Future Homes Standard for Builders: What Changes on Site

The Future Homes Standard (FHS) transforms on-site construction practice for new homes in England. Gas boilers are effectively replaced by heat pumps, homes built to the notional airtightness need continuous mechanical ventilation (dMEV or MVHR), rooftop solar PV becomes a functional requirement, the notional airtightness tightens to 4 m³/(h·m²) against an unchanged backstop of 8, and fabric specifications broadly carry over from the 2021 uplift, with high-performance glazing holding at around 1.2 W/m²K. These changes require new skills, new supply chains, and careful coordination between trades. This page summarises what changes on site and what builders need to prepare for.

For detailed guidance on each construction change, see our FHS Construction Changes Guide.

What Changes on Site

The table below summarises the key construction changes between current Part L 2021 homes and FHS-compliant homes:

Key Construction Areas

Heat Pump Installation

Almost every FHS home will be heated by a heat pump, typically an air source heat pump (ASHP); flats may instead connect to a heat network. Heat pumps introduce new requirements for builders:

• External unit: Concrete plinth, pipework penetrations, electrical connection, and adequate clearance for airflow
• Hot water cylinder: Internal space for a 150–250 litre cylinder (combi-style instant hot water is not compatible with heat pumps)
• Low-temperature emitters: Radiators sized for low flow temperatures (the notional building assumes 45°C; AD L1 sets 55°C as the regulatory maximum), requiring significantly larger emitters than conventional systems
• Commissioning: Heat pumps require specialist commissioning, including flow rates, refrigerant charge, and controller settings must all be verified

Airtightness

The FHS notional airtightness of 4 m³/(h·m²) at 50 Pa sits at the better end of current practice, and any dwelling testing worse must make the shortfall up elsewhere. Hitting it consistently requires a deliberate, continuous air barrier strategy:

• Every junction, penetration, and service route through the air barrier must be individually sealed
• Proprietary tapes, grommets, and membranes replace site-applied mastic for critical seals
• Mid-build airtightness tests are strongly recommended to catch issues before finishes are applied
• All trades must understand the air barrier location and their responsibility to maintain it

MVHR Installation

MVHR systems require dedicated duct routes from the central unit to every habitable room and wet room. For builders, this means:

• Duct installation is a first-fix activity: routes must be installed before ceilings and floor finishes
• 80–120 metres of ducting in a typical 3-bed home, requiring careful coordination with other services
• Rigid or semi-rigid ducting is preferred over flexible for performance and hygiene
• Commissioning: Airflow at every valve must be measured and balanced after completion

Solar PV

Solar PV installation must be coordinated with the roofing programme. Key considerations for builders include structural loading (12–15 kg/m²), weathertight panel mounting, DC cabling routes to the inverter location, and coordination with the electrician for grid connection.

Under HEM, solar PV is modelled very differently from SAP. Where SAP calculated annual PV generation as a single total, HEM models generation at half-hourly intervals and matches it against half-hourly household demand. This means the orientation and pitch of panels have a measurable impact on the energy rating: not just total output, but when that output occurs relative to demand.

• Orientation matters more: west-facing panels that generate during evening demand peaks may contribute more to self-consumption than south-facing panels that peak at midday when occupants are out. HEM captures this distinction; SAP could not
• Battery storage interactions: HEM can model battery charge and discharge cycles, meaning builders who install battery storage alongside PV will see this reflected in the energy rating. This opens up system design options that SAP simply ignored
• Practical installation: panels must be installed to the specified orientation and pitch. SAP already credited orientation and tilt, but only through an annual generation total. HEM carries the difference through into half-hourly self-consumption, so a roof-integrated system installed 20° off the design orientation shifts the result further

For builders, the key takeaway is that PV installation quality and accuracy now matter for compliance, not just quantity of panels installed.

Common Mistakes That HEM Will Expose

From our experience assessing thousands of new-build properties, many common construction shortcuts that went unnoticed under SAP will become visible under HEM's more detailed modelling. These are the areas where we see the most problems on site.

Airtight Membrane Failures

The air barrier is only as good as its weakest point, and in practice most failures occur at junctions and penetrations rather than in the membrane itself:

• Unsealed service penetrations: every pipe, cable, and duct that passes through the air barrier needs an individual seal. On a typical 3-bed house, there can be 50+ service penetrations. Missing even a handful will show up in the airtightness test
• Poor tape adhesion at junctions: proprietary airtightness tapes require clean, dry surfaces and firm pressure to bond properly. In wet or dusty site conditions, tapes applied quickly without surface preparation regularly fail within weeks. Junction details between membrane sheets, around window frames, and at wall-ceiling intersections are the most common failure points
• Membrane damage from follow-on trades: electricians routing cables, plumbers installing pipework, and carpenters fixing battens all work after the air barrier is installed. Without clear marking and awareness of the air barrier location, follow-on trades regularly puncture or tear the membrane without realising it

Under HEM, airtightness has a greater impact on the overall energy rating because infiltration is modelled dynamically rather than as a fixed assumption. A building that tests at 5 m³/(h·m²) instead of a design target of 4 m³/(h·m²) will show measurably higher heating demand in the HEM output.

