The Energy Performance Certificate is the single number that follows a home around when it is sold or let, and it is about to be rebuilt from the ground up. The government has set out a new Home Energy Model (HEM) that changes what an EPC actually measures, and the shift matters to anyone weighing up a window upgrade.
What the Home Energy Model measures
HEM scores a property on three things: fabric performance (insulation, draught-proofing and windows), the heating system, and smart readiness, such as solar. The order is deliberate. Fabric performance is scored first, which puts windows and insulation front and centre rather than treating them as an afterthought once the boiler has been counted.
That is a meaningful change from today. The current EPC system is based on estimated running costs and energy use, an approach that has been criticised for penalising homes heated by electricity. By leading with the physical fabric of the building, HEM rewards the parts of your home that hold heat in, and that is exactly where glazing sits. If you want the technical side of how a window's heat loss is measured, our guide to U-values explained walks through it.
Why windows come out ahead
Here is the part that survives the rule change. Windows count towards the fabric score under both the current EPC and HEM, so glazing is a durable improvement rather than a tactical one. A homeowner who upgrades now is not betting on one version of the rules. They are improving the bit of the assessment that both systems agree on, which makes the spend easier to justify whether the certificate is recalculated this year or in 2030.
I have watched homeowners chase a quick EPC point with a cheap fix, then find the rules had moved on by the time they sold. Fabric is different. Good glazing keeps the heat in whatever the model on the front of the certificate says, so it is one of the few upgrades I never hesitate to recommend on its own merits.
Tom Bradley, FENSA-registered installer
The timeline you need to know
HEM becomes compulsory for EPCs from 1 October 2029. A consultation ran from January 2026 and closed on 18 March 2026, and the full detail is expected before the end of 2026. That gives owners and landlords a long runway, but it is not a reason to wait, because the certificate sits alongside tougher minimum standards. Private landlords in particular face a separate deadline that we cover in the EPC C rules landlords must meet by 2030, and both rules trace back to the same programme, the Warm Homes Plan.
What it means for your next decision
If a new EPC is on your horizon, the practical move is to improve the fabric first, and glazing is a large, visible part of that. You can put rough numbers on the running-cost side with our energy savings calculator, and if cost is the barrier it is worth checking what help is available through current double glazing grants before you commit. The headline is simple. The certificate is changing, but the things that score well on it, warm fabric and good windows, are not.
Home Energy Model FAQs
The Home Energy Model (HEM) is the new system that will replace the way Energy Performance Certificates are calculated. It scores a property on three things: fabric performance (insulation, draught-proofing and windows), the heating system, and smart readiness, such as solar. Fabric performance is scored first.
HEM becomes compulsory for EPCs from 1 October 2029. A government consultation ran from January 2026 and closed on 18 March 2026, with full detail expected before the end of 2026.
The current EPC is based on estimated running costs and energy use. That approach has been criticised for penalising homes heated by electricity. The Home Energy Model leads with the physical fabric of the building instead.
Yes. Windows count towards the fabric score under both the current EPC and the Home Energy Model, so glazing is a durable improvement rather than a tactical fix tied to one version of the rules.