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 FHS compliance. 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. Reducing the form factor is often the most cost-effective way to improve compliance.
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 | Compliance 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 draft FHS specification tables cap 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
HEM's half-hourly solar modelling makes the distribution of glazing across orientations significantly more important than total area alone:
| Orientation | Recommended Share | Design Rationale |
|---|---|---|
| South | ~55% of total glazing | Maximum beneficial solar gain during heating season; managed by shading in summer |
| East | ~15% of total glazing | Morning solar gain; moderate overheating risk |
| West | ~15% of total glazing | Afternoon solar gain; higher overheating risk than east |
| North | ~15% of total glazing | Minimal solar gain; prioritise daylight only — minimise area |
Performance Specifications
The FHS notional dwelling uses window U-values of 0.8–1.2 W/m²K, which requires triple glazing in most cases. Key specifications to consider:
- U-value: Target 0.8–1.0 W/m²K for the whole window (including frame)
- 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
HEM calculates linear and point thermal bridges at every half-hourly timestep as part of the dynamic heat balance. This is a fundamental change from SAP, where a single y-value was applied to the total envelope area in the monthly calculation. Under HEM, thermal bridges contribute to heat loss dynamically and their impact varies with internal/external temperature differences through the year.
Key Junctions to Detail
| Junction | Typical Psi-Value (Standard) | Target Psi-Value (FHS) | Design Approach |
|---|---|---|---|
| Wall–floor (ground) | 0.16 W/mK | ≤ 0.08 W/mK | Insulation continuity at slab edge; proprietary thermal breaks |
| Wall–roof (eaves) | 0.06 W/mK | ≤ 0.04 W/mK | Continuous insulation from wall into roof; no gaps at wall plate |
| Window reveal | 0.05 W/mK | ≤ 0.02 W/mK | Insulated reveal detail; frame overlapping insulation layer |
| Window cill | 0.05 W/mK | ≤ 0.03 W/mK | Insulation returned below cill; avoid cold bridging through masonry |
| Corner (external) | 0.09 W/mK | ≤ 0.04 W/mK | Continuous insulation around corner; avoid returns in cavity |
Where junction psi-values are not provided, HEM applies default values from its thermal bridging tables. As with all HEM defaults, these tend towards the punitive end — providing calculated psi-values for all junctions will improve the compliance result.
Designing for Airtightness
The FHS notional dwelling targets 3 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
The FHS effectively mandates heat pumps for new homes. 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), compared to the 70–80°C typical of gas boilers. 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 40°C flow temperaturerather than the traditional 70°C. This typically means radiators 2–3 times the physical size of those in a gas-heated home
- 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 40% solar coverage of the building's floor area where feasible. Design considerations include:
- Roof orientation: South-facing roof pitches of 30–40° are optimal. 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 | Moderate — monthly calculation smooths impact | High — half-hourly calculation amplifies envelope heat loss |
| Orientation benefit | Limited — monthly solar averages | Significant — half-hourly solar modelling per orientation |
| Thermal mass credit | Minimal — simplified treatment | Genuine — dynamic modelling shows heat storage and release |
| Thermal bridge impact | Simplified y-value | Calculated at every timestep — junction detail matters |
| 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?
Target a form factor below 3 for straightforward compliance. Mid-terrace homes (1.5–2.0) comply most easily, while detached bungalows (3.0–4.5) face the greatest challenge. Where a higher form factor is unavoidable, compensate with enhanced fabric performance, increased renewables, or both.
Is triple glazing mandatory under the Future Homes Standard?
Not explicitly, but the notional dwelling uses window U-values of 0.8–1.2 W/m²K, which in practice requires triple glazing. Double-glazed units typically achieve 1.4–1.6 W/m²K — above the benchmark. While the whole-building performance approach allows trade-offs, triple glazing is expected to be 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 40°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.