The Home Energy Model is edging in to replace SAP on new-build homes in Britain, and the shift is more than a software swap. HEM’s goal is to represent how homes actually use energy across fabric, systems and controls, not just pass a compliance tick-box. For designers, site managers and commercial teams, it means earlier decisions on heating strategies, tighter evidence trails, and fewer assumptions left to chance. For clients, it points to more consistent, predictable operational outcomes once residents move in.
TL;DR
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– HEM is set to succeed SAP for new-build compliance, focusing on whole‑house performance and real operating conditions.
– Success on site hinges on early specification of heat sources, controls, ventilation and junction details, not just U‑values.
– Evidence quality becomes decisive: serial numbers, commissioning data, airtightness, and labelled photos all feed the as‑built model.
– Manage product substitutions and psi‑values proactively, or risk default penalties and redesign churn late in the programme.
What the Home Energy Model actually is
/> In plain terms, HEM is a more detailed way to predict a home’s energy use. Where SAP relied heavily on averaged factors and fixed profiles, HEM aims to reflect how building fabric, heating systems, hot water, ventilation, renewables and controls interact through the year. It’s designed to handle modern kit—heat pumps, MVHR, PV with smart inverters, batteries—alongside envelope performance and airtightness.
The workflow stays familiar—design model, build, as‑built model—but the inputs dig deeper. Things like low flow temperatures, distribution losses, real control strategies and manufacturer data for specific products carry more weight. HEM is also intended to be more transparent and open, which should help designers and contractors see exactly where a dwelling is winning or losing.
How HEM changes the day-to-day on new-build sites
/> The biggest shift is timing. With HEM, you need the actual heat source, emitter type, ventilation strategy and control philosophy resolved at design freeze, not vaguely “to be confirmed”. Design teams have to lock in details that used to be generic: flow temperatures, cylinder sizing, secondary circulation, and how zoning is delivered in practice.
On site, the as‑built model will lean on your evidence pack. Photos of insulation continuity and thermal bridge junctions, product labels for fans and heat pumps, and commissioning sheets with measured flow rates and setpoints now matter as much as a good airtightness test. If you substitute a product mid‑procurement, the assessor will need the new data sheet and control parameters or risk a performance penalty.
Expect closer ties between the energy assessor, M&E coordinator and site manager. Mechanical and electrical subcontractors must hand over real numbers—SFP for fans, pump inputs, inverter settings—rather than brochure claims. Plot‑by‑plot variation needs tracking as well, because HEM’s granularity means “similar but not the same” can change the outcome.
A live site scenario: getting from design to as-built without surprises
/> A mixed‑tenure housing scheme in the North West is aiming to hand over its first phase before winter. The architect issued coordinated layouts, and the assessor produced HEM design models assuming air‑to‑water heat pumps running at 45°C with oversized radiators, plus dMEV in wet rooms. Procurement then signals a six‑week delay on the specified heat pumps, offering an alternative unit with different control logic and defrost cycle. Meanwhile, the window supplier proposes a minor profile change to hit delivery windows, nudging the frame factor and installation depth.
The site manager is under pressure: show homes must open on time, scaffolds are booked, and airtightness testing is due on a windy week. The M&E subcontractor warns that the offered heat pump prefers 40°C to meet its efficiency, which clashes with radiator schedules already ordered. The energy assessor flags that default thermal bridge values will sting if junction photos and details aren’t aligned to the model. A hasty meeting locks in the alternative heat pump, swaps emitters in key plots, updates HEM inputs, and clarifies that commissioning data and labelled photos will be handed over plot-by-plot. The phase hands over with a consistent as‑built model and no last‑minute compliance scramble.
Pitfalls and fixes when feeding HEM with site reality
/> HEM rewards projects that treat product data and controls as design elements, not afterthoughts. If a heat pump shifts its performance at different weather and flow conditions, that needs to be reflected in schedules and setpoints. The same goes for ventilation: fan SFP, duct routes and terminals influence real electrical loads and heat loss, not just Part F paperwork.
