Glazing and windows in a commercial EPC upgrade

The interaction nobody prices

A building with large glazed areas loses heat through them in winter and overheats through them in summer. It also gets enough daylight that perimeter lighting can dim for much of the working day. Those facts point in opposite directions.

Heavily tinted or reduced glazing improves the fabric numbers and darkens the perimeter, which makes the lighting run harder and longer. In a deep-plan office with long hours, that trade can go either way, and it is worth modelling rather than assuming.

The options, cheapest first

Secondary glazing, which is far cheaper than replacement, reversible, and frequently the only option in a listed building.
Draught-proofing existing frames, which addresses air leakage rather than conduction and often matters more in an old building than the glass does.
Solar control film, which addresses overheating and cooling load without touching the frames, at the cost of some daylight.
Replacement units into sound existing frames, where the frames are good and the sealed units have failed.
Full replacement, which is the big capital option and usually a refurbishment decision.

Overheating is the half people forget

In a modern, well-insulated, heavily glazed commercial building the problem is frequently not heat loss but heat gain. Cooling that runs hard through the summer because of solar gain is both an energy cost and a plant cost, and solar control is often the cheaper fix than more cooling capacity.

This is also where lighting and glazing meet again: reducing the lighting load reduces the internal heat gain, which reduces the cooling demand, which is a second-order saving that rarely appears in a lighting quote.

When glazing is actually the right call

Where the existing units have failed and are misting, where frames are rotten or leaking air badly, where a facade is being replaced anyway, or where overheating is driving a cooling problem that is costing more than the glazing would.

Rarely as a standalone energy measure on its own numbers. Almost always as part of something larger that was happening regardless.

Questions we get asked

Will new windows improve a commercial EPC rating?

They improve the fabric inputs, so yes, but the capital is high relative to the rating movement and it is rarely the best value measure on its own. It makes far more sense as part of a refurbishment where the access is already being paid for.

Is secondary glazing worth considering?

Often, and particularly in a listed or historic building where replacement is not permitted. It is much cheaper than replacement, it is reversible, and it addresses both conduction and draughts.

Does reducing glazing help or hurt?

Both, which is the distinctive thing about this measure. Less solar transmission means less heat loss and less summer overheating, and also less daylight for daylight-linked dimming to use. In a long-hours deep-plan building that trade is worth modelling rather than assuming.

Last updated 8 October 2026.