Part L – Building Fabric Performance and More

If you are designing a new build or an extension, then since June you need to meet more stringent requirements with new Part L requirements as the government continues to push construction towards zero carbon.  And there is more to follow in 2025 with the Future Homes Standard, but for now what exactly does it mean for you?

There is good reason for Part L and the Future Homes Standard if we are serious about tackling climate change. After all an estimated 40% of all emissions come from our building stock, so if as an industry we can help cut this then we are doing our bit.

The revised Part L is part of that improvement and includes tougher limits on carbon emissions for new build with a 31% cut for dwellings compared to the previous version. The new standards are tougher and will be policed rigorously – so take note.

Part L requirements

As before there are two parts to Part L; Part L, Volume 1 for dwellings and Part L, Volume 2 deals with buildings other than dwellings.

For dwellings, the new regulation takes a more holistic approach to cutting emissions to reflect not only the ongoing improvements that building fabric performance makes but also the role of a building’s services.  This makes a switch to low or zero carbon heating systems plus photovoltaics an important consideration.

There are now four separate targets for Part L:

1. Primary energy target
2. CO2 emission rate
3. Fabric Energy Efficiency Standard
4. Minimum standards for fabric and fixed building services

There is also a new SAP 10 method of assessment to check that your design complies.  This government approved software creates a notional dwelling based on your information and calculates how much energy your proposed property will use. When the building is built, you will need to submit evidence that it has been built to meet the design criteria that you specified which will involve a fair amount of work – but more on that later.

Fabric Energy Efficiency Standard (FEES)

As a brick and blockwork manufacturer we can advise you on how to achieve the fabric efficiency requirements that you will need to comply.

There are two U values that you need to be aware of:

• Nominal or notional U values
• Backstop U values

If you achieve all of the notional specification figures, which includes the notional U values then your dwelling should meet the requirements for CO2 emissions and comply. The backstop U values are the maximum permitted values and you should only use these in areas where you cannot achieve the notional U values.

If you use a backstop U value, then improvements in other areas will be necessary.  You may need to make improvements across several parameters that you insert into your SAP calculation, for example by using zero carbon energy, increasing photovoltaic usage, reducing air leakage, enhancing PSI values or achieving lower than notional U values for other structural elements such as for the ground floor.

Nominal and backstop U values are set out in the table below:

Table 1 Part L values for new dwellings

A quick note about party walls from the above table.  To quote from the new regulations, you will achieve the notional value if these walls have “a fully filled cavity with effective sealing at all exposed edges and in alignment with insulation layers in abutting elements.”

If you are building an extension for an existing dwelling then you have to meet the following limiting standards – in this case there are no backstop values.

Table 2 Part L values for existing buildings.

As for the renovation of existing elements in an existing dwelling, the results need to be no worse than it was before and should meet new limiting standards.  The best advice is to reference the standard at this point because it also depends on the size of the renovation that you are doing.

Thermal bridging

When it comes to the performance of the building fabric, there is now a new focus on thermal bridging. This is known as the PSI value, which is the heat loss along a metre of a junction between two structural elements.

This can be a huge issue as poor detailing or incorrect construction of junctions can compromise the thermal performance of the building.  To help combat this, the new regulations require the submission of detailed design drawings for junctions and then on-site inspections and photographic evidence to show that your team has completed the work properly.

You can assess your thermal bridges in one of the following ways:

• Use construction joint details calculated by an experienced person
• Use junction details from reputable databases with independently assessed thermal junction details such as from LABC or NHBC.
• Use value in SAP, table K
• Use a default value of 0.20 W/m²K

The favoured method by the government is to calculate the PSI values for each property.

Part L – prove your compliance

It is no longer good enough to design a compliant building, you now need to prove that “as built” is consistent with the approved design of the notional building submitted in the design process.

This means that you need to:

• maintain material performance specifications,
• construct the building as it was designed,
• ensure you maintain quality and workmanship
• and use qualified and trained people to complete the work.

And you need to prove this through SAP 10 where you must list the specifications and notify any changes from the lists that you submitted at the design stage.

You will also need to submit a Building Regulations England Part L report or a BREL report.  This is new and provides building control bodies with the same information for every home. Not only will you need to state U value and PSI figures but you need to provide a complete breakdown of how they are achieved.  You will also need to provide high resolution, geo location photographic evidence and you, as the developer, and the energy assessor must both sign the “as built” report.

Delivering Part L

Not surprisingly there are lots of different ways of achieving Part L Volume 1.  Take external walls, although the government wants you to aim for their notional U value of 0.18, some properties will have other values due to offsetting in the SAP calculations.

And there are a number of options for achieving the U value in an external wall with different constructions, the type of brick and/or blockwork used, different cavity sizes and the possible use of insulated plasterboard or even new hybrid constructions.

Ultimately there are no hard and fast answers. You will need to work closely with an assessor and your suppliers to achieve Part L Volume 1.

If you think that the new Part L is demanding then you should have an eye on the future.  These changes are just a stepping stone towards the Future Homes Standard that the government will introduce in 2025.  This new standard intends to ensure that an average home will produce at least 75% lower CO2 emissions than one built to the previous standards; remember this compares to an improvement of 31% for the new Part L requirements.

While much of these improvements in the future will be driven by a move away from fossil fuels for heating, the fabric performance is vital since improvements here will reduce the energy needed by future homes.

So, while the standards are at first sight onerous, we have already and will find ways to comply. It’s worth remembering that it’s the same for everyone and as an industry we are major players in the future drive to net zero emissions.

To find out how to achieve the new Part L requirements for walls and floors download our guide here.

Remember you can also talk to our technical team for specific advice 0n 0330 123 1018.