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What you need to know about changes to Part L

Last week at BRE’s Part L launch event, DCLG announced how the 6 per cent and 9 per cent uplifts in the 2013 Part L (for England) will be implemented

The new requirements will see a major change in the way the National Calculation Methodology (NCM) works for new dwellings, with the introduction of additional targets, alongside the 6 per cent headline figure. The new targets are the Dwelling Fabric Energy Efficiency Standards (DFEES) and the Target Fabric Energy Efficiency Standards (TFEES) - pronounced Duffys and Tuffys for the lovers of acronyms. The introduction of these Fabric Energy Efficiency Standards (FEES) in the 2013 Part L provides a strong indicator of how the ‘fabric first’ approach to compliance will be used on the road towards the 2016 zero carbon targets.

There is also a change to the way the TER is calculated. Instead of updating the 2001 specification to create the 2013 target (as has previously been the case in 2006 and 2010), the specification has been completely redesigned from scratch for 2013. DCLG are referring to this new method as the ‘elemental recipe’.

In fact the system is not really that new, and bares a stunning similarity to the one used in Scotland for a few years now. It seeks to make it easier to understand how you can comply. As a result you can now be sure that you will show compliance just by building to the elemental recipe, although you will still need to produce the SAP calculation. The baseline assumes new homes will be constructed to the new ‘recipe’ of U-values: 0.13 for floors, 0.18 for walls, 0.00 for party walls, 0.13 for roofs and 1.40 for windows, external doors and rooflights. You also need to set the air permeability to five, use System 1 ventilation and assume 89.5 per cent efficient mains gas combi heating.

This is a huge step up from the 2010 requirements and would be very difficult to achieve in practice – a U-value of 0.18 for walls equates to around 180mm of mineral wool in a traditional brick/cavity/block wall for example. Fortunately that’s not quite the whole story as the software will still allow you to flex the design. In fact DCLG and BRE went to great  lengths to encourage flexing.

You’ll be pleased to hear that the backstop values for flexing remain effectively unchanged from the 2010 edition meaning U-values of 0.25 for floors, 0.30 for walls, 0.00 for party walls, 0.20 for roofs and 2.00 for windows, roof-lights and external doors. Air permeability also remains at 10. Therefore you can play around with the software model to find the most effective flexing for your project, so long as you don’t drop below the backstop values, and do not exceed the DFEE.

At the launch event, examples of flexing for detached and end terrace properties were presented. The first option to include triple glazing resulted in slightly relaxed requirements for walls and floors from the elemental recipe, whilst waste water heat recovery meant more significant relaxations to the wall and roof requirements.

Table showing example routes to meet the TER and TFEE for a 76m² end terrace

  End terraceEnd terrace
 Elemental recipeTriple glazingRelaxed fabric
External walls (W/m2K)0.180.220.26
Party walls (W/m2K)000
Floor (W/m2K)0.130.160.19
Roof (W/m2K)0.130.130.13
Windows (W/m2K)1.40.091.4
Air tightness (m3/hr.m2)555
Gas boiler89.5 per cent89.5 per cent89.5 per cent
Services -WWHR
TER (KgCO2/m2.yr) 18.7218.72
DER (KgCO2/m2.yr) 18.6818.67
TFEE (kWh/m2.yr) 54.2654.26
DFEE (kWh/m2.yr) 46.7354.20

 

The table also shows the key elements of this recipe approach and how new DFEE and TFEE will affect things – essentially the TFEE is a mandatory target and is set as 1.15 x the DFEE, meaning that you must be within 15% of the DFEE in order to get a pass value from the software. Put another way you are expected to achieve most of the pass marks using fabric first. In the first example, the TER and DER values are very close, whilst the TFEE and DFEE values are much wider apart as we are flexing fabric against improved fabric. In the second example the TER and DER are almost unchanged but the TFEE and DFEE become very close showing that flexing of services against fabric is at the limit of what can be achieved in this case.

I should also point out that the DFEE calculation is bespoke to the dwelling plot being considered. Unlike previous versions of SAP the new version considers heat gains rather than simply solar gains (therefore un-insulated pipework will have a negative effect) . Also thermal bridges will be calculated on the length of the actual bridge and psi values in each case rather than simply by default values.

Thermal bridges were shown to be the single most significant reason why as-built performance often drops significantly below the predicted performance and are expected to become the focus of the 2016 changes. In the meantime the option for quality assured construction detailing is being removed from the 2013 approved documents leaving the choice as ACDs (Accredited Construction Details) or nothing.

The new software isn’t available yet, and will only be available in a beta version for some time to come, until the commercial software companies release their updates. So the general consensus of advice was to ensure that projects are registered prior to 6 April 2014 in order to avoid the need to use the new software.

Outside of new dwellings the changes are less far reaching as commercial buildings will not be expected to meet the requirements for FEES, and therefore the SBEM approach is not noticeably different from 2010, apart from the 9 per cent uplift previously announced.

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