Energy Bills Are Rising Again: How Secondary Glazing Cuts Heat Loss by 64% in Period Homes

The Ofgem price cap rose 4% on 1 October 2026, taking a typical direct-debit household from £1,663 to £1,723 a year, with a further rise forecast for January. For period-home owners, that makes every source of avoidable heat loss worth a closer look.

Many period homes combine solid walls with single-glazed sash windows. Those windows can have U-values around 5.0 W/m²K, so heat escapes quickly through the glass and gaps around the frame. Thermal secondary glazing adds an insulated internal layer without asking you to remove the original window.

How secondary glazing cuts heat loss

A secondary unit creates a sealed cavity behind the existing sash or casement. The new frame, glass and seals slow conduction and reduce draught movement on the room side. With a suitable specification, the window U-value can improve from 5.0 to 1.8 W/m²K: a 64% reduction in heat loss through the window area.

That figure applies to the window area, not the whole house. Solid walls, roofs, floors, ventilation and heating controls still shape the energy demand. The practical benefit is that the original window stops being the coldest and leakiest part of the room.

A worked period-home example

Imagine a period home where the original sash windows are responsible for a significant share of the heat leaving the rooms. Before the work, the single glazing and draught paths allow heat to move out rapidly. After the secondary layer is fitted, the window area loses 64% less heat under the comparable performance calculation.

The exact saving depends on the property's window area, temperatures, occupancy, heating system and how much of the building fabric is upgraded. We do not attach an invented pound figure to a named home. Customers typically report noticeably lower bills and warmer rooms, but a survey and energy assessment are the honest way to estimate a particular property.

Condensation and EPC performance

By warming the inner surface of the window, secondary glazing can reduce the conditions that create condensation. It does not replace ventilation or fix damp, leaks or cold bridges, so the room and building still need to be assessed properly.

Secondary glazing can lift an EPC rating by 1–2 bands, depending on the whole property. That matters for landlords preparing for the confirmed 2030 Band C requirement: the policy is not yet in force, the current legal minimum remains Band E, the maximum required investment is up to £10,000 per property, and a 10-year cost-cap exemption can be registered if Band C still cannot be reached after the full spend.

Why it suits heritage homes

The system is fully reversible and preserves the original window. Planning permission is typically not required for internal secondary glazing, although listed building consent and local conservation guidance must be checked for the property. It is suitable for many listed buildings and conservation areas because the external elevation remains unchanged.

Read our thermal secondary glazing guide, compare current installed prices in our 2026 cost guide, and see the landlord context in the Band C deep-dive.

Cost and payback

Installed prices including VAT start at £675 for a small casement, £1,220 for a Victorian sash and £2,565 for a Georgian bay. The homepage cites a 3.5-year payback for thermal upgrades; treat that as the site's stated figure, not a guarantee. Use the calculator for a starting estimate and request a survey for a property-specific quote.

Frequently asked questions

How much heat does secondary glazing save?

A suitable specification can reduce heat loss through the window area by 64%, improving the U-value from 5.0 to 1.8 W/m²K. Whole-home savings depend on the property.

Does it work with original sash windows?

Yes. The secondary frame is fitted internally and the original sash can remain in place, preserving the external appearance and historic fabric.

How long does installation take?

A straightforward installation is typically completed in a single day once the bespoke units are ready.