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U-value calculator

Part L Building Regulations

Part L 2013, England

Transitional arrangements from Part L 2010

Part L 2010 – comprising Approved Documents L1A, L1B, L2A and L2B – applied in England until 6th April 2014. From that date, the new regulations came into force.

However, if a Building Regulation application was made prior to the new regulations taking effect, Part L 2010 still applies – providing that work on site commences within 12 months. ‘Starting work’ typically means digging foundations or laying drains, and only one plot on a site needs to start in order for all plots to be built to Part L 2010.

This results in a lag, where new properties need only conform to a previous set of regulations, some 12 to 18 months after the regulations were changed! And by that point, consultation is likely to have started on the next revision to the regulations…


  • Download our handy ‘at a glance’ reference to the U-value requirements of Part L 2010 for England and Wales.
  • Download our handy ‘at a glance’ reference to the U-value requirements of Part L 2013 for England.


L1A 2013 – new domestic buildings

Like the 2010 regulations, L1A 2013 requires new dwellings to meet a defined target for carbon emissions (the ‘target emission rate’, or TER). Unlike the 2010 regulations, there is an added emphasis on the basic fabric of the building, and in order to comply with L1A 2013, a new dwelling will also have to meet a ‘target fabric energy efficiency’ (TFEE).

‘Fabric First’

A building’s fabric is intended to be the longest standing part of the construction, so the adoption of a fabric first approach is designed to ensure long-term performance that will help reduce energy demand to the lowest possible level, and provide the energy in the most cost-effective way. SAP 2012 software calculates the TFEE for a dwelling by taking into account:


  • Building fabric U-values
  • Thermal bridging
  • Air permeability
  • Solar gains
  • Internal heat gains


Previously, a dwelling could achieve compliance by paying little attention to the fabric but taking advantage of costly renewable technologies such as heat pumps and photovoltaic arrays. Now, the aim is to reduce energy use and cost, reduce CO2 emissions, increase comfort, and rely less on those renewable technologies.

To achieve this, the calculation method rewards improved insulation standards and well-designed construction details that limit thermal bridging and air leakage. The overall intention is for L1A 2013 to result in a 6% reduction in CO2 emissions across the new homes build mix compared to L1A 2010 (although changes in the calculation methodology make it difficult to quantify the exact percentage).

For L1A 2013, the ‘limiting fabric parameters’ (essentially, the highest permissible U-values for individual building elements) from L1A 2010 remain. Designing to these values will not result in compliance and the building specification will need to be considerably better.


The Notional Dwelling

Appendix R of SAP 2012 sets out a specification for the ‘notional dwelling’ – a model that uses the same form and shape as the actual dwelling, and on which the TFEE is calculated. L1A 2013 provides an ‘elemental recipe’, including standards for U-values, airtightness and the heating system. Designing to the recipe ensures compliance with the regulations, although the values are not mandatory and alternative means of complying are possible.


Thermal bridging

Improving fabric heat loss standards over the years has uncovered the extent to which heat is lost through the junctions between thermal elements. It wasn’t until the introduction of Part L 2006 that the regulations even took it into account! The values ascribed to this sort of heat loss are known as psi-values.

L1A 2013 offers two routes for including thermal bridging in the SAP calculation. If psi-values are not specifically calculated for the proposed dwelling then default psi-values are used. These are very conservative and require significant compensation elsewhere to achieve the TER and TFEE. Alternatively, psi-values can be addressed, and junctions may be designed to the available accredited or enhanced details to improve the chances of complying.

Recticel Insulation does not currently offer a service for calculating psi-values. Please speak to the Approved Energy Assessor who is carrying out your SAP calculations.


L2A 2013 – new non-domestic buildings

For non-domestic construction, the target reduction in CO2 emissions is 9% across the building mix.  The focus is similar to domestic buildings, but the regulations define a wider range of notional buildings that take into account whether buildings are unlit, side lit, or top lit, and whether mechanical heating and cooling is specified.

There is no FEE requirement for new non-domestic buildings in England. However, there is a requirement to assess the “technical, environmental and economic feasibility of using high efficiency alternative systems, if available.” This may be in the form of decentralised energy from a renewable source, cogeneration, district heating or cooling, or heat pumps. A Building Inspector can request documentation to show the analysis has been done.

Airtightness values vary by floor area, due to lower levels of air permeability being easier to achieve in larger buildings.


