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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
Tweaks on the way for SAP10.3
Just six months to wait until the new Part L (the Future Homes Standard) goes live in England.
Five changes to the new methodology are under review with a fix expected before the end of the year. In some cases, these changes could affect existing SAP10.3 results.
Jon Ponting
Sep 302 min read
Part L 2026 - Revisions
If you’ve already downloaded your own copies of the Future Homes Standard and Future Buildings Standard (Part L 2026), do it again! There’s been some amendments.
Nothing too serious, just a few corrections and clarifications. All the big headlines (such as amount of PV and fossil fuel free requirements) remain the same.
Jon Ponting
Sep 271 min read
Part L 2026 PV requirements - FAQs
Since the new Approved Document Part L (the Future Homes Standard) was published, there's been uncertainty about the specifics of the new PV Functional Requirement.
We now have answers to some of those Frequently Asked Questions from the MHCLG.
Jon Ponting
Sep 131 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 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: Exhaust Air Heat Pumps
To comply with the Future Homes Standard, most new homes will require a heat pump. Apartment complexes might look at Exhaust Air Heat Pumps, but it seems the new SAP10.3 calculator isn’t so keen on those. I've been down a SAP rabbithole to find out why.
Jon Ponting
May 312 min read
SAP10.3 Oddities: WWHRS
We’ve had a few weeks to get under the bonnet of SAP10.3 to understand what a Future Homes Standard compliant home looks like, and we’ve uncovered a strange quirk which works against WWHRS.
To unpick what’s going on in the new SAP methodology, I met up with two of the biggest suppliers of Waste Water Heat Recovery Systems (WWHRS) in the UK: Showersave and ReCoup, at the Keystone Group offices in Derbyshire.
Jon Ponting
May 242 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
FHS Launch - Impact on EPCs
With the Future Homes Standard set to go live across England next March, housebuilders will need to design in PV panels and install heat pumps to the vast majority of new buildings.
That’s going to have a positive impact on Energy Performance Certificates... for now.
Jon Ponting
Apr 82 min read
FHS Launch - The gas boiler 'ban'
We’re told the Future Homes Standard will require most new homes to use heat pumps. That’s most, not all.
So does that mean there’s scope to sneak through the occasional gas boiler under the new Part L?
No. For two reasons...
Jon Ponting
Apr 52 min read
FHS Launch - Mandatory solar panels
One part of the Future Homes Standard has grabbed the media’s attention more than most: Mandatory solar panels on all new homes. But how will these new regs work in practice?
Jon Ponting
Apr 12 min read
FHS Launch - Fabric performance
Now the Future Homes Standard and Part L (2026) have been published, we can begin unpicking what compliance looks like for new building sites across England.
Changes to the fabric performance targets are under the spotlight this morning, or lack of changes may be more appropriate.
Jon Ponting
Mar 292 min read
FHS Launch - When does it apply?
When will the Future Homes Standard apply?
For most developments, we have a full calendar year to get ourselves ready. The new Part L goes live on 24th March 2027.
This is defined as the date the developer submits documents (initial notice, building notice, application with full plans) to the relevant building authority.
If paperwork is submitted and accepted before 24th March, the site can built to the 2021 (current) regs.
Jon Ponting
Mar 262 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
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