The Home Energy Model (HEM) changes the relationship between architectural design and energy compliance. Under SAP, the monthly method smoothed out the timing of solar gains and the dynamic effects of thermal mass. Under HEM's half-hourly dynamic simulation, these effects are modelled explicitly. Compact form factors and careful glazing design reduce energy demand and running costs, while thermal bridge-free detailing and robust airtightness strategies remain central to meeting 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 credited orientation through monthly orientation-specific solar gains, but it smoothed out their timing: it could not distinguish gains that arrive when the home needs heat from gains that arrive when it does not. Thermal mass was treated in a simplified way, with limited dynamic modelling of heat storage and release.
HEM changes this fundamentally. The half-hourly simulation tracks heat flows through every surface, models solar gains at each timestep using a standardised national weather year (the same file for every dwelling), 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 show genuinely lower modelled energy use
- Poor design shows up more clearly: complex forms, excessive glazing, and poorly detailed junctions all increase modelled energy use
- 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.
A form factor below 3 is a common industry design target. Because the FHS targets are set by a notional dwelling of the same size and shape as the actual dwelling, form factor mainly drives energy use, heat pump size, and running costs. Typical values by housing type:
| Housing Type | Typical Form Factor | Design Implication |
|---|---|---|
| Mid-terrace house | 1.5–2.0 | Lowest heat loss for its floor area; minimal exposed envelope |
| Semi-detached house | 2.0–2.5 | Low heat loss with standard fabric |
| End-terrace house | 2.0–2.8 | Moderate: additional exposed wall increases heat loss |
| Detached house | 2.5–3.5 | Higher heat loss; larger heat pump and higher running costs |
| Detached bungalow | 3.0–4.5 | Highest heat loss: 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 a standardised national weather year. This captures when solar gains arrive, keeping orientation a genuine design lever for real-world performance:
- 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, so large north-facing areas add little benefit
- East/west glazing provides moderate solar gain but increases overheating risk in summer, which is assessed separately under Part O (via the simplified method or dynamic thermal modelling)
Passive solar design practice favours generous south-facing glazing for winter gains and restrained north-facing glazing, with the notional dwelling capping total glazing at 25% of the total floor area and matching the actual dwelling's orientation. The FHS itself sets no orientation distribution: the compliance check uses the notional building comparison.
Thermal Bridges
In SAP, thermal bridging is calculated from psi-values and lengths for each junction, with the option of a simplified default y-value applied to the total envelope area. For new dwellings assessed in HEM, the default y-value route has been removed: junction psi-values must be modelled, calculated, or taken from standard values. In both methods the junctions combine into a heat loss coefficient used in the heat balance.
This makes evidencing junction detailing 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 assumes an airtightness of 4 m³/(h·m²) at 50 Pa, well inside the 8 m³/(h·m²) backstop limit, and designs testing worse must compensate elsewhere. Hitting it reliably 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: The notional dwelling pairs its airtightness with dMEV; designs going tighter than 4 m³/(h·m²) increasingly rely on mechanical ventilation with heat recovery 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 | Credited via monthly orientation-specific solar gains | Half-hourly modelling captures the timing of solar gains |
| Glazing distribution | Credited via monthly solar gains per orientation | Timing of gains modelled; overheating interaction clearer |
| Form factor | Drives envelope heat loss | Drives envelope heat loss; targets scale with the actual geometry |
| Thermal mass | Simplified: minimal benefit shown | Dynamically modelled: heavyweight construction shows real benefits |
| Thermal bridges | Per-junction psi-values, or a default y-value | Per-junction psi-values; the default y-value is removed |
| Airtightness | Simplified infiltration model | Mass-balance ventilation model; 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) remains a key driver of energy use. A compact building has less envelope area relative to its floor area, meaning less total heat loss, a smaller heat pump, and lower running costs. The FHS targets are set by a notional dwelling of the same size and shape as the actual dwelling, so compact form mainly reduces energy use rather than easing the target itself.
What are the glazing requirements under the Future Homes Standard?
The FHS notional dwelling caps glazing at 25% of the total floor area, with notional windows matching the actual dwelling's orientation. Its window U-value of 1.2 W/m²K is achievable with high-performance double glazing; triple glazing is a common choice for extra margin. The FHS sets no rule on how glazing is distributed between orientations, though favouring south-facing glazing for winter gains remains good practice.
How does HEM handle thermal bridges differently from SAP?
In SAP, thermal bridging is calculated from psi-values and lengths for each junction, with an optional default y-value as a fallback. For new dwellings assessed in HEM, the default y-value route has been removed, so junction psi-values must be modelled, calculated, or taken from standard values. The practical change is evidential: junction detailing must be demonstrated rather than covered by a default.
What airtightness level should architects target for FHS homes?
The FHS notional dwelling assumes 4 m³/(h·m²) at 50 Pa, significantly tighter than the 8 m³/(h·m²) backstop limit. Achieving it reliably means a continuous air barrier designed in from the outset. Many architects design tighter still, towards Passivhaus-equivalent levels using factory-finished cassettes. At those levels, MVHR becomes essential.
How important is building orientation under HEM?
HEM models direct and diffuse radiation at each half-hourly timestep using a standardised national weather year, capturing when solar gains arrive, while SAP already credited orientation through monthly orientation-specific solar gains. The notional dwelling's glazing matches the actual dwelling's orientation, so orientation moves the target as well as the result. Good orientation still reduces real heating demand and remains sound design practice.
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.