KJ Tait

‘Fabric First’ Building Regulations

11 September 2023

The Building Regulations were updated in England in June 2022 (Part L 2021) and in Scotland in February 2023 (Section 2022).  Whilst different, both sets of regulations employ a similar ethos of a ‘Fabric First’ approach when considering heating systems that are not refrigerant based, for example direct electric systems.   


We are currently designing our first electrically heated building under the new regulations in Scotland.  We have found there are major challenges with complying with the regulations, particularly if there is little budget or roof space for a PV installation.   


There are several reasons for this, firstly you have to understand that Part L/Section 6 tests the regulations against a Notional Building of the same activity type e.g. office, school etc.  This Notional Building has the same dimensions and zoning as the building being designed but has its own glazing ratios, fabric, air permeability, lighting and HVAC systems.  When we test against the building regulations a Target Delivered Energy Rate (TDER) in kWh/m2 is set by the Notional Building that the designed building must beat.  In England, the Target Emissions Rate (TER) in kgCO2/m2 must also be bettered. 


Where is the issue?  U-values for the Notional Buildings used by each set of regulations have improved significantly.  Scotland has gone further with particularly challenging U-values for external wall elements and glazing.   Air permeability figures under each set of regulations has also improved from the previous 10m3/h.m2.   


  England (W/m2K)  Scotland (W/m2K) 
External Wall  0.18  0.15 
Floor  0.15  0.13 
Roof  0.15  0.11 
Glazing  1.40  1.20 
Air Permeability (m3/h.m2)  3  4 


However, it is the behind the scenes change of thermal bridging factor that may cause Engineers striving for compliance the biggest challenge.  The new regulations for both England and Scotland add an additional 25% of the U-value for each of the thermal elements for the actual building but only 10% for the Notional Building.   


In effect, even if the Design Team were to match the U-value for an external wall in Scotland with the Notional Building at 0.15 W/m2K, the Notional Building’s U-value would be better due to the thermal bridging coefficient.  The rules around thermal bridges and on what basis it can be changed do not exist within the Building Regulations, it is our view that if the Design Team are calculating the thermal bridges with a view to reducing them then we can move to 10%.  However, we would subsequently need to make the calculations available to Building Control.   


There are other aspects at play that make compliance significantly harder under the new regulations>  In Scotland, the Notional Building has a provision of PV when the designed building is heated by direct electricity.  Other changes include the specification of an air source heat pump that is 286% efficient when a building has a heat pump for hot water.  The result of this is there is little to no win on the hot water as most heat pumps on the market will not better this for hot water generation.   


To alleviate any potential issues with gaining compliance under Section 6 2022 and Part L 2021, the Energy Modeller should be engaged during RIBA Stage 2 to build a model to test potential fabric combinations.   It is only by doing this that we can be confident of achieving compliance with these regulations.