L1B 2013 & L2B 2013 – existing domestic and non-domestic buildings

Some minor amendments have been made to L1B 2010 and L2B 2010, but the main point to note is that the U-value requirements are unchanged.

Part L 2014, Wales


Transitional arrangements from Part L 2010

Part L 2010 applied in Wales until 31st July 2014. From that date, the new regulations came into force. The transitional arrangements that exist in England also apply to Wales, with the same resultant lag for housing sites.


  • Download our handy ‘at a glance’ reference to the U-value requirements of Part L 2010 for England and Wales.
  • Download our handy ‘at a glance’ reference to the U-value requirements of Part L 2014 for Wales.


L1A 2014 – new domestic buildings

By introducing their own Building Regulations, Welsh Ministers could aim for an 8% CO2 reduction for new housing, rather than the 6% targeted in England. The separate regulations require their own version of SAP, developed by the BRE.

A similar recipe approach has been taken, with a notional gas-heated dwelling used as the reference. The recipe provides a compliant solution, or at least a sensible starting point if the proposed dwelling varies from the specification.

While a TER must still be met, the principle difference between Wales and England is that there is no FEE standard to be met. Instead, L1A 2014 specifies tighter limiting fabric parameters. These U-value ‘backstops’ are considerably more stringent than in England.

Where thermal bridging is concerned, L1A 2014 uses the same targets as L1A 2013 for England.


L2A 2014 – new non-domestic buildings

The regulations target a 20% reduction in CO2 emissions, with calculations carried out by a new version of SBEM software developed by the BRE.

On top of the notional recipe approach, the most notable change is the introduction of a Target Primary Energy Consumption (TPEC) alongside the TER. It is similar to the ‘fabric first’ approach adopted in England and aims to reduce ‘greenwashing’ by disadvantaging the use of renewables or biomass fuels over better fabric standards.


L1B 2014 – existing domestic buildings

The new regulations emphasise better compliance, as retrofit and refurbishment is seen as often failing to come up to standard. For new thermal elements (such as in extensions), the U-value requirements have been aligned with the backstop values in L1A 2014. Compliance is achieved by either following these reference values, demonstrating an area-weighted trade-off, or calculating using SAP.

For the renovation and upgrade of thermal elements, the standards remain unchanged from L1B 2010, albeit with threshold values removed. The regulations recommend minimum energy efficiencies where there is significant renovation of 50% of the element surface (or 25% of the building envelope). Where thermal elements are retained, minimum energy efficiencies are recommended where there is a material change of use, or where the existing element becomes part of the thermal envelope e.g. loft or garage conversion.

Consequential improvements have been introduced – a measure that was shied away from in the regulations for England. They are applied to buildings of all sizes, but work is only required where technically and economically feasible. The triggers have been tailored to the domestic market to produce a simple and low cost approach.

Where a dwelling is extended and habitable space increased by 10m2, the following measures will apply if not already in place:


  • Any uninsulated or part-insulated cavity walls should be filled where suitable
  • Loft insulation should be topped up to at least 250mm thick
  • Hot water cylinders should be insulated


Consequential Improvements should be appropriate and proportionate to the building works. Where floor area is increased by less than 10m2, only the loft insulation upgrade is required.


L2B 2014 – existing non-domestic buildings

Like L1B 2014, the requirements for new thermal elements have been tightened to match new build standards. An extra category has been introduced to differentiate “buildings essentially domestic in character” (such as care homes), and where these are extended, tighter standards apply.

Like L1B 2014, consequential improvements apply, but the triggers are an extension or an increase in fixed building services. Again, the improvements are only required where technically and economically feasible. Measures are capped at 10% of the value of the principal works and the payback period should be less than 15 years. They can be selected from either an EPC report, green deal assessment, or a table of recommendations in the regulations.


Section 6 2013, Scotland

The two volumes of the Technical Handbooks – one for domestic, one for non-domestic – set out their thermal requirements in Section 6. Scotland has been ahead of England and Wales for some years with a recipe approach based on notional buildings. If a building is constructed in accordance with the notional specifications, it will comply with the CO2 emissions target.

Section 6 is due to be updated in October 2015. During consultation (in early 2013), it was proposed that new homes will target a 21% in CO2 reduction and 43% reduction for non-domestic buildings.


  • Download our handy 'at a glance' reference to the U-value requirements of Section 6 2013 for Scotland.