Better foundations for new builds
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Paul Davidson on what the new Part L means for those working in the construction industry
The new version of Part L of the building regulations came into force in England on 6 April. The headline changes are a further 6% tightening of the carbon emissions target for new homes and a new mandatory target for the energy efficiency of building fabric. For non-domestic buildings, the overall CO2 target is reduced by 9%. Requirements for existing buildings being refurbished remain largely unchanged from 2010.
The new Part L is published within the context of a number of UK and EU policies – some of which are potentially at odds with each other. The most fundamental is the government target that all new homes in England are zero-carbon by 2016. In addition, the recast Energy Performance of Buildings Directive (EPBD, 2010/31/EC) requires member states to work towards “nearly zero energy” buildings by 2019 – an aspiration endorsed in UK building regulations.
The zero-carbon triangle
Consideration of these policy decisions have led to the concept of the zero-carbon triangle (see below) in which energy efficient design underpins the low- and zero-carbon energy and heat systems that together deliver compliance with the zero-carbon target. If this approach falls short, a number of “allowable solutions” will be open to the designer. The department for communities and local government (Dclg) has recently consulted on what these allowable solutions might be.
Pulling in the other direction is the so-called “growth commitment” of 2010 and the government’s red tape challenge, both of which aim to minimise the burden of legislation on industry.
The new Part L is seen as an important and technically meaningful step forward, which strikes a balance between the zero-carbon agenda and the growth commitment. As with previous incarnations of Part L, the latest version aims to further reduce energy costs for consumers and businesses, and makes an important contribution to delivering the UK’s carbon budgets.
There are two significant innovations in the requirements for new dwellings, as set out in the new approved document L1A. They are:
- a new regulation (26A), which requires new dwellings to achieve, or better, a fabric energy efficiency (FEE) target, as well as a CO2 target; and
- the introduction of a notional building specification, which sets the target for CO2 emissions (without the use of an improvement factor) and the target for fabric energy efficiency.
The CO2 target has been strengthened to deliver an overall 6% reduction in emissions across the new-build housing mix compared with Part L 2010. Between them, these requirements tackle the lower two segments of the zero-carbon triangle, with further work needed to decide on the approach for 2016.
The notional dwelling used to determine CO2 and FEE targets is based on the new home’s design in terms of size and shape. However, the notional dwelling’s construction specification is determined in Part L. For example, it details the building’s “U-values” (a measure of heat loss from elements such as walls, roofs or floors).
In the new Part L, the notional dwelling’s wall U-values are set at 0.18 W/m2/°C and roofs at 0.13 W/m2/°C. If the new home is constructed precisely to the notional dwelling’s design specifications, it will meet its carbon dioxide and fabric energy efficiency targets.
However, this specification is not intended to be prescriptive and may not always represent the most cost-effective solution. Developers are free to vary the specification as long as the same overall level of carbon emissions and FEE performance is achieved or bettered. Dclg is encouraging industry to compile a series of model designs to help in this process.
FEE is defined as the amount of energy needed for heating and cooling during the year, expressed as kWh/m2/yr. By setting a minimum performance standard, Part L is ensuring that the fundamental structure of the dwelling meets basic energy efficiency requirements, and that a low-carbon energy source cannot be used to “rescue” poor fabric design. This recognises that the fabric of a building – that is, the walls, floor, roof and glazing – is likely to remain in place untouched for long periods of its life and is often costly to upgrade. By contrast, heating, ventilation and air-conditioning (HVAC) equipment has a finite lifetime and will be replaced several times.
The Zero Carbon Hub (ZCH) – the organisation established in 2008 to take day-to-day responsibility for achieving the government’s target of delivering zero-carbon homes in England from 2016 – has proposed standard 2016 target values for FEE (known as FEES). However, in the light of consultation responses, Dclg decided not to set the bar as high as that proposed by ZCH. The new Part L target FEE is, therefore, increased by only 15% over the FEE value resulting from modelling the notional dwelling.
Two other components of the calculation procedure are worthy of mention. Fuel factors were originally introduced to relax the CO2 target for dwellings without mains gas. After consulting on the issue, Dclg has decided to retain the factors, amended for use in the new Part L. On a related issue, CO2 emission factors have been updated to reflect changes in the energy supply systems, most noticeably for electricity.
In addition to the two mandatory elements of the compliance test for new homes (which together comprise criterion 1), there remain four other compliance tests. The first of these is the setting of “elemental backstops” (criterion 2). The need for fabric backstops – such as, the maximum allowable U-values – has to some extent been overtaken by the FEE requirement, so these values remain unchanged from 2010. Standards for building services are, again, contained in the domestic building services compliance guide.
