The Home Energy Model (HEM) changes the relationship between architectural design and energy compliance. Under SAP, design decisions had a relatively muted impact on the compliance calculation — the monthly method smoothed out the effects of orientation, glazing placement, and thermal mass. Under HEM's half-hourly dynamic simulation, every design choice carries greater weight. Compact form factors, careful glazing distribution, thermal bridge-free detailing, and robust airtightness strategies are now critical to achieving compliance with the Future Homes Standard (FHS).
This page summarises the key design implications of HEM and the FHS for architects. For a detailed design checklist and worked examples, see our Architect Design Guide.
Why Design Decisions Matter More Under HEM
SAP's monthly calculation effectively averaged out many design-level differences. A south-facing living room and a north-facing living room of the same size produced similar compliance results because solar gains were averaged over the month. Thermal mass had minimal impact because there was no dynamic modelling of heat storage and release.
HEM changes this fundamentally. The half-hourly simulation tracks heat flows through every surface, models solar gains hour by hour using location-specific weather data, and calculates the dynamic interaction between thermal mass, heating systems, and occupant behaviour. This means:
- Good design is rewarded more accurately — well-oriented, compact buildings with optimised glazing genuinely perform better in the calculation
- Poor design is penalised more severely — complex forms, excessive north-facing glazing, and thermal bridges all show a greater impact on the compliance result
- Fabric performance is modelled dynamically — thermal mass, insulation, and airtightness interact in ways that SAP could not capture
Key Design Factors
Form Factor
Form factor — the ratio of the total thermal envelope area to the total floor area — is more critical than ever. A compact building loses less heat per square metre of floor area because it has proportionally less external surface through which heat can escape.
Under the FHS, compact forms with a form factor below 3 are strongly favoured. This has significant implications for housing typologies:
| Housing Type | Typical Form Factor | FHS Implication |
|---|---|---|
| Mid-terrace house | 1.5–2.0 | Easiest to achieve compliance — minimal exposed envelope |
| Semi-detached house | 2.0–2.5 | Good compliance potential with standard fabric |
| End-terrace house | 2.0–2.8 | Moderate — additional exposed wall increases heat loss |
| Detached house | 2.5–3.5 | More challenging — requires enhanced fabric or renewables |
| Detached bungalow | 3.0–4.5 | Most challenging — large roof and floor area relative to volume |
Bungalows and detached homes with complex roof forms are particularly affected. Architects should consider how to minimise the thermal envelope while maintaining the required floor area — for example, by favouring two-storey designs over single-storey, reducing dormers and extensions, and simplifying roof geometry.
Orientation and Solar Design
HEM's half-hourly solar calculation models direct and diffuse radiation using hourly weather data for the specific location. This makes orientation a genuine design lever:
- South-facing glazing delivers significant beneficial solar gains during heating season — HEM credits this accurately at each timestep
- North-facing glazing contributes minimal solar gain but still loses heat — the penalty is more visible in HEM than in SAP
- East/west glazing provides moderate solar gain but increases overheating risk in summer — HEM models this interaction with Part O requirements
The draft FHS specification recommends glazing of approximately 55% south-facing and 15% north-facing, with a total glazing cap of 25% of the total floor area. While these are guideline figures (the actual compliance check uses the notional building comparison), they indicate the design intent.
Thermal Bridges
In SAP, thermal bridging was handled through a single y-value multiplied by the total envelope area in the monthly calculation. HEM models linear and point thermal bridges at every half-hourly timestep, meaning their contribution to heat loss is calculated dynamically alongside fabric, ventilation, and solar gains.
