Part L is being updated – here’s how to comply with the proposed changes

In October 2019, the Ministry of Housing, Communities and Local Government (MHCLG) issued their new consultation on changes to Part L and Part F of the Building Regulations. Dubbed the Future Homes Standard, the consultation is intended to set out the changes to these Approved Documents and allow stakeholders to voice their opinions. It is worth noting the final decision on the changes has yet to be issued, but here at YDBC, we are prepared and passing this knowledge onto our clients. From the MHCLG’s proposals and the current version of SAP 10 – the methodology used to show compliance with Part L – we can begin to set out how compliance can be achieved and some of the main stumbling blocks developers are likely to encounter.

Future Homes Standard

The Future Homes Standard is a MHCLG commitment to ensure all new homes will have low carbon heating and “world-leading levels of energy efficiency” by 2025. Whether this can be implemented within the given time frame is questionable considering the delays in previous versions of Part L, plus the addition of a global pandemic and Brexit, and some of government’s own deadlines within the consultation have already been missed.

The delivery of the Future Homes Standard will initially be done via one of two specified options:

  • Option One – Future Homes Fabric: Intended to deliver a 20% improvement on current Part L standards via very high fabric standards
  • Option Two – Fabric Plus Technology: Intended to deliver a 31% improvement on current Part L standards by minor fabric increases alongside low-carbon heating and renewables

Which option MHCLG intends to adopt is still unknown. The original intention was for one of these options to be introduced into the Building Regulations in 2020. The latest expectation is that Part L will be released in October 2021 to come into effect in April 2022, however, this is not officially confirmed. 

Changes to Part L Compliance

To comply with the current version of Part L, you must achieve a Dwelling Emission Rate (DER) no higher than the Target Emission Rate (TER) and a Dwelling Fabric Energy Efficiency (DFEE) no higher than Target Fabric Energy Efficiency (TFEE). In the next version of Part L, whether Option One or Option Two is adopted, there will be a new form of compliance:

  • DER no higher than the TER

And:

  • Dwelling Primary Energy Rate (DPER) no higher than the Target Primary Energy Rate (TPER)

The replacement of a target fabric approach to a primary energy one is MHCLG’s change in emphasis. The focus will be on driving energy efficient decisions, which in some scenarios may not offer low carbon solutions but the retention of TER should prevent this.

To comply with current Part L legislation developers, architects and energy assessors should ideally be aware of the values which form part of the TER/TFEE (Summary in Table 1).  

U-Values Floor 0.13 W/m2/k Mech. Ventilation Extracts based on Floor Area
External Wall 0.18 W/m2/k Main Heating Gas Combi Efficiency = 89.5%
Party Wall 0.00 W/m2/k
Roof 0.13 W/m2/k
Doors 1.00 W/m2/k Main Heating Controls Programmer/Roomstat/TRVs
Windows 1.4 W/m2/k
Roof Lights 1.4 W/m2/k Water Heating Type Combi unless cylinder proposed
  Thermal Mass Medium
Thermal Bridging y-value = approx. 0.05 Low Energy Lights 100%
Air Permeability 5    

Table 1 – Current Part L1 TER/TFEE reference Values

Fabric and building services limits contained within Part L almost become irrelevant as the TER/TFEE reference values exceed them. For example, if a new dwelling has a wall U-value of 0.23 W/m2K then it would meet the Part L limit (0.30W/m2K). However, given it exceeds the 0.18 W/m2K value used in the TER/TFEE, it means the shortfall created would have to be made up elsewhere. The same approach can be applied to any part of the specification i.e., if the proposed specification is worse than the TER/TFEE value the difference has to be made up in another part of the specification. As a result, by reviewing the reference values it can be said that any new homes adopting the value set out in Table 1 will achieve compliance with current Part L requirements.

Some software vendors have released a Beta version of the SAP 10 methodology, but none are yet BRE approved therefore the results are likely to change. However, reviewing the two options of the Future Homes Standards and the reference value contained within, it should provide an insight into how compliance is achieved.

