Designing homes that comply with the Future Homes Standard (FHS) requires architects to think differently about energy performance from the earliest design stages. The Home Energy Model (HEM) replaces SAP's simplified monthly calculation with a half-hourly dynamic simulation that rewards good design and penalises poor decisions more accurately. This guide covers the key design parameters (form factor, glazing strategy, thermal bridging, airtightness, and heating system integration) and provides a practical design checklist for architects working on FHS projects.
Form Factor: The Starting Point
Form factor is the single most important geometric parameter for a home's energy efficiency. It is calculated as:
The form factor determines how much external surface area the building exposes to the elements relative to the usable space inside. Because HEM calculates heat loss through every surface at every half-hourly timestep, the total envelope area directly drives heating demand. Because the FHS targets are set by a notional dwelling of the same size and shape as the actual dwelling, a lower form factor mainly reduces energy use, heat pump size, and running costs.
Design Strategies for Low Form Factor
- Favour two-storey over single-storey: A two-storey home has a smaller roof and ground floor relative to its total floor area, significantly reducing the form factor compared to a bungalow of the same floor area
- Simplify roof geometry: Hipped roofs, dormers, and valley gutters all increase envelope area without adding floor area. Simple gable or mono-pitch roofs are more efficient
- Minimise extensions and set-backs: Every projection adds envelope surface. Integrate all accommodation within a simple, compact footprint where possible
- Consider terraced and semi-detached typologies: Party walls do not count as thermal envelope, so attached homes inherently achieve lower form factors
- Review the plan depth: Deeper plans (front to back) tend to produce lower form factors than shallow, wide plans of the same area
| Design Move | Effect on Form Factor | Heat Loss Impact |
|---|---|---|
| Convert bungalow to two-storey | Reduces by 0.5–1.5 | Significant improvement: less roof and floor heat loss |
| Remove dormer windows | Reduces by 0.1–0.3 | Moderate: eliminates additional wall and roof area |
| Attach two detached units | Reduces by 0.3–0.5 each | Significant: party wall removes heat loss surface |
| Simplify roof from hipped to gable | Reduces by 0.1–0.2 | Moderate: less roof area and fewer thermal bridges |
| Remove ground-floor bay window | Reduces by 0.05–0.15 | Minor, but cumulative with other changes |
Glazing Strategy
Glazing design under HEM requires balancing three competing demands: maximising beneficial solar gains, minimising fabric heat loss, and controlling overheating risk under Part O.
Total Glazing Area
The FHS notional dwelling specification caps glazing at 25% of the total floor area (TFA). Exceeding this threshold is possible but requires compensating performance elsewhere. In practice, most FHS-compliant designs will sit close to or below this figure.
Orientation Distribution
The FHS sets no requirement on how glazing is distributed between orientations: the notional dwelling's windows match the actual dwelling's orientation. Distribution still matters for real-world performance and for overheating, so passive solar design remains good practice:
| Orientation | Solar Behaviour | Design Approach |
|---|---|---|
| South | Strong beneficial gain during heating season | Favour for main living-space glazing; manage summer sun with shading |
| East | Morning solar gain | Moderate overheating risk; suits bedrooms and kitchens |
| West | Afternoon and evening solar gain | Higher overheating risk than east; size and shade with care |
| North | Minimal solar gain | Size for daylight only; keep areas restrained |
Performance Specifications
The FHS notional dwelling uses a window U-value of 1.2 W/m²K, which is achievable with high-performance double glazing; triple glazing is a common choice for extra margin. Key specifications to consider:
- U-value: The notional benchmark is 1.2 W/m²K for the whole window (including frame); 0.8–1.0 W/m²K gives headroom beyond the FHS
- g-value (solar transmittance): Balance heat gain against overheating. South-facing windows may need a lower g-value (0.4–0.5) while north-facing can use higher (0.5–0.6)
- Frame factor: The proportion of window area occupied by the frame affects both U-value and solar gain. Slimmer frames improve both metrics
- Air leakage: Windows are a critical air barrier junction. Specify high air-tightness performance and ensure consistent installation detailing
Thermal Bridge-Free Design
For new dwellings assessed in HEM, the default y-value route that SAP allowed has been removed: junction psi-values must be modelled, calculated, or taken from standard values (SAP retains the y-value option). In the FHS notional dwelling, junction lengths and types are the same as the actual dwelling, with psi-values taken from SAP 10 Table R2, so poorly detailed junctions in the actual dwelling compare directly against standard values in the notional.
