Sash Windows in London Renovations: Repair, Draught-Proofing, and Secondary Glazing in Listed and Conservation Buildings

The double-hung sliding sash window — a timber frame divided into two vertically sliding sashes, each glazed with multiple panes — is as close to an architectural constant as London has. From the Georgian terraces of Bloomsbury to the late-Victorian houses of South Kensington, the proportions of these windows define the rhythmic character of the streets. They are also, in their original single-glazed state, cold, draughty, noisy, and thermally inefficient by modern standards.

Managing sash windows in a prime London renovation requires balancing the conservation imperative (authentic windows are irreplaceable) against the practical requirements of comfortable modern habitation. The good news is that the two goals are more compatible than they might appear.

The Conservation Position

Most prime London postcodes are within conservation areas. Many properties are also listed — Grades I, II*, or II. The planning position is clear and consistent:

• —Listed buildings: Any replacement of original windows — regardless of the replacement material or glazing specification — is likely to constitute unauthorised alteration requiring Listed Building Consent (LBC). Applications to replace original windows with double-glazed uPVC or even timber double-glazed units are routinely refused. The presumption is in favour of repair.
• —Conservation areas: Replacing original windows with uPVC is not permitted in conservation areas; timber or aluminium double-glazed replacements in the same profile may be approved where the original windows are beyond economic repair, but the bar is high and requires Conservation Officer agreement.
• —Permitted development rights for window replacement are suspended in conservation areas and for listed buildings — any replacement requires formal consent.

The principle underlying conservation policy is that original fabric, once lost, cannot be recovered. A Victorian sash window removed and replaced with a modern double-glazed unit — even a high-quality one — is gone permanently. The planning system takes this seriously.

When Repair is the Answer

In the majority of cases, repair is both the planning-compliant and the economically rational choice. Original Victorian sash windows are made from dense, slow-grown Baltic redwood or pitch pine — timber of a quality that is no longer commercially available. Well-maintained original windows outlast modern replacement windows and, critically, are repairable indefinitely.

Common defects and their repair:

• —Rotten sills: External window sills bear the brunt of rain exposure. Sill rot is almost always localised — the bottom 50mm of the sill most commonly. Epoxy resin consolidant (Polyfilla Deep Fill, Repair Care Dry Flex) is applied to stabilise remaining sound timber; the rotten section is cut back to sound wood and a new section spliced in, either in hardwood or in epoxy. A full sill replacement (in hardwood) is required only where rot extends to the junction with the window frame.
• —Failed putty: Linseed oil putty on single-glazed sashes has a working life of 20–40 years before it shrinks, cracks, and falls away. Re-puttying all panes — removing old putty, priming the rebate, bedding new glass in linseed putty, facing off — is straightforward but time-consuming. Oil-based linseed putty is specified for traditional windows; acrylic putty is not appropriate on listed buildings.
• —Broken sash cords: Woven cotton or linen sash cords last 20–40 years; polyester cords are longer-lasting replacements. A broken cord causes one sash to drop or jam. Replacing requires removing the staff bead (the innermost moulding on the frame), removing the sash, accessing the weight pocket in the box frame, and re-threading the cord over the pulley to the cast iron weight. A competent joiner completes a cord replacement in 30–60 minutes per sash.
• —Swollen frames in summer / draughty frames in winter: Victorian sash windows were never draught-sealed; this is intrinsic to their design. The solution is draught-proofing, not replacement (see below).

Repainting:

Original sash windows should be painted with an oil-based primer and undercoat system, finished in an oil-based gloss. Solvent-based alkyd paints adhere better to timber in exposed conditions than water-based acrylics and provide better moisture resistance. The fashion for water-based finishes on windows is a false economy in a London climate — water-based paints crack and peel faster in the thermal cycling experienced by south-facing windows.

Draught-Proofing

A well-draught-proofed original sash window performs significantly better thermally than an unmodified one — draught infiltration is responsible for a large proportion of heat loss through traditional windows — and is indistinguishable in appearance from the outside.

The standard draught-proofing system for sash windows consists of:

• —Pile weatherstrip: A soft, dense-pile strip (typically 4mm or 6mm pile height) inserted into a kerf (groove) routed into the meeting rails, the parting bead, and the staff bead channels. The pile compresses against the opposing surface to form an air seal while allowing the sash to slide freely.
• —Brush seals at bottom rail: The bottom sash rail runs against the sill; a brush seal glued or pinned to the bottom rail seals the gap between rail and sill.
• —Parting bead replacement: The original parting bead (the central dividing bead between inner and outer sash tracks) is typically replaced with a new moulded section that incorporates a groove for the pile strip.

