Revised emissions target
Two years ago the government was considering a further 19% cut in the Part L1A emissions target which, if introduced, would have relied on the use of low carbon heat/power systems such as photo-voltaics. The DCLG has listened to the consultations and reason, and the target has been set at a more modest 6%, reflecting economic concerns and a need to encourage, not hinder, house-building.
In practical terms, the new emissions target can be met without the need for costly technologies, but does require a high standard of fabric performance to be achieved. However, the level of performance needed, is readily achievable with current materials and methods of construction, albeit with a greater emphasis on reducing thermal bridging.
New energy target
Alongside the revised Target Emissions Rate (TER), a new requirement has been introduced called the Target Fabric Energy Efficiency (TFEE), which sets a minimum standard for fabric performance measured in kWh/m2/y. This underpins the government’s ‘fabric first’ approach to reducing emissions by ensuring new homes cannot rely too heavily on low carbon heat/power systems to achieve compliance.
Revised method for setting targets
The way the Standard Assessment Procedure (SAP) sets targets has changed slightly, with the TER and TFEE both based upon the performance of a newly defined notional dwelling, which has a fixed specification for U-values, psi-values and heating etc., referred to as the Elemental Recipe. In setting the energy target, the calculated performance of the notional dwelling is increased by 15% to produce the TFEE. This is to be welcomed as it provides some design flexibility for the fabric performance. The full specification for the notional dwelling is set out in table 4 of the Approved Document L1A 2013.
Providing the TER and TFEE targets are satisfied and the existing fabric backstops are not exceeded, house builders are free to create their own fabric and services specification in the usual way. For those unsure of where to start, there is the option of simply adopting wholesale the Elemental Recipe, offering a straightforward way to ensure compliance from the outset. Whether it is implemented as it stands or is tweaked slightly to provide a more cost effective and practical specification, the new Elemental Recipe provides a good starting point for house builders that is simple to work with and not overly prescriptive.
|External walls||0.18 W/m2K|
|Party walls||0.0 W/m2K|
|Air tightness||5.0 m3/(h.m2)|
|Thermal Mass Parameter (TMP)||Medium (TMP=250)|
|Linear thermal transmittance||Standardised psi values – see SAP Appendix R, unless the actual dwelling uses the default y-value of 0.15 W/m2K, in which case the y-value for the notional dwelling is 0.05 W/m2K|
|Ventilation type||Natural (with extract fans)|
The main fabric-related values used in the specification for the notional dwelling (Elemental Recipe)
Implications for cavity walls
In terms of cavity wall thickness, the U-value of 0.18 W/m2K used in the Elemental Recipe can be achieved in cavities of around 150 mm using aggregate blocks with full-fill mineral wool insulation or partial fill phenolic insulation (with a 50 mm air gap). Alternatively, a more relaxed U-value of 0.2 – 0.22 W/m2K could be adopted, allowing a reduced cavity width. This can in turn be offset by improvements to other aspects of the fabric specification such as the roof insulation or windows. More relaxed U-values can also be offset by the addition of a low-carbon heat/power system such as photo-voltaics or flue gas heat recovery systems. Although this approach adds cost, it does allow advantage to be taken of the 15% performance margin included in the TFEE, whilst ensuring compliance with CO2 emissions limit set by the TER. A more detailed explanation of these options plus a range of worked examples can be found in the new guide from The Concrete Centre Thermal Performance: Part L1A 2013, which has been produced with input from across the sector, including the HBF, MMA, NHBC and numerous manufacturers.
This has become a particularly critical aspect of dwelling design, making the use of high performance construction details a sensible policy for house builders to adopt, particularly as they provide a low cost means of helping meet the energy and emissions targets. The masonry sector has developed thermally efficient construction details, the latest have been produced by the CBA. It is worth noting that if construction details such as these are not used, then the highly punitive default Y value for a dwelling’s overall thermal bridging must be applied instead. This can result in up to half of the calculated heat loss for the dwelling coming from thermal bridging alone. To help discourage this approach, the only Y value revised SAP software will accept in the future is the default value (0.15 W/m2K), which should encourage individual thermal bridges to be specified, leading to a much more favourable outcome.
There has been no change to the treatment of thermal mass since the last edition of Part L, although there was a revision to SAP in the intervening period that acknowledges the enhanced summertime performance of heavyweight dwellings where night-time ventilation is possible.
Speculation on future revisions to Part L1A
Towards the end of 2013 the government reaffirmed its commitment to the challenging deadline of new homes being zero carbon by 2016. However, this is now looking increasingly unlikely due to the lack of time to address the legislative issues involved. In reality the deadline may slip back to 2019, which aligns with the non-domestic zero carbon targets and meets the requirements of the European Energy Performance of Buildings Directive (EPBD). In practical terms, there are limits to what can be achieved on-site to meet this target, so the plan is for emissions from new homes to be offset through a combination of fabric energy efficiency, on-site low-carbon heat/power systems, and a range of additional, mostly off-site systems known as Allowable Solutions that will bridge the shortfall. The Allowable Solutions scheme could support a range of carbon saving industries and programmes, such as the upgrading of insulation in existing buildings and offshore wind power. These would be invested in by the scheme, with funding coming from a levy on housing developers.
In terms of a future uplift to the fabric performance required by Part L1A, it seems unlikely that this will go much further, as the new requirements are already approaching practical limits. However, it is not inconceivable that the helpful 15% margin added to the fabric energy efficiency target in Part L1A 2013 could be reduced or removed altogether, which would reduce the flexibility currently provided to the house builder.
Written by Tom De Saulles.
Read the article online at: https://www.worldcement.com/europe-cis/27032014/uk_building_regulations_for_new_homes_956/