between the box frame and brickwork.
• Areas hidden behind architraves where accessible.
• Evidence of damp, rot or failed pointing.

Sealing these junctions forms part of effective draught proofing for period properties. It is important to use compatible materials and avoid trapping moisture against historic timber.

6. Paint build-up is preventing the sashes from closing properly

Repeated painting can reduce the clearance between the sash and frame. The result is often a window that sticks, closes unevenly or remains slightly open at one corner.

This affects thermal efficiency in two ways. First, it creates direct air gaps. Second, it can damage seals when the window is forced open or closed.

Careful easing, adjustment and local paint removal can restore correct operation. If the cords are worn or broken, sash cord replacement may also be required before draught proofing is installed.

7. Uninsulated weight boxes are acting as cold zones

Traditional box sash windows contain cavities for weights, pulleys and cords. These cavities are an essential part of the original mechanism, but poorly sealed weight pockets can contribute to cold surfaces and air movement.

A refurbishment survey should determine whether the pockets, access panels and surrounding joints are sound. Any remedial work must preserve access for future maintenance and avoid damaging the original frame.

This is one reason a comprehensive refurbishment can be more effective than applying seals to the visible sash edges alone. Sash window refurbishment can combine timber repairs, cord replacement, hardware servicing and integrated draught proofing.

8. The windows have no secondary thermal barrier

Draught proofing reduces air leakage, but it does not change the fact that the original window remains single glazed. If the room is still cold after sealing, the next step may be secondary glazing for sash windows.

Secondary glazing adds a separate internal glazed panel behind the original sash. The trapped air space improves insulation and also reduces external noise. Systems can be sliding, hinged, lift-out or fixed, depending on the window design and room requirements.

Choose secondary glazing if:

• Heat retention is the primary objective.
• The property faces a busy road, railway or flight path.
• The original windows must remain untouched.
• The building is listed or located in a sensitive Conservation Area.
• You want a reversible internal improvement.

For local reference, homeowners can compare heritage secondary-glazing approaches in Kensington, Chelsea, Islington and Camden. A further guide covers secondary glazing in Hampstead and Belsize Park, where period architecture and traffic noise are common considerations.

9. Curtains and blinds are not being used effectively

Window coverings are not a substitute for draught proofing or glazing, but they can reduce heat loss at night.

Consider:

• Full-length, lined curtains that overlap the window reveal.
• Thermal blinds fitted close to the glass.
• Internal shutters where they are historically appropriate.
• Closing curtains at dusk rather than after the room has already cooled.
• Keeping curtains clear of radiators to avoid blocking warm air circulation.

Avoid covering ventilation routes or creating condensation traps. Window coverings should complement, not conceal, unresolved damp or air-leakage issues.

10. The rest of the property is undermining the window upgrade

Windows are only one part of the building envelope. Draughts through external doors, floorboards, skirting boards, loft hatches, fireplaces and service penetrations can reduce the benefit of sash-window improvements.

A sensible order of work is:

1. Check loft insulation and roof-level air leakage.
2. Seal obvious gaps around doors and floors.
3. Repair defective pointing and moisture pathways.
4. Draught-proof the sash windows.
5. Add secondary glazing to the coldest or noisiest rooms.
6. Review heating controls and radiator performance.
7. Reassess the EPC after improvements are complete.

This staged approach usually provides better value than replacing windows before the main sources of heat loss have been identified.

Draught proofing or secondary glazing: which should you choose?

Professional sash window draught proofing commonly costs around £150–£300 per window, depending on size, condition and access. Secondary glazing is a larger investment, often around £400–£800 per window for bespoke heritage applications.

Draught proofing can reduce heat loss associated with air leakage by up to 30%. Secondary glazing can reduce heat loss through the window by up to 70%. These figures are maximum indicative performance claims, not guaranteed savings on every property. The final result depends on installation quality, window size, glass specification, room use and the condition of the wider building.

For a detailed comparison, see our guide to secondary glazing versus draught proofing.

Can these improvements raise an EPC rating?

They can contribute to an improved EPC rating, and a one- or two-band increase may be achievable in some properties. However, the rating is calculated from the building’s complete energy profile. Window upgrades alone do not guarantee a particular band.

The strongest EPC improvement usually comes from combining:

• Professional sash window draught proofing.
• Secondary glazing where heat loss is greatest.
• Loft and roof insulation.
• Efficient heating controls.
• Hot-water and pipe insulation.
• Door and floor draught reduction.
• Appropriate low-energy lighting and appliances.

External appearance is usually preserved because draught-proofing seals are discreet and secondary glazing is installed internally. For additional examples of heritage-sensitive solutions, see the area guides for Westminster and Kensington and Chelsea, as well as these local guides for Kensington secondary glazing and Chelsea secondary glazing.

A practical next step for London period-property owners

Start with a window survey rather than assuming replacement is necessary. The survey should identify air gaps, timber defects, failed cords, glazing limitations, condensation risks and conservation requirements.

For many Victorian and Georgian terraces, the most proportionate solution is to restore and draught-proof the original sashes first, then add secondary glazing in bedrooms, front rooms or elevations exposed to traffic. This approach improves comfort and thermal efficiency while retaining the timber, glass and architectural details that define the property.

Contact Sash Window Draught Proofing to discuss a heritage-sensitive assessment and receive a specification based on your windows, property condition and performance priorities.