For years, SAP has been the compliance workhorse for UK new-build energy ratings. That’s changing. The Home Energy Model (HEM) is being lined up to replace SAP and better reflect modern homes with heat pumps, smarter controls and variable tariffs. For builders, the shift isn’t just a new calculator—it’s a different way of organising design data, site evidence and commissioning, with more emphasis on as-built reality.
TL;DR
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– HEM replaces SAP for domestic energy assessments and is geared to heat pumps, smart controls and real usage patterns.
– Expect tighter joins between design models, product data, site evidence and commissioning outcomes.
– You’ll need HEM-ready modelling tools, thermal bridging detail libraries, robust photo/evidence capture and airtight commissioning records.
– Start earlier with fabric and junction details; avoid default assumptions that can wreck the as-built result.
– Build a repeatable workflow with your energy assessor, M&E, envelope trades and QS to control cost and compliance risk.
HEM in plain English: what changes from SAP and why it matters
/> HEM is being developed to replace SAP as the way homes’ energy use and carbon performance are modelled at design and as-built stages. Instead of the more static assumptions common in SAP assessments, HEM is structured to reflect hourly operation, system control strategies and new technologies like heat pumps, low-temperature emitters, PV and batteries. It is also intended to be more transparent, with clear links between the model inputs and the installed products and details on site.
For site teams, this means fewer “black box” allowances and more direct responsibility for the quality of what’s built and commissioned. Default values and generous tolerances that used to be propped up in SAP can be harder to justify. Junction psi-values, airtightness, ventilation commissioning, emitter sizing and controls setup matter more to the final rating. HEM also encourages digital continuity: product data from design should flow into procurement and then into the as-built model, backed with photos, certificates and test results.
If you’ve delivered to the latest Part L requirements, some of the groundwork will feel familiar: photographic evidence of fabric, airtightness targets and services commissioning. The difference with HEM is scope and sensitivity. System interactions—think heat pump curves, cylinder losses, time-of-use tariffs—play a bigger part. The “as designed” and “as built” gap becomes more visible, which means QA and change control need to be tight.
What shifts on real sites, from design freeze to handover
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– Design stage gets more granular. Fabric performance isn’t just U-values; junctions, cavity closers, lintel details and slab edges feed into the energy model with psi-values from details your designer or supplier must stand behind. If you don’t have a detail library, you’ll be stuck with punitive defaults.
– M&E coordination starts earlier. Heat pump emitter sizing, pipework insulation thicknesses, cylinder siting and controls sequences influence the model outcome. These need freezing alongside layouts, not bolted on during second fix.
– Procurement must mirror the model. Substitutions are risky if they change lambda values, system efficiencies or controls capabilities. Align specifications, datasheets and subbie scopes so the kit that arrives matches the inputs your assessor used.
– Site evidence is not an afterthought. Expect to capture consistent photo sets at each plot showing insulation continuity, airtightness taping, thermal break installations, duct routes and plant specification plates. Tie these to plots and timestamps in a shared CDE.
– Commissioning becomes compliance-critical. Heat pump flow temperatures, weather compensation, ventilation flow rates and zone control functionality all need to be proven. Handwritten sheets in a site cabin won’t cut it; clean, exportable records are needed.
– As-built models are living documents. When designs change, or a supplier swaps goods, your assessor must update the model and you must update the evidence pack. Leave it to the end and programme pain follows.
Scenario: On a 62-plot timber frame development in the North West, the site manager is pushing to get plasterboard on before a cold snap. The energy assessor flags that several window-jamb details were built using a standard closer not in the agreed thermal bridging library. The M&E subcontractor also wants to switch to different emitters due to a delivery delay, nudging flow temperatures up. The QS is juggling cost pressure and is tempted to approve the substitution “like-for-like.” Meanwhile, airtightness testing is booked for three plots, but the penetrations for ASHP pipework weren’t sealed internally. Photos for the part-built walls are thin, as the assistant site manager was covering logistics after a crane slot moved. By the time the first as-built HEM run is attempted, the plaster is on and rework will mean lost days and damage.
The toolkit: digital and on-site tools to land HEM compliance
/> You don’t need to become modellers, but you do need the right stack and habits.
– HEM-ready assessment software and a competent assessor. Ensure they can handle design and as-built versions, import detail libraries, and show you sensitivity results so you know what really moves the needle.