Option 1 – Future Homes Fabric

As noted above, it is intended to deliver a 20% improvement on current Part L standards via very high fabric standards. Table 2 notes the reference values used in the TER and TPER.

U-Values Floor 0.11 W/m2/k Mech. Ventilation Extracts based on Floor Area
External Wall 0.15 W/m2/k Main Heating Gas Combi Efficiency = 89.5%
Party Wall 0.00 W/m2/k
Roof 0.11 W/m2/k
Doors 1.00 W/m2/k Main Heating Controls Time and Temperature Zone, TRVs
Windows 0.80 W/m2/k
Roof Lights 1.10 W/m2/k Water Heating Type Combi unless cylinder proposed
  Thermal Mass Same as Dwelling
Thermal Bridging y-value = approx. 0.04 – 0.05 Waste Water Heat Recovery Minimum 36% Efficient
Air Permeability 5 Low Energy Lights Likely LEDs Throughout

Table 2 – Option 1 TER/TPER Reference Values

One point to note from the above is the wall U-value of 0.15 W/m2K. This is very low and most likely a typical 100mm masonry cavity wall construction will not suffice. For developers, a wider wall may have implications on floor areas and/or plot sizes. If it is decided not to achieve 0.15 W/m2K then the shortfall will need to be made up elsewhere, however with a set of reference values so tight this may prove more difficult.  Other key points to note from Table 2 are: window U-values of 0.08 W/m2K which will include triple glazed units, the inclusion of a waste water heat recovery system which are not currently implemented regularly, the allocation of low energy lighting rather than the assumption of defaults and the enhanced heating control via time and temperature zone controls for most dwellings.

Should Option One come into effect then any new dwelling will be expected to have a very high fabric specification. As shown above the trade-off between the elements is likely to be minimal given the reference values are so long but as part of this option renewables are not considered; unlike Option Two.

Option 2 – Fabric Plus Technology

Option Two is intended to deliver a 31% improvement on current Part L standards by minor fabric increases alongside low-carbon heating and renewables. Similarly to the above, the reference values for the TER and TPER are available within SAP 10 and are shown below in Table 3.

U-Values Floor 0.13 W/m2/k Mech. Ventilation Extracts based on Floor Area
External Wall 0.18 W/m2/k Main Heating Gas Combi Efficiency = 89.5%
Party Wall 0.00 W/m2/k
Roof 0.11 W/m2/k
Doors 1.00 W/m2/k Main Heating Controls Time and Temperature Zone, TRVs
Windows 1.20 W/m2/k
Roof Lights 1.50 W/m2/k Water Heating Type Combi unless cylinder proposed
  Thermal Mass Same as Dwelling
Thermal Bridging y-value = approx. 0.04 – 0.05 Waste Water Heat Recovery Minimum 36% Efficient
Air Permeability 5 Low Energy Lights Likely LEDs Throughout
Photovoltaic (PV) System Allocated based on Ground Floor Area

Table 3 – Option 2 TER/TPER Reference Values

Firstly, both the floor and wall U-values are the same as the current reference values allowing for some continuity with construction methods. Secondly, the window U-values allow for double glazed units. A waste water heat recovery system is still being adopted but most importantly the TER/TPER will include a PV system. As a result, in order to achieve a DER/DPER no higher than their target values it is expected that most new dwellings under Option Two will require some form of PV or low carbon technology, such as a heat pump.

Summary

Whilst there is no release date for new the Part L regulations, unless the consultation changes significantly, there is enough within the latest SAP 10 methodology to give an indication on compliance. Reviewing the reference values used for the TER and the new TPER will assist developers, architects and energy assessors with understanding how compliance is achieved and where the short falls are within their specification.

What is clear, whichever of the two options (Fabric or Renewables) is adopted new considerations at design stage on fabric and use of low carbon/renewable technologies will be needed.