Secondary Glazing Thermal Insulation

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.

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.

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.

A single-glazed London sash loses heat two ways: conduction through the glass, and air leakage around the sashes and perimeter. In older windows the leakage is frequently the larger share, and it is the part that makes a room feel cold even when the thermostat says otherwise, because moving air strips heat from skin far faster than a cold surface does.

Secondary glazing addresses both. The trapped air cavity is an insulating layer that raises the internal surface temperature, and the compression seal removes the draught path completely. In practice, occupants notice the draught removal within hours and the surface-temperature change over the first cold week.

A typical single-glazed timber sash sits around 4.8-5.4 W/m²K. Adding a sealed secondary layer with standard laminate commonly brings the combined assembly to roughly 1.8-2.2 W/m²K, and using low-emissivity coated glass in the secondary panel can take it lower again. We model the figure for your specific window rather than quoting a brochure number, because cavity depth, glass choice and the condition of the primary window all move it.

For context, that improvement is in the same territory as replacing the window with a modern sealed unit — achieved without removing the original joinery, and without the consent problem that replacement creates in a conservation area or listed building.

Windows are one component of a building's heat loss, so the honest framing is that secondary glazing reduces the window's contribution substantially and the whole-house bill proportionally less. In solid-walled London stock, glazed area is often a significant share of the loss, and households commonly report both a lower run time on the heating and being able to use rooms they previously avoided in winter.

For EPC purposes, secondary glazing is recognised in the assessment methodology and can contribute to a band improvement, which matters to landlords working toward minimum standards. Delivery and U-value modelling sit with our thermal insulation service.