Guide to Part L Changes: England’s New Regs Focus on Fabric First Route to Low Carbon Buildings

20th September 2022

Changes to building regulations have come into effect in England which will help transition the building landscape to reaching the government’s target to half energy use in all new builds by 2030. The headline figures from the new Part L document for builders and specifiers are the requirement to reduce CO2 emissions compared to 2013 requirements by:

  • 31% for dwellings
  • 27% for buildings other than dwellings.

Where building plans were submitted before 15th June 2022 and work on the unit commences begore 15th June 2023, the 2013 version of Part L still applies. For any plans submitted after 15th June 2022, or where work on an individual building has not commenced before 15th June 2023, the new Regulations must be applied. The latter part of this is a significant point to note for housing developments in particular. The transition arrangements only apply to individual buildings, rather than a group of buildings, such as a housing development. So in such scenarios, building must have commenced on each individual house by 15th June 2023 for the 2013 standards to apply.

Fabric First Approach

The new regulations firmly place the focus on adopting a fabric first approach to achieving better performing buildings, and rightly so. By ensuring the walls, floors and roofs of any new build are more energy efficient, it lessens the risk of dependence on technology, and also lessens the potential impact of severe energy price hikes, such as the increases we are currently experiencing.

Thermal bridging is another area of focus, and the reg changes move to address the design to build gap, an issue which has been all too common on sites across the country. There is now a requirement to provide photographic evidence and air pressure testing for each building, as well as a checklist to ensure work is in accordance with specifications and critical junctions satisfy thermal bridging values.

New U-Values

The table below outlines the Mannok recommended U-values for Part L 2021 for domestic builds:

* To pass overall SAP calculation the U-values will need to be significantly better than the limiting values.

** 0.30W/m2K for internal or external insulation, 0.55W/m2K for cavity insulation.


Mannok New Build Solutions

Cavity Walls

Option 1: 100mm Mannok Therm Cavity MC in 150mm cavity with 100mm Mannok Aircrete Standard inner leaf and 100mm dense block or brick outer leaf, internally finished with plasterboard on dabs.

Option 2: 122mm Mannok Isoshield (full fill) in 125mm cavity with 100mm Mannok Aircrete Standard inner leaf and 100mm dense block or brick outer leaf, internally finished with plasterboard on dabs.

Pitched Roof – insulation between and below rafters

125mm Mannok Therm MR between rafters plus 92.5mm Mannok Therm Laminate MLK below rafters.

Pitched Roof – insulation at ceiling joist level

150mm mineral wool, λ 0.040W/mK, between joists plus 120mm Mannok Therm Roof MR above joists.


150mm Mannok Therm Floor MF above or below ground bearing concrete slab – based on P/A of 0.4

Thermal Bridging

Mannok Aircrete Thermal Blocks are a proven excellent solution to thermal bridging at all key junctions, reducing heat loss by up to 80% versus traditional dense concrete blocks.

The Technical by Mannok Team are available to work with specifiers, builders and contractors to help maximise the energy efficiency of a build using Mannok products, and to advise on the most effective route to Part L compliance.


Call +44 28 67748866 or email