FHS Specifications: The Full Technical Breakdown
Every U-value, airtightness target, and performance metric that defines Future Homes Standard compliance — from fabric and heating to ventilation and overheating.
Fabric Efficiency Requirements
The thermal performance targets that define the FHS building envelope — significantly tighter than Part L 2021.
| Element | FHS Target | Comparison |
|---|---|---|
| External Walls | 0.18 W/m²K | 0.26 under Part L 2021 |
| Roof | 0.13 W/m²K | 0.16 under Part L 2021 |
| Floor | 0.13 W/m²K | 0.18 under Part L 2021 |
| Windows | 1.2 W/m²K | 1.6 under Part L 2021 — requires triple glazing |
| Doors | 1.0 W/m²K | 1.6 under Part L 2021 |
| Airtightness | 3 m³/h/m² at 50Pa | 8 m³/h/m² under Part L 2021 — 3× tighter |
Heating Systems
The 75–80% carbon reduction target effectively mandates low-carbon heating. While the regulations do not explicitly ban gas boilers, compliance is virtually impossible with combustion as the primary heat source.
Heat Pump Requirement
Air source heat pumps (ASHP) and ground source heat pumps (GSHP) are the primary compliance route. Systems must be sized to the calculated heat loss of the dwelling using the Home Energy Model. MCS certification is required for all installations.
Alternative Low-Carbon Systems
Heat networks (district heating), shared ground loop systems, and potentially hydrogen-ready boilers in specific circumstances may achieve compliance. However, these represent a small minority of expected installations.
No More Gas Connections
FHS homes will not be connected to the gas grid. This eliminates gas hobs and gas fires alongside gas boilers. All energy needs must be met by electricity, with the assumption that the grid continues to decarbonise.
Ventilation (Part F)
The dramatically improved airtightness of FHS homes (3 m³/h/m² at 50Pa) means controlled mechanical ventilation is essential. Natural ventilation through gaps and cracks is no longer sufficient.
MVHR Requirement
Mechanical Ventilation with Heat Recovery (MVHR) recovers 85–95% of the heat from extracted air and transfers it to incoming fresh air. This is the primary ventilation strategy for FHS homes, maintaining indoor air quality while minimising heat loss.
Part F Compliance
Part F of the Building Regulations governs ventilation and works in conjunction with FHS. Extract rates for kitchens, bathrooms, and utility rooms must meet minimum specifications. Whole-dwelling ventilation rates are calculated based on floor area and number of bedrooms.
Commissioning Requirements
MVHR systems must be properly commissioned with measured airflow rates at each terminal. Imbalanced systems reduce efficiency, increase noise, and can cause moisture problems. Commissioning certificates are required for building control sign-off.
Overheating (Part O & TM59)
Highly insulated, airtight homes are at increased risk of overheating — particularly south-facing properties. FHS requires compliance with Part O and TM59 overheating assessment.
TM59 Assessment
CIBSE TM59 provides the methodology for assessing overheating risk. Living rooms must not exceed 28°C for more than 3% of occupied hours. Bedrooms must not exceed 26°C for more than 1% of occupied hours, with an additional adaptive thermal comfort criterion.
Passive Cooling Strategies
FHS homes should incorporate passive cooling: external shading (brise-soleil, deep reveals, overhangs), cross-ventilation design, appropriate glazing ratios, and low g-value glass on south and west elevations. MVHR summer bypass modes also contribute.
The Home Energy Model (HEM) & SAP 10.3
The government has confirmed a dual approach to energy assessment under the FHS. An updated SAP (SAP 10.3) will be available at launch, with the new Home Energy Model (HEM) following. Both can be used for FHS compliance during a dual running period of at least 24 months.
SAP 10.3 at Launch
The FHS will launch with SAP 10.3 as the approved calculation methodology. This updated version of SAP has been produced in a format suitable for the FHS notional building specification. The heat pump in the SAP notional building is set with a coefficient of performance (COP) of 2.5 for all dwellings, as SAP cannot dynamically size heat pumps to individual dwellings.
What HEM Does Differently
HEM models the complete thermal behaviour of a dwelling — including thermal mass, realistic occupancy patterns, actual heating system performance curves, and the interaction between fabric, ventilation, and heating. Unlike SAP, HEM can create a heat pump in the notional building which is optimally sized to the dwelling. HEM also supports voluntary delivered energy reporting via the BREL report.
Dual Running & Transition
HEM is expected to become an approved calculation methodology no earlier than 3 months after March 2026. Once available, either HEM or SAP 10.3 can be used for FHS compliance. SAP 10.3 will have a minimum lifetime of 24 months from when the regulations are laid. The government will give industry 6 months' notice before setting a date beyond which SAP 10.3 can no longer be used for new building control plans.
Technical Questions
What are the insulation requirements under FHS?
The FHS requires significantly improved thermal performance: external wall U-values of 0.18 W/m²K (compared to 0.26 under Part L 2021), roof U-values of 0.13 W/m²K, floor U-values of 0.13 W/m²K, and window U-values of 1.2 W/m²K (effectively mandating triple glazing). Airtightness must achieve 3 m³/h/m² at 50Pa, approximately three times tighter than the 2021 standard.
Are gas boilers banned under the Future Homes Standard?
The FHS does not explicitly ban gas boilers, but the 75–80% carbon reduction target makes it virtually impossible to comply using a gas boiler as the primary heating system. In practice, this means heat pumps (air source, ground source, or hybrid) become the default heating solution for FHS-compliant homes. Some hybrid systems combining a heat pump with a gas boiler may technically comply in specific circumstances.
What is the Home Energy Model?
The Home Energy Model (HEM) is a new, more sophisticated whole-building energy model developed by government. It accounts for thermal mass, realistic occupancy patterns, actual heating system performance curves, and the interaction between building systems — factors SAP simplified or ignored. HEM is expected to become an approved calculation methodology no earlier than June 2026. However, the FHS launches with SAP 10.3 as the approved methodology, and both systems can be used during a dual running period of at least 24 months. HEM remains the government's preferred long-term methodology.
What U-values are required under FHS?
FHS U-value requirements are: walls 0.18 W/m²K, roof 0.13 W/m²K, floor 0.13 W/m²K, windows 1.2 W/m²K, and doors 1.0 W/m²K. These represent a significant improvement over Part L 2021 values and effectively mandate specific construction approaches — for example, the window U-value of 1.2 W/m²K requires triple glazing as standard.