# Glazing and windows in a commercial EPC upgrade

> Why glazing is both the biggest fabric weakness and the source of the daylight that lets you dim the lights, and how to weigh the two against each other.

Source: https://commercialepcupgrades.co.uk/measures/glazing

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# Glazing and windows in a commercial EPC upgrade

Glazing is the only measure on this site that can make another measure worse. It is usually the weakest thing in the fabric, and it is also where the daylight comes from. Reduce the glazing or its transmission and you cut heat loss, and you also cut the daylight that daylight-linked dimming was harvesting.

High capital, and it interacts with other measures

## 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?

### Is secondary glazing worth considering?

### Does reducing glazing help or hurt?

Sources

- A non-domestic EPC is an asset rating: it is based on the fabric of the building and its services rather than on metered consumption. [A guide to energy performance certificates for the construction, sale and let of non-dwellings, DCLG, 2014](https://www.gov.uk/government/publications/energy-performance-certificates-for-the-construction-sale-and-let-of-non-dwellings)

Last updated 8 October 2026 .

- [Insulation and fabric The rest of the building envelope](https://commercialepcupgrades.co.uk/measures/insulation-and-fabric)
- [Daylight dimming The measure glazing feeds](https://commercialepcupgrades.co.uk/measures/controls)
- [Listed buildings Where replacement is usually off the table](https://commercialepcupgrades.co.uk/buildings/listed-building)

Next step

## Get glazing and windows priced for your building

Tell us the building type, the floor area and the rating you are on now. One commercial contractor comes back to you with a quote off a survey.

Or call [07367 293494](tel:+447367293494). Monday to Friday, 9am to 5.30pm.