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The HEM-line blog
The Home Energy Model (HEM) is a new way of forecasting the energy performance of residential buildings in the United Kingdom. In the coming years, HEM will replace the Standard Assessment Procedure (SAP) for generating Energy Performance Certificates (EPC) and checking compliance with Part L / Section 6 of Building Regulations.
HEM is currently in development. When it is approved for use, this blog will include tutorials and advice for how to use the new calculation tool.
While we're waiting, you can browse through this archive of my LinkedIn articles, covering changes to Building Regulations, Energy Performance Certification, national planning policy, and everything in between.
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HEM Deepdive: Colours
I’ve been digging into areas of the Home Energy Model which us humble SAP assessors have never had to bother ourselves with previously.
One of the new questions HEM asks is:
What colour is the house?
It asks, because black surfaces absorb more heat than white surfaces.
Jon Ponting
Oct 262 min read
HEM Deepdive: Obstacles
With the Home Energy Model edging towards its official release, I’ve been playing with some of the new inputs to understand how impactful they are on a dwelling’s energy performance.
The first section that will be unfamiliar territory to SAP assessors is Obstacles.
Jon Ponting
Oct 182 min read
Scotland Section 6 and PassivHaus consultation
The Scottish Government has published new proposals for changing Section 6 of the Technical Handbooks to create a ‘Scottish equivalent to the PassivHaus standard’.
A public consultation on these changes is open until 16th October 2026.
Jon Ponting
Aug 23 min read
HEM Deep-Dive: Energy Flex
Data from a Home Energy Model studying three 48-hour snapshots of energy flex in a small house.
The fabric performance meets Future Homes Standard, but this model uses direct heating (panel heaters) and direct electric immersion (instead of a heat pump).
The model includes a PV array (4kWp), a 5 kW battery, and an immersion diverter.
Jon Ponting
Jul 293 min read
HEM Deep-Dive: Thermal Mass
Is it better to build our homes with a low thermal mass and lightweight materials, or use dense materials to create a high thermal mass?
I've built a HEM model to look into this.
Jon Ponting
Jul 153 min read
HEM Deep-Dive: PV angles
In last week's post about PV output from the Home Energy Model, I said south facing panels generate more electricity. But then I realised I’ve been hard-wired into thinking that.
Ask any SAP assessor 'What is the best orientation for PV?'. They’ll tell you due south at 30 degrees.
But HEM isn’t SAP. So does HEM agree with SAP?
Turns out, nope!
Jon Ponting
Jul 82 min read
HEM Deep-Dive: PV performance
This deep dive uses output data from SAP and HEM calculations which predict how much electricity is generated by a 4kWp PV system.
I modelled this four times, changing the orientation of the PV through north, east, south and west. The house design has a 35 degree pitched roof.
Jon Ponting
Jul 13 min read
SAP10.3 Oddities: Low Air; Don't Care
SAP10.3, which checks homes are compliant with the Future Homes Standard, does a very strange thing with air leakage results. You could have an air test of 1.5 or 4.5, and your fabric performance (FEE) won't change.
Logically, a house with a better air tightness will lose less heat, so you’d expect the fabric performance to score better. That’s always been the case in previous versions of SAP.
Jon Ponting
May 172 min read
New Part L for Wales (Targets and Notional)
This revised Approved Document Part L will apply to any building work where the relevant building inspector accepts applications from 4th March 2027.
Any unstarted buildings on existing sites after 4th March 2028 will need to meet the new regs, regardless of when the site was submitted.
As with the current Part L, most of the English (UK) and Welsh versions are identical, but there are a few curveballs developers need to be aware of.
Jon Ponting
May 102 min read
EPC Reform postponed until 2027
Last week, the UK Government (representing England and Wales), and the Scottish Government both released statements saying EPCs will remain unchanged until late 2027.
To recap, the way Energy Performance Certificates look, how results are graded, and the way they are calculated, are all changing. When it happens, it will be the biggest refresh to EPCs since their creation in 2008.
Jon Ponting
Mar 232 min read
HEM User Guides updated
A fresh set of Home Energy Model User Guides have been published - this will be particularly useful for manufacturers of domestic heating, cooling, hot water, ventilation and renewable systems.
Jon Ponting
Mar 161 min read
Future Homes Standard Update for Wales
The Senedd has published its response to last year's Approved Document Part L consultation, confirming new energy standards for building sites from 2027.
Strictly speaking, FHS doesn't apply in Wales, but this news broadly follows what we're expecting from the UK Government in the coming weeks.
Jon Ponting
Mar 92 min read
New EPC Targets for Social Housing
The new Minimum Energy Efficiency Standards (MEES) for social housing in England will go live from April 2030.
Landlords must ensure all homes comply by this deadline.
If a property scores A-C using a current EPC, that's OK.
If a property scores A-C under the fabric metric of the new EPC (available from the end of 2026), that's OK.
For properties with a D, E, F or G rating, the landlord will need to make improvements to the energy performance.
Jon Ponting
Feb 42 min read
EPC Reform: England and Wales
EPC certificates in England and Wales will be changing from October 2026. There will still be an A-G rating. But the single rating is being replaced with a dashboard of information, primarily focusing on fabric performance.
Jon Ponting
Jan 263 min read
Scotland plans to add PassivHaus to Building Regs
The Scottish Government has outlined plans to update Technical Handbook Section 6 to scrap SAP and further tighten fabric performance standards. They're calling it a 'PassivHaus equivalent standard'.
Specific details and timeframes will be confirmed later this year, with the final ‘Go Live’ date already confirmed as 31st March 2028.
Jon Ponting
Jan 122 min read
HEM - Outputs and Troubleshooting
When a SAP calculation spits out unexpected results, the assessor can look through the worksheets and, 99% of the time, they can find the reason and fix it. The Home Energy Model doesn’t produce simple worksheets that assessors can scrutinise, and it doesn’t have a single methodology you can thumb through. So how will we troubleshoot in HEM?
Jon Ponting
Nov 17, 20252 min read
Is 2026 the new 'Autumn 2025'?
The Government's standard response about the release of the Future Homes Standard has changed recently from 'Autumn 2025' to 'in the coming months'.
Jon Ponting
Nov 13, 20251 min read
New HEM Technical Documents published
This is only going to be exciting for us high-level Home Energy Model nerds, but new Home Energy Model Technical Documents have just been published on the DESNZ website.
These docs explain the logic behind HEM.. a bit like the SAP methodology manual.
Jon Ponting
Nov 10, 20252 min read
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