Key Junctions to Detail
| Junction | Design Approach |
|---|---|
| Wall–floor (ground) | Insulation continuity at slab edge; proprietary thermal breaks |
| Wall–roof (eaves) | Continuous insulation from wall into roof; no gaps at wall plate |
| Window reveal | Insulated reveal detail; frame overlapping insulation layer |
| Window cill | Insulation returned below cill; avoid cold bridging through masonry |
| Corner (external) | Continuous insulation around corner; avoid returns in cavity |
Where junction psi-values are not provided, standard default values are used instead of the junction's real performance. Providing calculated psi-values for all junctions will generally improve the compliance result.
Designing for Airtightness
The FHS notional dwelling assumes 4 m³/(h·m²) at 50 Pa. Achieving this consistently requires the airtightness strategy to be an integral part of the architectural design, not a construction-stage afterthought.
Air Barrier Strategy
The air barrier must be a single, continuous, identifiable layer throughout the building envelope. It should be clearly identified on all drawings and its continuity maintained at every junction, penetration, and change of construction:
- Timber frame / SIPs / CLT: Air barrier typically on the warm side of the insulation, often a dedicated membrane or the structural sheathing board with taped joints
- Masonry cavity: Inner leaf of blockwork with parge coat or dedicated membrane on the inner face. Wet plastering the inner leaf provides a good air barrier but must be continuous (not just a skim)
- Modern methods of construction (MMC):Factory-finished panels and volumetric modules typically offer the best airtightness performance and consistency
Service Penetration Management
Every pipe, cable, duct, and flue that passes through the air barrier must be individually sealed. Strategies to minimise penetrations:
- Create a service zone on the warm side of the air barrier for electrical and plumbing distribution, avoiding the need to penetrate the membrane
- Group penetrations where possible and use proprietary grommets and seals rather than site-applied mastic
- Design MVHR duct routes that avoid crossing the air barrier more than necessary, typically two penetrations (supply and exhaust to outside) per dwelling
Designing for Heat Pump Heating
For most new houses the practical route to the FHS targets is a heat pump; flats and some urban sites may instead connect to a heat network. Architectural design must accommodate heat pump systems from the outset:
Heat Emitter Design
Heat pumps operate most efficiently at low flow temperatures (35–45°C). AD L1 has required new and fully replaced wet heating systems to be sized for a maximum flow temperature of 55°C since 2021, and the FHS notional dwelling assumes 45°C. This requires:
- Underfloor heating (UFH): Naturally suited to low flow temperatures (typically 35–40°C). Works well with the thermal mass of screeded floors. Best specified for ground floors, potentially upper floors too
- Oversized radiators: If radiators are used, they must be sized for a 45°C flow temperaturerather than the 70°C once typical of gas-heated homes. This means radiators 2–3 times the physical size of those older installations
- Hybrid approach: UFH on the ground floor with oversized radiators on upper floors is a common and effective strategy
Space Planning for Heat Pumps
- External unit: Allow at least 1 m clearance around the air source heat pump for airflow. Consider acoustic impact on the dwelling and neighbours. Avoid locating directly below bedroom windows
- Hot water cylinder: Heat pumps require a cylinder (typically 150–250 litres). Allow space in a utility room or airing cupboard. Combi-style instant hot water is not compatible with heat pump systems
- Buffer vessel: Some systems benefit from a buffer tank (50–100 litres) to prevent short-cycling. Allow space in plant areas
Solar PV Integration
The FHS requires rooftop solar PV on most new homes, with a target of PV coverage equivalent to 40% of the dwelling's ground floor area where feasible. Design considerations include:
- Roof orientation: AD L1's benchmark array (40% of ground floor area, orientated south-east to south-west, 45° pitch, unshaded) sets the output the real installation must at least match. East-west split roofs can also work well, providing more consistent generation throughout the day
- Unshaded area: Ensure sufficient unshaded roof area for the target PV capacity. Avoid locating vents, flues, and dormers where they would shade panels
- Structural loading: PV panels add approximately 12–15 kg/m² to the roof. Specify roof structure accordingly
- Inverter location: Allow space for the inverter (typically near the consumer unit). HEM requires specific inverter specifications for the calculation
HEM's half-hourly modelling of solar generation and self-consumption means that the orientation and tilt of PV panels, combined with the dwelling's electricity demand profile, produce a more accurate picture of the real energy benefit than SAP's monthly approach. Battery storage can also be modelled, further improving self-consumption rates.
