Secondary Glazing vs Double Glazing on Cost

The headline difference

Per opening, secondary glazing typically lands well below a like for like replacement window, and on period property the gap widens because the replacement has to be a bespoke timber or heritage aluminium unit rather than a standard white plastic one. The comparison people usually make, secondary glazing against the cheapest available replacement, is not a fair one on a Victorian terrace, because that cheap replacement is not something you would want in the opening.

The costs that only replacement carries

Taking a window out is a building job, not a glazing job. There is removal and disposal of the old frame, making good the reveal internally and externally, redecoration, occasional lintel or cill repair discovered once the frame is out, external access and sometimes scaffolding, and in a conservation area or listed building a consent application with drawings. Secondary glazing carries none of these because nothing is removed and nothing outside the building changes.

• Removal and disposal of the original window
• Internal making good and redecoration
• External access or scaffolding
• Consent applications on protected property

Performance per pound on noise

If your problem is noise, secondary glazing is not merely cheaper, it is better. A sealed double glazed unit has a cavity of 12mm to 20mm and the two panes are effectively coupled through it. A secondary system gives you 100mm or more of decoupling. Spending double for a worse acoustic outcome is the mistake we most often talk people out of.

Performance per pound on heat

On pure thermal performance a modern replacement window with a warm edge sealed unit will usually edge ahead of secondary glazing on the centre pane U value. Where secondary glazing catches up in practice is on air leakage: sealing an old draughty window on the room side removes the uncontrolled air changes that account for a large share of the heat you actually lose, and it does it for a fraction of the outlay. For most older houses the pounds per degree of comfort favour secondary glazing.

The asset you keep

There is a value in the original window that does not show up on either quote. Original glass with its slight distortion, original joinery, original glazing bar profiles and original ironmongery are not reproducible and are part of why a period house is worth what it is. Replacement removes them permanently. Secondary glazing is fully reversible: unscrew the frames and the house is as it was.

When replacement is genuinely the better buy

We will say so when it is. If the existing frames are rotten beyond economic repair, if the house is a modern build with no character to preserve, if the opening is already a failed sealed unit, or if you need the whole window replaced for structural reasons, then paying once for a good replacement beats paying for secondary glazing over a window that needs rebuilding anyway. Secondary glazing improves a window, it does not repair one.

How to decide

Ask what the window is currently costing you: noise, draughts, cold, condensation, or all four. Then ask what condition the frame is genuinely in. If the frame is sound and the problem is comfort, secondary glazing gives you far more improvement per pound. If the frame is failing, budget for repair or replacement and consider secondary glazing afterwards for acoustic performance.