MVHR Duct Coordination

MVHR duct installation is one of the most coordination-intensive activities in an FHS build, and the most common source of performance problems:

• Routing conflicts: MVHR ducts compete for space with plumbing waste pipes, hot and cold water supplies, heat pump pipework, and electrical cables. Without early coordination between M&E trades, ducts end up squeezed into inadequate voids, creating excessive bends and restrictions
• Duct length and pressure drop: every metre of ductwork and every bend adds resistance. If duct runs exceed the manufacturer's specification, the MVHR unit cannot deliver the designed airflow. HEM models ventilation rates at each timestep, so an under-performing system will produce higher modelled demand
• Crushing and kinking: semi-rigid ducts routed through tight spaces or beneath heavy cable bundles get flattened, reducing airflow. This is particularly common in floor voids and ceiling spaces where ducts are installed before other services and then compressed by follow-on work
• Condensation and hygiene: poorly insulated duct runs through cold spaces (lofts, unheated voids) can develop condensation. Rigid ducting with proper insulation is strongly preferred over flexible ducting for this reason

Thermal Bridging at Junctions

HEM models thermal bridging from psi-values at each junction type, as SAP does, but removes SAP's option of a default y-value in place of junction calculations. The common weak points are:

• Window reveals: where insulation wraps around the window opening. Insufficient return of insulation at the reveal creates a cold bridge that HEM will model as increased heat loss
• Floor-wall junctions, particularly at ground floor level where the wall insulation meets the floor insulation. A gap or misalignment between the two creates a thermal bridge running the entire perimeter of the building
• Balcony and canopy connections: structural elements that penetrate the insulation layer, such as steel balcony supports or concrete canopy slabs, create point thermal bridges that can dominate the heat loss at that junction

Commissioning Failures

Even well-installed systems can underperform if commissioning is inadequate:

• MVHR not commissioned: airflow at each supply and extract valve must be measured and balanced after installation. An uncommissioned system will not deliver its design airflow, leading to poor air quality and moisture problems. HEM assumes the system operates as designed; if it does not, the gap between modelled and actual performance will be significant
• Heat pumps on default settings: heat pumps must be commissioned to the specific building: flow temperatures, weather compensation curves, hot water schedules, and defrost settings all need configuring. A heat pump left on factory defaults will run less efficiently than HEM predicts, increasing energy bills for the occupant

Build Programme Implications

The FHS does not just add new technologies; it changes the sequence and coordination of the build programme. Builders who plan for these changes will avoid costly rework and delays.

Earlier Coordination Between Trades

Under current practice, M&E coordination often happens reactively, with services routed around whatever structure is already in place. Under the FHS, three systems need early coordination:

• MVHR duct routes must be designed and agreed before first fix begins. Duct paths through floor voids, ceiling spaces, and risers need to be clearly marked on drawings and protected during construction
• Heat pump pipework routes from the external unit to the cylinder and distribution system must be planned alongside the MVHR ducting, not in competition with it
• Air barrier continuity must be understood by every trade that works near or through it. This requires a clear air barrier strategy drawing that all trades can reference

Airtightness as a Whole-Team Responsibility

Achieving 4 m³/(h·m²) is not the responsibility of a single trade. Every operative who makes a penetration, fixes a bracket, or routes a service through the thermal envelope affects airtightness. Practical steps include:

• Site induction covering the air barrier location and the requirement to seal all penetrations
• Proprietary grommets and sealants available on site at all times (not ordered as an afterthought)
• Mid-build airtightness testing after the air barrier is complete but before plasterboard and finishes cover it. Finding problems at this stage is far cheaper than after completion

Testing and Commissioning Requirements

FHS homes require more testing and commissioning than current Part L homes:

• Airtightness testing: mandatory, with the result feeding directly into the HEM calculation
• MVHR commissioning: measured airflow at every terminal, documented and signed off
• Heat pump commissioning: commissioning per MIS 3005-I or equivalent, with documented flow rates, refrigerant charge, and controller configuration
• As-built SAP/HEM assessment: the energy assessor's as-built calculation must reflect what was actually constructed, not just the design intent

Building these testing and commissioning activities into the programme, rather than treating them as last-minute items before handover, is essential for FHS compliance.

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