Thermal bridges matter more too. If you use bespoke details, secure psi‑values from a competent source and tie them to buildable sequences. If you can’t, carry the right risk allowance instead of assuming best‑case. Evidence must be structured: photos with plot numbers, location tags and dates; commissioning records with actual measurements; and a clear record of substitutions with their data.
# Common mistakes
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– Treating the heating system as “TBC” until late procurement. HEM needs the specific kit and control strategy baked into the design model.
– Assuming default psi‑values won’t hurt. Defaults can push energy use up; secure calculated values for key junctions or adjust the fabric to compensate.
– Commissioning forms without numbers. “Set to manufacturer’s recommendation” won’t cut it—record flow temps, flow rates, fan speeds and setpoints.
– Untracked plot variation. Swapping one window type or cylinder on a few plots without updating the model creates compliance drift and rework at handover.
Checklist for getting HEM-ready specifications and QA
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– Lock in heat source, emitter sizes and design flow temperatures at RIBA Stage 3/4, with manufacturer data sheets appended to the energy model pack.
– Define ventilation type, routes and terminals early; capture fan SFP and duct leakage assumptions that match what can be built and commissioned.
– Obtain psi‑values for repeating junctions from a competent source and align them with buildable details; plan photo evidence for each junction type per plot.
– Establish an airtightness target with realistic sequencing; book tests to suit envelope readiness and weather windows, not just programme lines.
– Set up a substitutions protocol: no product swap without energy assessor input, data sheets, and an assessment of impacts on flow temps and controls.
– Build a digital evidence folder structure by plot and elevation: labelled photos, serial numbers, commissioning sheets, and as‑built drawings.
– Brief sales and customer care so homeowner packs reflect installed controls and heat pump operating temperatures, reducing call-backs and resets.
What to watch as HEM rolls through the UK market
/> Expect software toolchains and assessor training to evolve fast. Interfaces with overheating assessments, electric vehicle charging provision, batteries and smart tariffs are likely to firm up, bringing design and M&E coordination closer together. Planning authorities may begin to reference HEM outputs directly for local requirements, and MMC providers will push for standardised detail libraries to secure better psi‑values. Transitional arrangements from SAP to HEM will create mixed portfolios for a while; keep a tight handle on which scheme each phase sits under and how your evidence differs.
If you’re leading delivery, assume HEM will surface sloppy assumptions that used to slip through. The prize for getting ahead is smoother handover and fewer compliance shocks; the cost of waiting is late redesign and extra kit to claw back performance.
FAQ
# When will SAP actually be replaced on my project?
/> Timelines depend on when your scheme is registered and which regulations it falls under. Many teams will see overlap as transitional rules apply, so some plots may stay on SAP while later phases move to HEM. Check your building control adviser’s view early and plan evidence requirements for both.
# Do I need different subcontractors or assessors for HEM?
/> You don’t need new trades, but you do need trades that can provide measured data, not just catalogue sheets. Your energy assessor must be competent in the new methodology and the chosen software. Build tighter links between the assessor, M&E coordinator and site manager so data flows both ways during construction.
# How should product substitutions be handled under HEM?
/> Treat substitutions as design changes, not purchasing admin. Before approving a swap, get the manufacturer data, confirm control logic and operating temps, and ask the assessor to rerun impacts on the affected plots. Document the decision, update drawings and commissioning targets, and file product labels for the as‑built pack.
# What evidence will building control want to see for HEM compliance?
/> Expect a similar principle to current practice: a design-stage model, an as-built model, and supporting evidence. Provide airtightness results, thermal bridge information, labelled photos of key junctions, and commissioning sheets with actual settings and flow rates. Good file naming by plot and elevation speeds sign-off.
# Who owns the HEM data and can it support post-occupancy checks?
/> Typically the client owns the design information, with the assessor producing reports as part of their appointment. If you want the outputs to inform post‑occupancy tuning, agree data formats and access at procurement stage. Linking HEM assumptions to metering and controls later can help diagnose gaps between predicted and actual use without guesswork.
