The UK’s Home Energy Model is edging into view as a more dynamic way to predict dwelling energy use, and it’s being discussed as the likely successor to the long‑standing SAP method for compliance. For design teams and site managers used to SAP workflows tied to Part L sign‑off, this shift means a different conversation about fabric, services, and occupant behaviour—earlier in RIBA stages and with tighter links to installation quality. The modelling step won’t sit neatly in the background anymore; it will drive choices on air‑tightness targets, ventilation strategies, and heat pump sizing that ripple into procurement, programme, and QA.
TL;DR
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– Home Energy Model (HEM) is a more granular, time‑step approach than SAP, influencing design options earlier and testing how homes behave through the day and seasons.
– Expect stronger emphasis on airtightness, ventilation balance, and real‑world installation quality; performance assumptions will be harder to “average out”.
– Procurement will shift: validated product data, thermal bridge evidence, and commissioning records become critical inputs to compliance reporting.
– Site teams will need clearer coordination between envelope trades and M&E, with airtightness and MVHR/ASHP interfaces becoming sign‑off gatekeepers.
– Start dual‑running HEM and SAP on live schemes to understand design sensitivities before the policy switch matures.
Home Energy Model vs SAP in plain English
/> SAP (Standard Assessment Procedure) has served as the UK’s method for calculating dwelling energy performance for years. It works with monthly or seasonal averages and relies on standardised assumptions for occupancy and weather. The Home Energy Model aims to represent a home’s energy profile more dynamically, typically using shorter time steps that capture daily swings in temperature, solar gain, ventilation, and system controls.
In practice, that means HEM will scrutinise things SAP tended to smooth out. For example, how an MVHR actually balances supply and extract over a winter evening, or how a heat pump copes with a cold snap at dawn. It also makes unintended gaps—leaky loft hatches, poorly sealed service penetrations, underperforming window installation—more visible in the numbers. This isn’t about making schemes fail; it’s about making designs and site delivery more faithful to how homes are really used. The expected compliance route will lean more heavily on evidence: product data, psi‑values, commissioning records, and airtightness tests aligned to the model’s assumptions.
On real sites: how the workflow changes
/> HEM nudges critical decisions into Stage 2/3 and demands more joined‑up thinking between architecture and M&E. Instead of “fabric first, services later”, you’ll see parallel iteration: form factor, glazing pattern, shading, and thermal bridge strategies tested alongside ventilation rates, emitter sizing, and control logic. That affects procurement timing—teams will need earlier market engagement for validated window performance, MVHR with known specific fan power, and heat pumps with transparent seasonal data.
Commissioning gains weight. If the model assumes balanced ventilation and set‑back temperatures, the commissioning plan and O&M material must make that achievable in reality. Airtightness becomes a programme activity, not a day‑before‑PC panic: pre‑linings tests, smoke‑pen checks, and a clear owner for sealing responsibility at each junction will save rework and retests. Expect the energy modeller to sit closer to coordination meetings, flagging design tweaks that look trivial on drawings but shift the dynamic loads materially.
Scenario: A design‑and‑build contractor delivers 38 timber‑frame homes on a suburban site with tight access and staggered handovers. The architect pushes for generous south‑facing glazing; the M&E consultant proposes compact ASHP units and MVHR with summer bypass. The energy modeller runs HEM iterations and finds evening overheating risk in mid‑terrace plots due to party wall heat retention and limited cross‑ventilation. Procurement has already tendered windows on headline U‑values, but the HEM shows frame factors and installation details swing performance more than expected. The site manager asks for an earlier airtightness pre‑test; the frame contractor needs two extra days to seal service penetrations before plasterboard. A late RFI seeks external shading for four plots on a façade already in the scaffold strike plan, triggering a sequencing rethink and a temporary works tweak. The team re‑issues the commissioning plan to include measured MVHR flow verification before kitchens and wardrobes go in.
Checklist for an HEM‑ready housing scheme
– Lock in airtightness responsibility by junction (frame, window, MEP, roofers) and schedule pre‑plaster tests, not just a final test.
– Specify MVHR and heat pumps with declared performance under test conditions comparable to HEM inputs; capture data sheets in submittals.
