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  • Writer's pictureEvolusion Innovation

Part L, TGD L, and Fabric Insulation

Updated: Jun 7, 2019

Part L of the Building Regulations in Ireland, and the associated Technical Guidance Document (TGD L), have been subject to the most substantive changes of any section of the regulations in recent years. From a situation where compliance was a relatively straightforward matter of satisfying discrete minimum standards (e.g. elemental u-values), it is now necessary to view the relevant building in a holistic manner, and to carry out complex calculations using the Dwelling Energy Assessment Procedure (DEAP) for dwellings, and NEAP or SBEM for Buildings other than dwellings, to demonstrate that the primary energy and the CO2 emissions do not exceed specified maxima. The latest aim is to have near-zero energy buildings (nZEB), and this has been summarised in another article in this news section of our website.

The drive to substantially improve the energy performance standards of buildings, and the changes in energy consumption this has led to, can be illustrated using the 2005 requirements as a baseline.

To achieve these performance improvements each aspect of Part L contributing to building performance has incrementally changed over the past number of iterations of the TGDs.

Part L requirements cover a range of issues that affect overall energy and carbon emissions performance, including requirements relating to the fabric of the building. A “fabric first” approach to energy conservation and efficiency is frequently advocated, and some of the key guidance of TGD L (dwellings) in that regard include:


Maximum backstop U-values for building elements are prescribed in the TGDs.

The maximum u-value for walls under 2011 guidance is 0.21 W/m2K. To comply with nZEB requirements, a wall u-value of 0.18 will be needed, a 15% reduction. The nZEB u-value for floors is 0.18, and for roofs 0.16. Good practices on site to ensure insulation is installed correctly (such as avoiding gaps, positioning wall ties etc.) is critical to achieving theoretical calculated u-values.

Back in 1991, the maximum u-values for walls was 0.55 W/m2K, for ground floors 0.45 W/m2K, and for roofs 0.35W/m2K.


The airtightness limit specified in the 2011 TGD L is 7m3/(hr.m2). To achieve nZEB compliance this is reduced to 5m3/(hr.m2). Where airtightness is below 3m3/(hr.m2), mechanical ventilation is recommended. Airtightness compliance can be indicated by testing – the TGD specifies the numbers of units to be tested and who can do the tests.


Thermal bridging is now a critical element in the overall DEAP calculation, as breaks in the insulation envelope now have, proportionally, a much greater impact on the overall energy efficiency and general performance of a building than heretofore (e.g. because of the much-improved elemental u-values, gaps/breaks in insulation result in localised much colder surfaces and therefore increase the potential for condensation). Thus it is essential to ensure continuity of insulation and to reduce local thermal bridging, such as around windows, doors and other openings, and at junctions between building elements. Where thermal bridges occur, they must not cause surface or interstitial condensation. Thermal bridging heat loss is factored in to DEAP calculations, and the default thermal bridging factor in DEAP is 0.15, but it is generally expedient to achieve a much lower factor. If certain conditions are met, as described in TGD L and outlined below, a factor of 0.08 can be used, otherwise the factor can be calculated from first principles (a service Evolusion provides). A lower value can make a significant difference in achieving compliance with CPC and EPC limits, and it is interesting to note that in the example dwelling specifications of a compliant dwelling house provided in Appendix E of TGD L a thermal bridging factor of 0.05 is specified.

Appendix D of TGD L provides detailed information in relation to thermal bridging, and provides four approaches to compliance:

(i) adopt Acceptable Construction Details for typical constructions as shown in sections 1 to 6 in the document “Limiting Thermal Bridging and Air Infiltration – Acceptable Construction Details” for all key junctions;

(ii) adopt Acceptable Construction Details sections 1 to 6 combined with details from Appendix 2 of the document “Limiting Thermal Bridging and Air Infiltration – Acceptable Construction Details” or other certified details (as defined in (iii) below) for all key junctions;

(iii) use certified details which have been assessed in accordance, and comply, with Appendix D, e.g. certified by a third-party certification body such as Agrément or equivalent or certified by a member of an approved thermal modellers scheme or equivalent for all key junctions;

(iv) use alternative details which limit the risk of mould growth and surface condensation to an acceptable level as set out in paragraph D.2 of Appendix D for all junctions.

To be clear, it is not enough to simply comply with the fabric requirements. DEAP must be used to calculate the Energy Performance Coefficient and Carbon Performance Coefficient and compare the EPC and MPC to the Maximum Permitted Energy Performance Coefficient (MPEPC) and the Maximum Permitted Carbon Performance Coefficient (MPCPC). Along with the fabric issues outlined above, the DEAP calculation takes into account renewables, heating systems, solar gains, cooling systems, controls, building ventilation, lighting etc.

The TGDs (with full guidance on methods of compliance with the Part L requirements) are available for download here.

Our Building Physics team at Evolusion are leading experts in building energy efficiency and sustainability. Certified in u-value and condensation risk calculations by the BBA, approved by NSAI in thermal modelling, and licensed to carry out BREEAM and LEED sustainability assessments, we have the qualifications and experience to provide you with high quality and cost-effective services.

Our team of construction technology and offsite construction experts at Evolusion Innovation has been involved in over €2 billion worth of offsite construction projects across Ireland, the UK and Europe over the past 15 years. The company is growing quickly, and we recently opened our newest office in Birmingham in the UK.

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