黑洞社区

Introduction

The 2013 revision to Part L of the 黑洞社区 Regulations is a key milestone towards the government鈥檚 target for all new homes to be zero-carbon by 2016, and new non-domestic buildings by 2019. Following a long consultation period, new carbon emission targets were published in November 2013. These set a 6% improvement on the 2010 Part L targets for housing on aggregate - which means there may be flexibility across different types of homes - and a 9% improvement on aggregate for non-housing. As the industry gears up for the introduction of the revised Part L on 6 April 2014, this CPD sets out to explain the method behind it, and show how it can be implemented.

Changes to Part L1A for new dwellings

• The TFEE Previous versions of Part L only required the designer to hit a target emission rate (TER) for carbon dioxide (CO2). In Approved Document L 2013 there is now a second standard called the fabric energy efficiency standard (FEES), for which a target fabric energy efficiency (TFEE) is set. This focuses on the energy demand of the building, and will therefore drive efficiency through measures that reduce energy use - such as a 鈥渇abric first鈥� approach - rather than mitigating CO2 through building systems and renewable technologies. The TFEE is measured in kWh/m2/yr and is therefore not affected by the relative carbon emissions of different fuel types.

The key factors that influence a building鈥檚 fabric energy efficiency are:

• U-values of floors, walls and roofs
• U-values of windows and doors
• Thermal bridging
• Air permeability
• Thermal mass
• External heat gain (solar gain)
• Internal heat gains

The TFEE is calculated by determining the fabric energy efficiency from a notional dwelling of the same size and shape as the actual dwelling. This value is then increased by 15% to give the TFEE.

• New notional dwelling There is a new notional domestic building for Part L1A 2013, replacing the 2002 version used previously. The specifications for the notional home are published for the first time within the Part L document itself in Section 5: Model Designs. Some of the key values include:

• Limiting fabric U-values The limiting fabric U-values remain unchanged from those in Part L 2010 and are shown below.

How to comply

• Notional or limiting U-values? Part L1A 2013 allows flexibility in design, so that elements of the building can be varied according to requirements, as long as overall targets for CO2 and energy are met, and the limiting fabric U-values are not exceeded. This is referred to as the 鈥渆lemental recipe鈥� approach.
  Part L1A 2013 states that 鈥渢o achieve the TER and the TFEE rate, a significantly better fabric performance than that set out in Table 2 (limiting fabric U-values) is likely to be required鈥�. So designers are best advised to start not from the limiting fabric U-values, but from the notional building U-values in Section 5: Model Designs.
• Thermal bridging Thermal bridging must be addressed to give the best chance of compliance. If no attention is paid to thermal bridging and the default value of y = 0.15 is taken, it will be very difficult to compensate elsewhere in the design.
• Party walls If the dwelling has a party wall, the best course of action is to ensure that it is either of solid design or is fully filled and sealed. Both of these approaches will enable a U-value of 0 to be used. Failure to do this will require further measures elsewhere in the design to compensate for the increase in CO2, and will make compliance more difficult
• Flexing the fabric The table below shows an example of how the ingredients of the elemental recipe can be adjusted, or 鈥渇lexed鈥�, when triple glazing is added to a 76m2 end-of-terrace home.

In the elemental recipe column, the DFEE represents the energy use of the notional dwelling; the TFEE is this figure multiplied by 1.15. The second column shows that by introducing higher performance glazing into the recipe, the walls and floor can be allowed slightly higher U-values without adversely affecting the DER or DFEE.

• Aggregate approach The 6% domestic target is an aggregate figure across the mix of housing types. Those dwellings with the most fabric, such as detached properties, must achieve greater emissions reductions than those with less fabric and therefore less scope for improvement, such as a mid-floor flat.

Changes for existing homes

Part L 2013 makes no changes to the U-values required when making home improvements. The maximum values are shown in the table below:

Changes to Part L2A for new non-domestic buildings

Commercial and other non-domestic buildings will not be expected to meet the requirements for FEES, and therefore the SBEM approach is not noticeably different from 2010. The main change is the 9% aggregate increase on the TER, and the fact that greater variation is included between building types, as shown below:

Another change is that there is a wider set of notional buildings used to determine CO2 targets. The three building types are:

• Side-lit or unlit, heating only (SLU-H)
• Side-lit or unlit, includes cooling (SLU-C)
• Top-lit.

Limiting U-values are unchanged from 2010, regardless of the notional building type. These are: 0.26W/m2K for walls, 0.22W/m2K for floors, 0.18W/m2K for roofs and 1.6W/m2K for windows (although this does not apply to top-lit buildings).

However, there is variation in the notional buildings鈥� air permeability, which have been subdivided by size. This is because the larger the building, the easier it is to achieve airtightness. The new values are as follows:

A summary of the Part L2A notional buildings is published in Section 5: Model Designs of the Approved Document. If the actual building is constructed to the notional specifications it will meet the CO2 targets and the limiting fabric and building services parameters. However, developers are free to vary the specification, provided the same overall level of CO2 emissions is achieved or bettered.