FHS Compliance Design Checklist
SAP vs HEM: What Architects Need to Know
| Design Aspect | Under SAP | Under HEM |
|---|---|---|
| Form factor sensitivity | Envelope heat loss from U-values and areas | Same, at half-hourly resolution; targets scale with the actual geometry |
| Orientation benefit | Monthly orientation-specific solar gains | Half-hourly solar modelling captures gain timing |
| Thermal mass credit | Minimal: simplified treatment | Genuine: dynamic modelling shows heat storage and release |
| Thermal bridge impact | Per-junction psi-values or a default y-value | Per-junction psi-values; the default y-value is removed |
| Glazing g-value | Limited influence | Important: affects solar gains and overheating at each timestep |
| MVHR performance | Simplified credit | Detailed: SFP, heat recovery, and duct losses modelled |
| Heat pump COP | Simplified seasonal average | Variable: modelled with source/sink temps at each timestep |
| PV self-consumption | Monthly estimate | Half-hourly generation vs demand matching |
For a comprehensive comparison of the two calculation methodologies, see our SAP vs HEM page.
Frequently Asked Questions
What form factor should I target for FHS compliance?
A form factor below 3 is a common industry design target. Compact, simple shapes lose the least heat for their floor area. The FHS targets are set by a notional dwelling of the same size and shape as the actual dwelling, so form factor mainly drives energy use, heat pump size, and running costs rather than the pass margin.
Is triple glazing mandatory under the Future Homes Standard?
No. The notional dwelling uses a window U-value of 1.2 W/m²K, which is achievable with high-performance double glazing; standard double units at 1.4–1.6 W/m²K fall short of the benchmark. The whole-building performance approach allows trade-offs, and triple glazing is a common choice for extra margin rather than the expected standard.
Does thermal mass help with HEM compliance?
Yes. HEM dynamically models thermal mass at every timestep, meaning heavyweight construction shows a genuine benefit that SAP could not capture. Thermal mass smooths internal temperature swings, reduces peak heating demand, and improves heat pump efficiency. However, longer heat-up times may affect intermittent heating patterns.
How should architects design for heat pump heating?
Heat pumps operate most efficiently at low flow temperatures (35–45°C), so emitters must be sized accordingly: either underfloor heating or oversized radiators designed for a 45°C flow temperature. Allow space for a hot water cylinder (heat pumps cannot provide instant hot water), ensure space for the external unit with adequate acoustic separation, and design the distribution system for low-temperature operation from the outset.
Why must MVHR be designed in from the start?
MVHR requires dedicated duct routes to every room, typically 80–120 metres of ducting in a 3-bedroom home. These need straight runs with minimal bends, routed through floor voids or dedicated service zones. The unit needs a central location with maintenance access and acoustic separation from bedrooms. Retrofitting MVHR into a design not planned for it is expensive and compromises performance.
Related Pages
Part L Changes
Detailed breakdown of fabric specifications, U-values, and heating requirements under the FHS.
Ventilation & Part F
MVHR requirements, airtightness interaction, and HEM's pressure-driven ventilation model.
Compliance Pathways
HEM vs SAP 10.3 routes, the notional building approach, and how to demonstrate compliance.
SAP vs HEM
Side-by-side comparison of the old and new calculation methodologies.