What U-value does secondary glazing achieve?

Around 1.8 W/m²K, down from roughly 5.0 W/m²K for single glazing — comparable to modern double glazing. Adding a low-emissivity coating to the secondary pane takes it to approximately 1.6 W/m²K.

How much heat does secondary glazing save?

Up to 60% less heat loss through the treated window. In a period home where single glazing accounts for 15-25% of total heat demand, whole-house installations commonly deliver a 15-25% reduction in heating use.

Does secondary glazing improve an EPC rating?

Yes. It is recognised in SAP assessments and typically contributes several points in a period property, which is often enough to move a band — relevant for landlords under minimum energy efficiency standards.

What is the best cavity depth for thermal performance?

20-100mm. Beyond that, convection currents begin inside the cavity itself. Where noise is also a priority we use the deeper acoustic cavity and accept a marginal thermal trade-off, since the draught-sealing benefit is unaffected.

Secondary Glazing Thermal Insulation

U-value from 5.0 to 1.8 W/m²K — without replacing a single window.

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5.0 → 1.8

W/m²K U-value

Up to 60%

Less heat loss per window

15-25%

Of a period home's heat demand

£480

From, per window

Single glazing has a U-value of roughly 5.0 W/m²K, meaning it loses heat about six times faster per square metre than an insulated cavity wall. In a Victorian terrace with ten single-glazed windows, that is commonly 15-25% of the entire heating demand of the house.

Adding a secondary unit creates a still air layer between two panes. Still air is an excellent insulator, and the result is a window U-value around 1.8 W/m²K — comparable to modern double glazing, achieved without removing the original joinery.

How the still air layer works

Heat crosses a window by conduction through the glass, convection in any air movement, and radiation. A sealed secondary cavity attacks all three: it adds a second conductive barrier, it stops the convective air exchange that draughty sashes rely on, and low-E coated glass options reflect radiant heat back into the room.

Cavity depth for thermal purposes behaves differently from acoustic depth. Thermal performance optimises around 20-100mm; beyond that, convection currents start within the cavity itself. Where a window is being specified for both noise and heat, we use the acoustic depth and accept a marginal thermal trade-off.

Conduction: second glass barrier plus trapped air
Convection: perimeter seals stop air exchange entirely
Radiation: optional low-E coating reflects heat inward
Draught: full-perimeter EPDM compression sealing

Low-E glass and when it is worth it

A low-emissivity coating on the room-side face of the secondary pane reflects long-wave radiant heat back into the room and can take the assembly to around 1.6 W/m²K. It is worth specifying on north-facing rooms and large glazed areas; on a small window in a warm room the payback is marginal.

EPC and heating bills

Secondary glazing is recognised in SAP assessments and typically contributes several points to an EPC rating in a period property, which matters for landlords facing minimum energy efficiency standards. On heating bills, customers commonly report a 15-25% reduction after a whole-house installation — driven as much by draught elimination as by the U-value itself.

Thermal specification

Single glazing baseline: ~5.0 W/m²K
With secondary glazing: ~1.8 W/m²K
With low-E coated secondary: ~1.6 W/m²K
Optimal thermal cavity: 20-100mm
Draught reduction: Full-perimeter EPDM compression seals
EPC: Recognised in SAP; contributes to rating uplift

Pricing guide

Specification	Price	Notes
Thermal specification, 4mm with low-E	from £420	Heat-loss led, no acoustic priority
Combined thermal + acoustic, 6.4mm	from £480	Most common specification
Low-E coating upgrade	+£60	Per window
Whole-house package (6+ windows)	10% discount	Best payback

All prices supplied and fitted in London, inclusive of survey and 25-year warranty. See the full secondary glazing price list.

Case study: Whole-house thermal retrofit in Ealing

Ealing, W5

Problem: Twelve single-glazed windows, EPC band E, and a landlord facing minimum energy efficiency standards.

Solution: Twelve low-E secondary units on a thermal-led specification, 6.4mm laminate to the road frontage.

Result: EPC lifted to band C on reassessment, with the owner reporting a 22% fall in winter gas consumption.

Frequently asked questions

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