– Obtain psi‑value evidence for key thermal bridges or commission project‑specific calculations where catalogue data doesn’t match your details.
– Agree a glazing and shading strategy early, including frame factors, g‑values, and any external shading or brise‑soleil fixings and programme impact.
– Build a commissioning script that measures actual ventilation flow, system pressures, and emitter balancing, with placeholders for O&M and user guidance.
– Set up an as‑built data pack for the modeller: product SKUs, installation photos, test certificates, and variation logs that might affect performance.
Pitfalls and fixes when moving from SAP to HEM
/> Teams coming from a SAP mindset often assume standardised inputs will absorb late design shifts. HEM is less forgiving of vague edges. A change in stairwell volume, a slightly different window‑to‑wall ratio, or a substituted MVHR model can alter dynamic loads and ventilation balance in ways that show up in compliance margins. The fix is cultural as much as technical: treat the model as a live control document and assign change control the same weight as fire strategy or structure.
A second pitfall is assuming commissioning can recover weak detailing. If the fabric is leaky or thermal bridges are sloppy, post‑handover set‑up has limited room to compensate. Getting pre‑lining checks into the programme, and giving the sealing trades time and materials, is cheaper than chasing a moving target later.
Third, product data gaps cause delays. HEM wants numbers that reflect actual kit behaviour across temperature bands and operating modes. Engaging suppliers early and making data completeness a contract deliverable keeps the model credible.
Finally, user operation matters more. If controls are baffling, set‑points drift, and energy use climbs. A short handover session, clear O&Ms, and simple defaults do more for long‑term performance than any last‑minute modelling tweak.
# Common mistakes
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– Treating HEM as a back‑of‑house submittal rather than a design tool, which leads to late surprises when margins evaporate.
– Assuming catalogue U‑values and generic psi‑values reflect your installed details; they often don’t once frames, packers, and fixings are considered.
– Leaving airtightness ownership to “everyone and no‑one”; leaks then sit in the gaps between trades and are found too late.
– Swapping M&E kit post‑tender without re‑running the model; small differences in fan power or defrost strategy can tip compliance.
What to fix first: run trial HEM models on a current SAP‑based scheme to learn where your sensitivities lie—glazing ratio, infiltration, ventilation power, shading. Build an airtightness and commissioning rehearsal into the next show home. Get the modeller into coordination meetings and give them authority to flag design risks early. Write product data requirements into supply contracts with a named contact responsible for timely, accurate information.
The direction of travel is clear: more realistic modelling that rewards good detailing, tidy interfaces, and intelligible controls. Three questions to take into your next project meeting: Which design choices move our HEM result most? Who owns airtightness at every junction? What’s our plan for capturing as‑built data that actually matches the model?
FAQ
# Will HEM replace SAP for all new homes straight away?
/> Not overnight. Policy direction suggests HEM will become central for demonstrating energy performance of new dwellings, but transitions take time and may run in parallel with familiar processes. Plan to dual‑run where possible so teams can build confidence before any formal switch.
# How does HEM change what I specify for windows and doors?
/> Frame factors, installation details, and solar control become more prominent alongside U‑values. Consider g‑values, shading devices, and verified psi‑values for jambs, sills, and heads. Ask suppliers for data that reflects installed performance, not just centre‑pane numbers.
# Do I need different subcontractors or just tighter coordination?
/> You likely need the same core trades, but with clearer scopes and sequencing. Assign airtightness responsibilities at junction level and align M&E and envelope programmes to allow pre‑lining tests and ventilation commissioning before finishes. Coordination meetings should include the energy modeller when design changes are aired.
# Who owns the HEM model and data on a D&B job?
/> Ownership should be set out in appointments and contracts. Typically the energy modeller maintains the file, with inputs from design consultants and product suppliers, while the main contractor controls as‑built evidence. Make sure change control requires model updates when products or details shift.
# What happens if a supplier can’t provide the performance data HEM needs?
/> Build contingency into procurement. You can commission additional testing or seek alternate products with transparent data, but factor in time and programme risk. Include data completeness as a deliverable and hold a pre‑award meeting to confirm what will be provided and when.
