With EPC band C now confirmed for October 2030, single-glazed sash windows are one of the hardest — and most overlooked — places to claw the points back. Here is what works on heritage stock. On 21 January 2026 the government ended years of on-again, off-again uncertainty: every privately rented home in England and Wales will need an Energy Performance Certificate rating of at least C by 1 October 2030.
One date, new and existing tenancies alike, with the cost of getting there capped at £10,000 per property (or 10% of its value, if that is lower); a landlord who spends to the cap without reaching C can register a ten-year exemption.
For landlords with modern stock, that is a manageable to-do list. For anyone letting a Victorian terrace, a Georgian conversion or a period flat, a single line item tends to decide the whole certificate: the windows.⧉
Single glazing is the quiet EPC killer. A traditional single-glazed timber sash has a U-value (a measure of how quickly heat escapes) of around 4.8 W/m²K. Modern insulated glazing sits nearer 1.2 to 1.6. Because the assessment behind an EPC reads every window in the property, a home full of original glass is shedding points before the assessor has even looked at the boiler.
And the way EPCs are calculated is being overhauled: the reformed certificates — now expected in the second half of 2027, after the original 2026 target slipped — will score a home on several measures rather than one, among them a dedicated fabric rating for how well the building holds heat. That puts the fabric itself, glazing included, front and centre in a way today’s cost-based rating never has.
The heritage catch
The obvious fix — tear it all out and drop in uPVC double glazing — is off the table for a large slice of the rental market. A great deal of period stock sits within conservation areas, and listed status adds a further layer of control on top.
Swapping characterful timber sashes for plastic units is frequently refused by planners, drags down value, and on a listed building can amount to a criminal offence. Historic England’s guidance is unambiguous: original windows should be retained and upgraded wherever possible, not replaced.
Fortunately, the compliant routes are also the effective ones — and there are two that do most of the heavy lifting.
What actually works on a sash window
Secondary glazing — a slim, discreet internal pane fitted behind the existing window — is one of the very few upgrades acceptable almost everywhere, conservation areas and most listed buildings included.
Because it is entirely internal and reversible, it needs no planning permission and rarely listed building consent — though on a listed property it is worth a quick word with the conservation officer first. It roughly halves the U-value of a single-glazed sash, to around 2.2–2.9, cuts draughts and street noise, and on a borderline property can be enough to lift it a band on its own.
Where the timber frames are deep enough and the existing glass is of no historic interest, the second option is to reglaze the sashes themselves with slimline or vacuum double glazing — modern sealed units a fraction of the thickness of standard double glazing that sit inside the original frames.
Done properly, the window is indistinguishable from the street but performs like a modern one. On a listed building this usually requires listed building consent, so the first call is to the local conservation officer, not the installer.
None of this is a silver bullet, and landlords should treat “new windows will get you to C” with caution. Glazing typically adds two to three points to an EPC score in isolation. But on a period property sitting at D or E, those points are frequently the ones that are missing at a band boundary — and, unlike a heat pump or external wall insulation, they are available without disruptive work between tenancies or a raid on the whole £10,000 budget.
Pair heritage glazing with loft insulation, draught-proofing and better heating controls, and band C becomes realistic even for stock built long before anyone had heard of an EPC.
Why the early movers win
The case for acting now rather than in 2029 is straightforward. Heritage glazing lasts for decades, cuts tenants’ heating bills, tends to add value rather than strip it, and doesn’t force a property off the market while the work is done. Leave it late and you will be competing for a shrinking pool of specialist installers against every other landlord who also waited.
The work is genuinely skilled — matching sightlines, keeping the original box frames and weights, getting the acoustic and thermal detail right — so it pays to use a restoration firm such as Scott James Sash Windows Specialists, which retrofits slimline double glazing or secondary glazing into the existing frames, rather than a company that simply sells replacements.⧉
For landlords, the first move costs nothing: pull the current EPC and see where the property sits today. If the windows are single-glazed and the rating is D or below, glazing is very likely to be the single most effective and most planning-friendly upgrade on the table, and the one that turns a looming 2030 problem into a solved one.