Specialist sash window draught-proofing companies (Ventrolla, Selectaglaze, The Sash Window Workshop) carry out draught-proofing as a dedicated service. A thorough draught-proofing of a single window takes 3–4 hours and costs £200–£400. A typical London terraced house has 12–20 sash windows; total draught-proofing cost: £3,000–£8,000. Energy savings and comfort improvement are substantial.

Secondary Glazing

Where thermal and acoustic performance must be improved beyond what draught-proofing alone achieves, secondary glazing — a separate inner window unit installed behind the original sash — is the conservation-approved solution.

What secondary glazing does:

• —Creates an insulating air gap between the original single-glazed window and the inner secondary unit — achieving effective U-values of 1.8–2.2 W/m²K (compared to 5.0–5.5 W/m²K for an unmodified single-glazed window)
• —Significantly reduces sound transmission — the air gap between original and secondary unit is the critical acoustic variable; a 100mm gap achieves approximately 45 dB Rw sound reduction, suitable for properties on busy streets
• —Eliminates condensation on the inner surface of the original window

System types:

Secondary glazing is installed as a separate framed unit fixed to the window reveal or surround:

• —Sliding horizontal secondary units: The inner secondary unit mirrors the sash configuration — two horizontally sliding panels. Suitable for windows where opening access is required.
• —Lift-out panels: Single fixed panel, lifted out for ventilation. Appropriate for windows where access is infrequent.
• —Hinged casement: The secondary unit is hinged at one side, opening inward for access. Appropriate for larger windows.
• —Vertical sliding secondary: The inner unit mirrors the original sash action. Most visually coherent but most complex mechanically.

Frame materials:

Secondary glazing frames in prime London renovations are typically: - Aluminium (slim-line, powder-coated): The standard choice. Slim 25–35mm frame profiles are unobtrusive. Colour-matched to the window frame (typically white or off-white) or in a contrasting metal finish. - Timber: Less common; appropriate in listed buildings where the conservation officer requires it.

Glazing specification:

The secondary glazing unit itself is typically: - 6.8mm laminated glass (two 3mm panes bonded with a PVB interlayer) — provides both thermal benefit and acoustic improvement; the laminate layer damps sound transmission differently from the original single glass, improving overall attenuation - 6.4mm acoustic laminated glass — enhanced acoustic specification, appropriate for properties on major roads

Planning and listed buildings:

Secondary glazing is considered reversible (it does not alter the original window) and is generally acceptable to conservation officers without formal consent. For listed buildings, it is best practice to notify the local authority and obtain written confirmation that consent is not required — this protects the owner against any future ambiguity.

Replacement Windows: When Is It Permissible?

In a small number of cases — where the original windows are genuinely beyond economic repair (extensive structural rot throughout the frame, failed joints at all corners, sills rotted through to the box frame) — replacement is appropriate.

In conservation areas (not listed):

Replacement requires planning permission. The replacement window must match the original in: - External profile and section dimensions - Glazing bar arrangement and proportions - Opening configuration - Material: timber is the standard expectation; slim-section aluminium may be acceptable where the profile closely matches timber; uPVC is not acceptable

Slim-profile double-glazed timber windows — made by specialist joiners to match the original section dimensions — are the standard replacement specification in conservation areas. Suppliers include The Sash Window Workshop, Roseview Windows (uPVC, not applicable here), and bespoke joiners working from measured drawings of the original.

Double glazing in slim profiles:

Modern slim-profile sealed units (6/12/4mm or 4/16/4mm) can be manufactured in the thicknesses appropriate for traditional sash frames. The sight lines and shadow lines differ slightly from the original putty-glazed single-pane appearance but are acceptable to most conservation officers when the total frame section dimensions are matched.

Vacuum glazing:

Vacuum-glazed units (Pilkington Spacia, LandVac) achieve double-glazing-equivalent thermal performance (U-value ~1.0 W/m²K centre pane) at a total unit thickness of 6–8mm — thin enough to be installed within the traditional sash frame section without altering the proportions. Vacuum glazing is significantly more expensive than conventional sealed units (£300–£600/m² vs £80–£150/m²) but is the most sympathetic upgrading option for original windows where replacement glazing is considered.

Summary

The hierarchy of intervention for original sash windows in prime London renovation is:

1. 1.Repair and draught-proof — appropriate for all windows in good structural condition; achieves substantial comfort improvement at modest cost
2. 2.Repair, draught-proof, and add secondary glazing — achieves near-modern thermal and acoustic performance while retaining the original window; appropriate for properties on busy streets or with high-specification comfort requirements
3. 3.Replace with matched timber double-glazed units — appropriate only where the original window is beyond economic repair; requires planning permission in conservation areas
4. 4.Vacuum glazing in original frames — premium option where the frame is sound but single glazing is inadequate; avoids the planning complexity of full replacement

The common error — specifying replacement windows as the default — is both planning-risky and typically unnecessary. Original London sash windows, properly repaired and draught-proofed, are excellent windows.