Reducing Heating Bills with Secondary Glazing
A single glazed sash window loses heat in three separate ways, and only one of them is about the glass. Understanding all three explains why secondary glazing often makes a house feel warmer than its U value improvement alone would suggest.
The three routes heat takes
First, conduction through the glass itself, which is the number a U value describes. Second, radiation from your body and your radiators to a cold pane, which is why sitting near a single glazed window feels chilly even in a warm room. Third, air leakage: uncontrolled infiltration through the meeting rail, the sash pulley pockets, the gaps around the beads and the joint between frame and reveal. On an untouched Victorian window the third route is frequently the largest.
- Conduction through the pane
- Radiant loss to a cold surface
- Air leakage through gaps and pockets
What secondary glazing changes
It attacks all three. The trapped air layer between panes slows conduction. The room side pane sits far closer to room temperature, so the radiant chill disappears and the space next to the window becomes usable again. Most importantly, a continuously sealed frame stops the infiltration completely, because the new frame is sealed to the reveal in a way the original sash never was and never will be.
Realistic numbers on the glazing element
A well specified secondary system with Low E soft coat glass and a generous cavity takes a single glazed opening from roughly 5.4 W/m²K into the region of 1.9 W/m²K. That is a large proportional improvement, comparable in magnitude to fitting a modern sealed unit, and it is achieved without touching the original window. The exact figure depends on cavity, glass and frame, and we will state it on the quote rather than generalising.
Why the felt difference exceeds the calculated one
Energy calculations weight conduction heavily and treat air leakage crudely. In lived experience, stopping draughts changes a room more than a modest U value gain. Curtains stop moving, the sofa can sit under the window, the thermostat can come down a degree because the perceived temperature has risen. Comfort is not a rounding error, it is why most people notice the change within a day.
Payback, honestly
Payback depends on your heating fuel, your usage pattern, how many windows you treat and how draughty they were to start with. On a large, cold, heavily glazed period house heated to a comfortable temperature all winter, the payback period is genuinely attractive. On a small, already reasonably tight house that is barely heated, it is longer, and comfort rather than money is the honest justification. We would rather say that than promise a figure we cannot stand behind.
Getting the most from the specification
Choose Low E soft coat glass, which reflects long wave heat back into the room and typically adds a small amount to the cost for a worthwhile gain. Take the deepest cavity your reveal allows, though thermally the returns flatten beyond about 100mm. Make sure the perimeter is properly sealed rather than trimmed, and keep radiators and shelving out of the cavity so warm air is not trapped against the cold pane.
What to do alongside it
Secondary glazing works best as part of a sequence: loft insulation first because it is the cheapest heat you will ever save, then draught proofing of doors and floors, then glazing, then heating controls. Windows are usually the most visible problem but rarely the only one, and a house that has had all four done feels transformed rather than merely improved.
Frequently asked questions
On the pure conduction figure a modern replacement usually edges ahead. Once air leakage and cost are included, secondary glazing is often the better value improvement on an older house.
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