Criterion 3 has changed slightly to limit the effects of heat gains in summer, with the emphasis widened from just solar gains. This, for example, encourages the proper insulation of domestic hot water pipes. Lastly, criterion 4 has also been revised. Although it still deals with the quality of construction and commissioning, it now recognises the vital importance of doing everything possible to ensure the design intent is translated into practice and that the resulting performance is consistent with the calculated building emission and FEE rate.
One change from 2010 is the removal of the reference to separate quality assured and accredited construction details for thermal bridging. Designers are now encouraged to use approved construction details from the Dclg for the junctions between fabric elements and at the edges of openings.
The provision of information to householders (criterion 5) remains an important route to ensuring that dwellings perform to their design potential, and the new Part L provides additional guidance on how builders can comply with this element.
For non-domestic buildings, the aim of the revised Part L is again to take a sensible, cost-effective step towards zero-carbon buildings. This has been achieved by tightening the specification for the notional building, the result of which is to deliver a 9% reduction in carbon emissions compared with 2010, when aggregated across the expected mix of building types (see below).
|Building type||% improvement|
|Deep-plan office with air conditioning||12%|
|Aggregate across build mix||
An extra category of notional non-domestic building has been introduced, so there are now three, based, as in 2010, on the source of daylight. These are:
- side-lit (or unlit), where the HVAC system provides heating only;
- side-lit (or unlit), where the HVAC includes cooling; and
In addition, the air-permeability for the notional building now varies according to the gross internal floor area, following concerns raised during the consultation.
The carbon emissions target (TER) set using the relevant notional building’s performance is achievable in most developments by focusing on the quality of the fabric and building services without the necessity for renewable energy sources.
However, as in the domestic sector, the notional building specifications are not intended to be prescriptive, but allow significant flexibility in design options. Unlike with new homes, the variability of non-domestic building types does not lend itself to the introduction of a mandatory FEE target. Instead, the elemental backstops are used to ensure a minimum level of energy efficiency in each building. These have been left unchanged from 2010.
Minimum standards for building services are set out in the non-domestic building services compliance guide, which has been updated to take account of, for example, standards set under the Ecodesign and Energy Labelling Directives (2009/125/EC and 2010/30/EU). An addition since 2010 is the option to demonstrate that the lighting system installed complies with criterion 2 by using the LENI method – a way of calculating the energy efficiency of lighting expressed in kWh/m²/yr.
Another new feature in the revised Part L is the requirement from the recast EPBD for designers to give proper consideration to high-efficiency alternative systems, such as renewable energy technologies, district heating, heat pumps or combined heat and power. For Part L, this will be covered by a facility in the compliance tools (see below) to record that this has been done and where evidence of analysis can be found.
After talks with industry, the TER can, in some cases, be increased by a factor related to the building’s age. This higher target can be set where a modular or portable building is to be used with a service life of longer than two years, but with at least 70% of its external envelope composed of modules built before the latest Part L came into force.
The government announced in December 2012 that there would be no changes in requirements for consequential improvements to existing buildings and no uplift in standards for extensions or windows. As a result, the two approved documents relating to existing buildings – ADL1B and ADL2B – have undergone only very minor changes for clarification and to provide additional guidance. In the case of non-domestic buildings, updates to the compliance guide may have an impact on the specification of replacement service items.
New dwellings’ compliance with the revised Part L will be demonstrated through new commercial software tools, with the calculation method based on a standard assessment procedure (SAP 2012). Pending the creation of the new software tools, Dclg asked BRE to produce a temporary, updated version of its existing software cSAP to allow industry to experiment with house designs and building products. This has been available since December 2013.
For non-domestic buildings, one of the compliance routes will be a new version of SBEM – the “simplified building energy model” software tool that analyses a building’s energy consumption. A revised version of SBEM, which complies with the new regulations, was released by BRE on 3 April (ncm.bre.co.uk).
Neither SBEM or SAP 2012 will be capable of producing an official compliance report or energy performance certificate, but they should prove useful to the construction and product supply industries.
The core guidance on the new Part L is contained in a few key documents. They have been produced in a new format and an index has been introduced to each:
- ADL1A – new homes
- ADL2A – new buildings other than dwellings
- Changes to ADL1B (existing homes) and ADL2B (existing buildings other than dwellings)
- Domestic building services compliance guide
- Non-domestic building services compliance guide
- NCM modelling guide (available from 2013ncm.bre.co.uk)
All are available at planningportal.gov.uk.
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