Secondary glazing in a listed house

Secondary glazing is the alteration most conservation officers will accept in a listed house, and replacement double glazing is the one they will usually refuse. The reason is reversibility. A secondary unit fitted inside the reveal can be taken out in an afternoon and leaves the historic sash, the glazing bars and the original glass untouched, so the special interest of the building survives the intervention. That is the argument your application has to make, and it has to be made with drawings rather than assertions.

You will still need listed building consent for most secondary glazing schemes. Internal works count, because section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 catches any works for the alteration of a listed building that would affect its character as a building of special architectural or historic interest.

Why replacing the windows is usually resisted

Historic windows carry a disproportionate share of a building's significance. Hand-drawn cylinder and crown glass has visible distortion and a liveliness that float glass does not reproduce. Georgian and early Victorian sashes have glazing bars of 15mm to 20mm with a fine moulded profile, and the sealed units in a modern replacement cannot be held in anything that slender without the bar thickening. Original joinery also tells you about the building's date and its sequence of alteration.

Replacing all of that to gain a thermal improvement fails the test in section 16(2) of the 1990 Act, which requires the decision-maker to have special regard to the desirability of preserving the building and any features of special architectural or historic interest it possesses. Officers will also point out that most heat loss from a sash window is air leakage around the meeting rail and the sash boxes rather than conduction through the glass. Draught-proofing and repair deal with that at a fraction of the cost and with no loss of fabric.

And the repair case is strong on its own terms. A sound softwood sash from 1830 has heartwood pine in it that will outlast anything you can buy now, provided the paint film is maintained.

Secondary glazing as a reversible alteration

Reversibility is not a magic word that makes consent automatic. It has to be designed in. The questions an officer asks are where the new frame is fixed, what is cut, and what the room looks like when the unit is in place.

• Fix to the window board, the lining or a timber subframe, never through historic joinery or into the sash box
• Avoid cutting shutters, shutter boxes or panelling, and show in section how the unit clears them
• Set the frame back far enough that the architrave and the reveal mouldings stay legible
• Use a slim aluminium or timber frame in a dark recessive finish rather than white, which reads as a bright line across the opening
• Keep the opening method appropriate: vertical sliders for sashes, side hung casements for casement openings, lift-out panels where access is rare

Horizontal sliders are inexpensive and awkward in a vertical sash opening, because the meeting style never lines up with anything in the host window. In a tall Georgian sash the better answer is a two-panel vertical slider with the horizontal division set at the meeting rail of the original.

Frame position, sightlines and the sash

The detail that decides whether a scheme looks acceptable is the alignment between the new framing members and the existing ones. Where the secondary frame member sits directly behind an existing glazing bar or meeting rail, it disappears. Where it sits 80mm away from it, you see two lines.

Ask for a measured survey of the existing window before the secondary system is specified, and plot the new framing on the same drawing. On a sash of unequal lights, which is common where a Victorian owner replaced Georgian glazing with two-over-two, the division will not sit at mid-height and a standard product will not fit.

Ventilation matters too. A sealed secondary unit over a single glazed sash creates a cold cavity where condensation will form on the outer pane in winter. The usual answer is to allow the cavity to breathe to the room rather than to the outside, and to keep the secondary unit itself reasonably airtight. Where there is a history of condensation and decay in the sash boxes, the moisture problem should be solved before the glazing goes in.

Thermal performance against appearance

Approved Document L accepts that in a listed building or a building in a conservation area, special considerations apply and work should not unacceptably alter the character or appearance of the building. Secondary glazing is a recognised route to improvement in exactly those circumstances, which means you are not arguing against the Building Regulations when you propose it.

The acoustic benefit is the part owners underestimate. An air gap of around 100mm or more between the original glass and the secondary pane does far more for sound insulation than for heat loss, which is why secondary glazing is the standard answer for a listed house on a main road. If traffic noise is the real problem, say so in the application and specify the cavity depth deliberately rather than taking whatever the system allows.

Where slim profile units are accepted

Some authorities will accept slim profile sealed units in existing sashes, usually where the windows are later replacements of no particular interest, where the glass is modern float, or where the building is listed at grade II with a much altered interior. The arguments against are practical as well as philosophical. Deepening the rebate to take a unit weakens a slender bar, the sash gets heavier and needs rebalancing, and the sealed unit has a service life measured in decades while the joinery has a life measured in centuries. Edge seal failure in a thin cavity is the usual failure mode.

Where this route is proposed, the application should identify window by window which sashes are original and which are not, and limit the intervention to the later ones. A blanket proposal across an elevation invites a refusal.

The listed building consent application

There is no fee for an application for listed building consent. What you submit decides how long it takes. For a secondary glazing scheme expect to provide a location plan, existing and proposed elevations and sections through the window at a scale of 1:5 or 1:10 showing the frame position and fixings, a window-by-window schedule, photographs of each window type, product information for the proposed system, and a Heritage Statement that identifies the significance of the windows and explains the effect of the proposal on it.

The statement is the document officers read first. It should set out what the windows contribute to the special interest, what is original and what is not, and why the chosen system causes the least harm of the available options. A short Design and Access Statement is not required for listed building consent on a dwelling but it does no harm where the scheme is extensive.

Talk to the conservation officer before you apply. Most authorities offer pre-application advice on listed buildings, and a fifteen minute conversation about frame colour and fixing positions saves a round of amended drawings. Unauthorised works to a listed building are a criminal offence under section 9 of the 1990 Act, so do not start on the basis that nobody will see an internal alteration.

Recording the windows before work starts

Photograph and measure every window before anything is touched, with a scale in shot and a plan showing which window is which. If any historic glass has to come out, number it and keep it. For a graded building where the joinery is a notable part of the interest, a short written and photographic record along the lines of Historic England's recording levels gives you a baseline, and Historic Building Recording is sometimes required by condition where fabric is affected.

Where you have also applied for planning permission for other external works, a Planning Statement can carry the policy case while the heritage statement deals with significance. Keep the two documents consistent on what is being removed.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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