Contractors are starting to feel the shift from SAP assessments to the emerging Home Energy Model, and it isn’t just a software swap. The new approach aims to reflect how homes actually use energy across the day, making choices about heat pumps, MVHR, PV and airtightness more consequential. That pulls compliance risk closer to site delivery and commercial control: product substitutions that used to be benign can now tip a dwelling over the line, and commissioning records become as important as drawings when the assessor runs the numbers.
TL;DR
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– Home Energy Model moves away from static averages and is more sensitive to heat pump performance, ventilation efficiency, PV and airtightness.
– Product data and commissioning evidence carry more weight; defaults will hurt more than under SAP.
– Expect tighter coordination between design, procurement and energy assessors, with earlier lock-in of M&E kits and fabric details.
– QA artefacts (photos, air test logs, MVHR commissioning sheets) become primary inputs to compliance, not afterthoughts.
– Plan change control rigorously: late swaps of plant or openings can trigger a rerun with real programme impacts.
What the Home Energy Model changes, in plain English
/> HEM is designed to model energy use across time rather than relying on broad seasonal averages. That means it pays attention to when heat is needed, what the outside temperature is doing, and how well the chosen kit actually performs in those conditions. Heat pumps with different efficiency curves, PV arrays with varying orientation and inverter behaviour, and MVHR systems with real-world efficiencies will move the needle more than they did under SAP.
The model is more explicit about thermal bridges and airtightness. Generic y-values and optimistic assumptions on junctions are less forgiving, nudging teams towards robust details, third-party thermal bridging calculations, and evidence that airtightness targets are deliverable in the sequence you’ve planned. It also expects better alignment between design intent and what’s installed: a nominal “low-temperature” system that needs higher flow temperatures on site won’t score the same.
Digital workflows matter. Expect assessors to ask for machine-readable schedules, product data sheets with performance at stated conditions, and photos or logs that tie back to unique dwelling IDs. Where SAP might have ridden through a gap with a standard default, HEM is set up to make those gaps visible and costly.
How this plays out on real UK housing sites
/> Picture a 54-home suburban timber-frame scheme outside a regional city. The design manager is pushing for design freeze while the M&E subcontractor is waiting on final heat pump selections from procurement. The energy assessor has trialled HEM on a sample plot and flags that the proposed MVHR model needs a different ducting arrangement to achieve the assumed efficiency. Meanwhile, the window supplier offers a late swap on glazing that improves U-values but kills lead-in for trickle vents, colliding with ventilation strategy and Part F coordination.
On week 28, a price spike triggers an alternative heat pump brand with a different control regime and declared efficiency curve. The assessor reruns the model and reports a downturn that forces lower flow temperatures, which in turn asks for larger emitters in a few plots. Programme tightens: plasterboards are booked, but now wall radiator positions and pipe spacing need confirming. Commissioning sheets for MVHR and heating are raised from “hand-over paperwork” to “compliance-critical.” The site manager reshuffles access to ensure the airtightness test for a terrace happens before boxing-in, and the QS rings round for thermal bridge calculations to avoid punitive defaults on tricky balcony junctions.
Pitfalls and fixes for the HEM era
/> HEM’s sensitivity exposes thin spots in delivery. The cure is to bring the assessor into design earlier, push for product certainty sooner, and treat site QA as evidence, not admin. Fixes are mostly about workflow: align specifications with installable details, secure product data that the model recognises, and lock in change control that doesn’t leave design, procurement and site at odds.
# Common mistakes
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– Assuming a like-for-like plant swap will pass unchanged. Performance curves, control logic and set-point assumptions differ by model.
– Relying on generic thermal bridge values without checking junction buildability. Sequencing and tolerances can erode assumed performance.
– Leaving MVHR design to “contractor’s design” with no space allowance. Poor duct runs will drop efficiency below modelled expectations.
– Treating airtightness as a final-week event. Late fixes rarely recover the leakage difference that HEM will penalise.
Site-ready checklist for HEM-driven compliance
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– Lock down heat source selection with declared performance data at relevant outdoor temperatures and flow temps; avoid provisional picks.
– Agree MVHR layouts early with realistic space for ductwork, bends and silencers; secure commissioning access in every plot.
– Obtain thermal bridge details with psi-values for key junctions (e.g. sill, lintel, eaves, balconies) and confirm they’re buildable in sequence.
– Build an evidence pack plan: photo points for insulation continuity, service penetrations, airtight layers, and junctions, mapped to plot IDs.
– Fix an airtightness strategy with interim testing on sample units before finishes; adjust details based on measured leakage paths.
– Set strict product substitution rules: no changes without a quick HEM check and sign-off by design and commercial.
– Capture commissioning data digitally: flow temps, emitter balancing records, MVHR airflows and fan powers against the design.
What to watch as HEM beds in
/> Assessor capability and software maturity will vary for a while, so expect different interpretations of the same inputs. Manufacturers will be under pressure to publish data in formats models can use; kits without clear performance declarations may quietly fall out of spec lists. Design-and-build teams that connect BIM data to assessments will cut friction, while those running parallel spreadsheets will burn time on re-entry.
Procurement will shift earlier. Heat pump and MVHR brands will need selecting before construction to avoid default penalties and courier-lead panic buys that don’t meet assumptions. Site teams will feel more tests creep into the critical path: airtightness before lining, MVHR commissioning before ceiling close-up. Get those in the programme with float, not as last-day tasks.
The upshot is simple: better alignment of design, kit and workmanship leads to smoother compliance. Sloppy substitutions and rushed QA will be punished faster and more visibly under HEM.
Keep an eye on how Scotland and Wales align timelines, how software vendors refine tools, and what data formats manufacturers standardise on. In the next three months, ask suppliers for machine-readable performance data, ask assessors to mock-run a sample plot, and ask your programme lead where HEM-critical inspections sit on the look-ahead.
FAQ
# Will HEM mean I have to change my standard house types?
/> Not necessarily, but expect tweaks to details and kit selection. House types that passed under SAP using generous defaults may need tighter airtightness, better thermal bridge treatment or a specific MVHR model. Run a representative plot early and be ready to adjust before the design freeze.
# How should I handle product substitutions under HEM?
/> Treat substitutions as design changes, not buying decisions. Make sure the assessor gets the actual performance data in a usable format and reruns the model for the affected plots. Build a quick-turn approval route so procurement doesn’t outpace compliance.
# Who owns and maintains the data needed for HEM on a D&B job?
/> Typically the main contractor coordinates inputs, with the energy assessor curating the model. M&E and envelope subcontractors should supply product data sheets and commissioning records, while designers provide junction details and specifications. Agree a single evidence repository and naming convention at project start to avoid lost or unusable files.
# Does HEM change what I test on site?
/> The familiar tests still matter, but timing and completeness are more critical. Airtightness should be proven before coverings, not just at the end, and MVHR commissioning needs measured flows and fan powers that reflect how the system will actually run. Expect more emphasis on photo evidence of insulation continuity and services penetrations.
# How do I brief my supply chain for HEM without overcomplicating things?
/> Keep it simple and specific: share the target flow temperatures, MVHR efficiency assumptions, airtightness target and key thermal junctions. Request product data that aligns with those assumptions and make commissioning sheets contract deliverables. Set a clear rule that no changes occur without sign-off from design and the energy assessor, and include time for rechecks in the programme.
