Xtratherm | A Fabric First route to Compliance

Xtratherm delivered their CPD presentation on Part L - A Fabric First Approach to achieving Compliance on October 30th. The presentation explores how a fabric first approach can help designers achieve energy performance through good detailing and a low thermal bridging factor. We explain the principles of energy efficiency using materials and methods that are readily available from your builders merchants with every day on site best practices.

What is Part L?

Part L1A gives technical guidance on how to meet the energy efficient requirements of the Building Regulations 2010. The 1984 Building Act laid the groundwork for Part L1A, a functional requirement to "make reasonable provision for the conversation of fuel and power."

What does Part L apply to?

There are four approved documents L:

  • L1A is specific to dwellings
  • L1B relates to existing dwellings
  • L2A is for new buildings other than dwellings
  • L2B covers existing buildings other than dwellings

What is the current version of Part L1A?

Part L has been significantly altered since 2010. Amendments were made to Part L in 2013 and 2016 Part L1A 2013-2016 Amendments is the current version. However, for the purposes of this article we'll focus on the main changes brough forward in Part L 2013 which seen a 6% improvement over the 2010 Part L1A and the introduction of FEEs targets (Fabric Energy Efficiency).

The criteria for compliance with the energy efficiency requirements of the Building Regulations are a mix of mandatory and statutory guidance. Those significant for insulation are touched on below:

Compliance requirements:

  • The design carbon dioxide emission rate (DER) for the whole of the dwelling must be shown as not exceeding a defined maximum allowable emission rate (TER).
  • In addition, the Dwelling Fabric Energy Efficiency or DFEE must not exceed a maximum allowable energy loss (TFEE). The DFEE and TFEE takes account of energy loss through the building elements such as roofs, walls and floors, thermal bridges at junctions and air movement from leakage and ventilation. The thermal mass of the dwelling as well as heat gains from the sun, lighting and appliances are also factored in.
  • Individual fabric elements must achieve specified energy efficiency backstop standards
  • Lastly the "as built" energy performance of the dwelling must match or exceed the design version.

All calculations should be verified by the 2012 edition of the Standard Assessment Procedure (SAP 2012). The recommendations are for two sets of evidence, one submitted before the start of the works and the second as built shortly after works have been completed. The "as built" calculations are used to provide information for the Energy Performance Certificate (EPC) for the completed dwelling.

Design Flexibility Limits

Fabric Standards: There are limiting U-Value standards for different fabric elements of the building - including walls, roofs, windows, rooflights etc. It should be noted that adopting the values set out in Table 3 of L1A will most likely fail to achieve the required TER and TFEE.

Air permeability: Limiting values are set for air permeability

Building services also have set limits

Thermal Bridges: As U-values fall and airtightness levels increases, heat loss through 'thermal bridges' becomes more significant. The regulations require that the building fabric should have no reasonably avoidable thermal bridges. Thermal transmittance values are much more stringent (and increased in number) than those in the 2007 Accredited Construction Details (ACDs). Part L1A 2013 has been strengthened to deliver a 6% reduction in the C02 emission rate across the new home build mix relative to the 2010 regulation. The upshot of this is that the designer will have to compensate for any poor performing junctions elsewhere in the fabric. Thermal bridges must be identified, calculated and inputted into SAP calculations.

Air Permeability: On each building development, an air-pressure test should be carried out on three units of each building or 50% of that development, whichever is less. Regulation requirements are complex and should be adhered to carefully depending on size and type of dwelling. All dwelling results must be reported to the BCB.

Compliance with the reuquirements is demonstrated if:

  • The air permeability is not worse than the limiting value of 10/m3/hr/m2 at 50 Pa
  • The DER and DFEE calculated using the measured air-permeability is not worse than the corresponding TER and the TFEE respectively using SAP 2012

Building Services: Part L1A also contains requirements for the analysis of feasibility of high efficiency alternative systems such as renewable energy, or heating and colling systems. The provision of operating and maintenance instructions is also required.

If you have questions or require further information, please call Xtratherm's Technical Team on 0371 222 1055.