– A thermal bridging detail library. Either from your designer, system provider or a specialist, with psi-values and installation notes you can build repeatably. Store drawings in your CDE and reference them in plot packs.
– BIM-to-energy data workflow. Even a simple parameter export helps push U-values, areas and product tags from your model into the HEM tool, reducing transcription errors that come back to bite on as-built.
– Site evidence capture app. Set up plot-level templates for required photos: ground floor insulation, edge insulation, cavity closures, window/door interfaces, M&E penetrations, ducting, plant model plates, controls wiring centres.
– Airtightness and ventilation testing partners who can provide digital logs. Aim for early diagnostic testing on typologies to catch issues before finishes go on.
– Commissioning playbooks for heat pumps and MVHR. Agree the exact checks, target settings and documentation format with your M&E subcontractor and assessor before first fix.
– Change control baked into procurement. Any substitution triggers an impact note to the assessor and designer, with a go/no-go decision based on model deltas and programme impact.
# Quick toolkit checklist
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– Lock in a HEM-capable assessor at RIBA Stage 3 and agree data/evidence formats.
– Publish a junction detail schedule and psi-values to all trades before start on site.
– Pre-book airtightness diagnostics on first plots before plasterboard.
– Include product data sheets and model plates in the evidence pack requirements.
– Make commissioning sheets digital, with photos of controller settings and measured flows.
– Tie any M&E or fabric substitution to a formal model update before approval.
Pitfalls and fixes when moving from SAP to HEM
/> A few areas consistently trip projects up—and they’re avoidable with planning.
– Over-reliance on defaults: Using generic psi-values, pipe insulation allowances or controls descriptions can drag performance down. Fix by investing early in verified junction details and product-specific data.
– Late-stage M&E design: Emitter sizing and control strategies matter more under HEM. Fix by bringing the M&E designer and supplier into the energy conversation at design freeze, not during second fix.
– Evidence gaps: Missing photos or commissioning records force conservative assumptions. Fix by templating evidence by plot and stage, and auditing weekly on a sample of homes.
– Disconnected teams: Assessors, designers, QS and site management often operate in silos. Fix with a short, recurring HEM coordination meeting that tracks model assumptions, pending changes and evidence status.
# Common mistakes
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– Treating HEM as a paperwork swap for SAP. It changes risk allocation; fabric and commissioning quality influence outcomes more directly.
– Assuming heat pumps “pass on their own.” Without low-temperature emitters and proper controls setup, the model can worsen, not improve.
– Skipping early air-leak checks. Waiting for final tests misses hidden voids and service penetrations that become costly to fix post-plaster.
– Approving “like-for-like” substitutions informally. Small spec shifts—lambda values, cylinder losses, fan powers—compound in the model.
What to watch next for UK builders and specifiers
/> Expect the model and approved tools to iterate as Future Homes Standard requirements bed in. Integration with EPCs, smarter metering data and possibly more robust as-built verification are on the horizon; keep procurement flexible and invest in detail libraries and commissioning competence now.
FAQ
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When will HEM fully replace SAP on live projects?
Roll-out is expected to align with updates to new home standards, but dates can move as policy and software mature. Treat the current period as a transition: upskill teams, align your workflows, and run pilot HEM assessments alongside SAP on a few plots to de-risk.
# Do I need different subcontractors to deliver under HEM?
/> Not necessarily, but expectations rise. Choose envelope and M&E partners who are comfortable with documented detail libraries, airtightness diagnostics and digital commissioning records, and make those deliverables explicit in scopes.
# How should I handle design changes once procurement is underway?
/> Treat every change that touches fabric or services as a controlled event. Route it through your assessor for a quick model impact check, assess cost and programme effects, then either proceed with updated evidence requirements or hold the line.
# Who owns the data and evidence produced for HEM?
/> Project teams should agree this up front in appointment documents. Store evidence in a shared CDE with clear naming conventions; the client will usually expect a complete as-built pack, while the assessor needs reliable, version-controlled inputs to sign off.
# Will HEM mean more site testing?
/> It may not increase the number of statutory tests, but it raises the value of early diagnostics and robust commissioning. Plan for sample airtightness checks before finishes, verified ventilation flow measurements, and clear records of system settings at handover.