This makes junction detailing significantly more important. Architects should target thermal bridge-free construction where possible, approaching Passivhaus-standard psi-values at all key junctions. Common focus areas include:
- Wall-to-floor junctions
- Wall-to-roof junctions
- Window and door reveals, heads, and cills
- Balcony and canopy penetrations
- Corner details and party wall junctions
Airtightness as Architecture
The FHS notional dwelling targets an airtightness of 3 m³/(h·m²) at 50 Pa. This is a fundamental shift from the current regulatory maximum of 8 m³/(h·m²) and cannot be achieved through site-applied taping alone. It requires a continuous air barrier strategy designed into the building from the outset:
- Continuous air barrier: A single, identifiable air barrier layer throughout the building envelope — typically a membrane, structural sheathing board, or the inner leaf of masonry
- Factory-finished cassettes: Timber frame, SIPs, and closed-panel systems offer better airtightness consistency than traditional site-built methods
- Service penetration strategy: Every pipe, cable, and duct passing through the air barrier must be sealed. Design service zones that minimise penetrations
- MVHR integration: At 3 m³/(h·m²), mechanical ventilation with heat recovery is essential for indoor air quality. Duct routes and unit location must be planned from the design stage
How SAP and HEM Differ for Design Decisions
| Design Decision | Impact in SAP | Impact in HEM |
|---|---|---|
| Building orientation | Minimal — monthly solar averages | Significant — half-hourly solar modelling rewards good orientation |
| Glazing distribution | Limited impact on compliance | Major impact — south-facing glazing strongly rewarded |
| Form factor | Moderate impact | Critical — compact forms strongly favoured |
| Thermal mass | Simplified — minimal benefit shown | Dynamically modelled — heavyweight construction shows real benefits |
| Thermal bridges | Single y-value in monthly calc | Calculated at every timestep — junction detail matters more |
| Airtightness | Important but undervalued | Critical — tighter envelopes properly credited |
| Heat emitter design | Generic categories | Specific sizing affects heat pump performance modelling |
Frequently Asked Questions
Why does form factor matter more under HEM?
Form factor — the ratio of the thermal envelope area to the floor area — matters more because HEM's half-hourly simulation calculates heat loss through every surface at each timestep. A compact building with a form factor below 3 has less envelope area relative to its floor area, meaning less total heat loss. HEM amplifies the advantage of compact forms compared to SAP's simplified monthly method.
What are the glazing requirements under the Future Homes Standard?
Draft FHS tables cap glazing at 25% of the total floor area. The recommended distribution is approximately 55% south-facing and 15% north-facing, to maximise beneficial solar gains while minimising heat loss. Triple glazing with U-values of 0.8–1.2 W/m²K is expected to be standard. HEM's half-hourly solar modelling makes glazing placement significantly more important than under SAP.
How does HEM handle thermal bridges differently from SAP?
In SAP, thermal bridging uses a simplified y-value applied once in the monthly calculation. In HEM, both linear and point thermal bridges are calculated at every half-hourly timestep as part of the dynamic heat balance. This means thermal bridging has a much greater impact on the compliance result, making thermal bridge-free construction increasingly important.
What airtightness level should architects target for FHS homes?
The FHS notional dwelling targets 3 m³/(h·m²) at 50 Pa — significantly tighter than the current Part L maximum of 8 m³/(h·m²). Achieving this requires a continuous air barrier designed in from the outset. Many architects are targeting Passivhaus-equivalent levels using factory-finished cassettes. At these levels, MVHR becomes essential.
How important is building orientation under HEM?
Orientation is significantly more important under HEM. The half-hourly solar calculation models direct and diffuse radiation at each timestep using hourly weather data, rather than SAP's monthly averages. South-facing glazing delivers more useful solar gain in the model, and poorly oriented buildings show a real penalty. Prioritise south-facing living spaces with generous glazing and minimise north-facing glazing.
Related Pages
Architect Design Guide
Form factor, glazing strategy, thermal bridges, airtightness, and a compliance checklist.
Part L Changes
Detailed breakdown of fabric specs, heating requirements, and what’s new in Part L.
For Developers
Cost impact, transitional arrangements, and compliance strategy for FHS projects.
Technical Reference
Deep dive into fabric heat loss, thermal mass, and solar gains modelling in HEM.