EPC Band Uplift by Glazing Type: Evidence from 23 Million UK Homes
Most window-replacement guides quote a vague “you will save energy” and stop. We pulled the full DCLG domestic Energy Performance Certificate dataset — 23.1 million certificates across 326 English and Welsh local authorities — and calculated how many EPC bands the current → potential rating actually shifts for each glazing type.
The results are uncomfortable for the triple-glazing sales pitch. They are very good for secondary glazing and for the kind of listed-property work that gets dismissed as a heritage compromise.
How a SAP band uplift actually works
The Standard Assessment Procedure scores each home on a 1–100 scale, then bands it A through G. Your current rating is what the assessor recorded the day they visited. Your potential rating is what SAP estimates the same home would score after every recommended improvement is applied. The gap between the two is the uplift number we aggregated.
The avg band uplift numbers below are the population mean of that gap, segmented by what the assessor recorded for GLAZED_TYPE on inspection day.
Average EPC band uplift, by current glazing type
The number below is the average difference between a property’s current energy efficiency band (A through G) and its modelled potential band, across every UK home with that glazing type on file. Higher is better. A figure of 2.00 would mean the typical home moves two whole bands — for example, E to C — once recommended improvements are applied.
| Current glazing type | Avg band uplift | Sample size |
|---|---|---|
| Secondary glazing | 1.93 | 100,456 |
| Single glazing | 1.68 | 10,214 |
| Double glazing — installed before 2002 | 1.77 | 1,949,971 |
| Double glazing — unspecified vintage | 1.68 | 5,208,819 |
| Double glazing — installed 2002 or later | 1.50 | 3,463,235 |
| Triple glazing | 1.44 | 27,226 |
Why secondary glazing tops the list
The hierarchy looks wrong on first read. Triple glazing sits at the bottom; secondary glazing — usually treated as a listed-property compromise — sits at the top.
The data is not measuring how good each glazing type is in absolute terms. It is measuring how much headroom is left in the property for improvement. Triple glazing is fitted overwhelmingly into already well-insulated post-2010 stock. Those homes are close to their potential ceiling already, so the EPC model has little room to push them higher. Secondary glazing is fitted overwhelmingly into Victorian and Edwardian period properties whose other measures (loft, walls, heating) also have huge improvement headroom — so the whole-property modelled uplift is large.
Read another way: if you live in a 1970s estate that already has 2002-plus double glazing, do not expect a third pane to lift your EPC much. If you live in a Victorian terrace with original sashes, secondary glazing is doing a lot of heavy lifting in the SAP scoring methodology — even before you start the conversation about timber repair versus full replacement.
Why pre-2002 double glazing has more uplift than post-2002 double glazing
Window Energy Rating arrived in 2004 and Part L 2002 sharpened the U-value rules earlier than that. Anything fitted before April 2002 is unlikely to carry low-emissivity coating, modern argon fill, or a warm-edge spacer. Replacing it usually moves U-value from around 2.8 W/m²K to under 1.4 W/m²K — a ~50% improvement in glazing-fabric heat loss. Post-2002 double glazing is already much closer to A-rated spec, so the marginal gain from a like-for-like replacement is small.
If you are scoping a window replacement quote for a 1980s or 1990s property with sealed units that have never been changed, the pre-2002 row is your reference number — not the post-2002 row.
Triple glazing is for comfort, not the EPC
Our 1.44 uplift figure for current triple glazing is low because the population already lives in well-insulated homes. The sample size — 27,226 — is also small relative to the 9-million-plus double-glazed sample, because UK uptake is still nascent.
If you are asking whether to upgrade from A-rated double glazing to triple glazing purely to lift the EPC, the data says: marginally. Triple glazing’s case is acoustic, draught-elimination near the window, and Passivhaus-grade U-values in low-energy new builds. None of those show in a SAP band score.
Glazing comparison matrix
EPC band uplift is one lens. Here is how each glazing type scores on the three trade-offs that drive most quoting decisions — thermal performance, acoustic performance, and listed-building compatibility:
The single line that surprises most quote-shoppers: secondary glazing is the only intervention that scores top marks on thermal and acoustic and stays compatible with listed consent. The only thing it sacrifices versus full replacement is air-tightness.
How we ran the numbers
We pulled the full domestic EPC bulk export from api.get-energy-performance-data.communities.gov.uk/api/files/domestic/csv and streamed the 5.5 GB zip through a one-pass aggregator. For every certificate we read GLAZED_TYPE, CURRENT_ENERGY_RATING and POTENTIAL_ENERGY_RATING and calculated current_band_index − potential_band_index on the A-G scale. We averaged that delta per glazing-type bucket. Records with missing ratings or where current equals potential were excluded.
The full national rollup file lives at src/data/epc-glazing-uplift.json in the open-source codebase. The build script that regenerates it monthly when DCLG refreshes the export is at scripts/build-epc-aggregates.mjs.
What this means when scoping a quote
Three practical takeaways for anyone weighing a quote against an installer’s pitch:
- The single biggest band uplift in our dataset comes from replacing or supplementing pre-2002 double glazing. If the original sealed units are still in your house, a quote that lands inside the standard A-rated double glazing price band is doing real work.
- Secondary glazing on period properties punches well above its reputation. The whole-property SAP model rewards it because it lets the rest of the building fabric upgrades land. If a listed building consent rules out replacement, do not write off the remaining uplift.
- Triple glazing’s case is rarely the EPC. If the quote pitches triple glazing primarily as a band booster, ask for the U-value delta and the comfort metrics. Do not pay the 20-30% premium on energy-band logic alone.
Compare your scenario against the double vs triple glazing pillar, the secondary glazing for listed buildings deep dive, or the Part L 2022 window U-value rules.
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