CPD: Part L 2010

The next milestone on the road to zero carbon

Geoff Wilkinson, vice chair of the CIOB’s Building Standards Faculty, takes a look at the long-awaited changes to Part L 2010 and their practical effects

The 2010 amendments to Part L are significant, and bring an estimated capital cost increase of around 4-8%, depending on the type of project. But the good news is that it should still be possible to gain compliance using traditional construction methods, and there is no specific requirement to incorporate renewable energy sources.

The first major change is to Part L itself (as opposed to the Approved Documents) in which a new requirement, 20D, has been introduced. As part of the application package (ie prior to works commencing), the designer must submit:

• target CO₂ emissions rate for the building (TER)

• calculated CO₂ emissions rate for the building as designed (DER);

• a list of specifications to which the building will be constructed.

This is a significant issue for contractors as any value engineering changes to the specification may result in the need to resubmit to Building Control, as minor product changes can result in significant variations in the TER.

The only method for complying with this requirement remains the National Calculation Method, which includes software tools such as the Standard Assessment Procedure for the Energy Rating of Dwellings (SAP), the Simplified Building Energy Model (SBEM) and Dynamic Simulation Modelling (DSM). The National Calculation Method was recently revised in line with the 2010 changes, and the relevant software tools should automatically produce the specification list required to comply with the final part of 20D.

The Approved Documents retain the familiar four-volume set introduced in 2006, separating new and existing work in commercial and residential developments. The key changes are as follows:

Approved Document L1A — Dwellings

For dwellings, the TER is still calculated using the Standard Assessment Procedure (SAP) in the same way as it was in Part L1A 2006. The SAP methodology is based on the BRE’s Domestic Energy Model (BREDEM), which provides a framework for calculating the energy consumption of dwellings. SAP works by assessing how much energy a dwelling will consume and how much carbon dioxide (CO₂) will be emitted in delivering a defined level of comfort and service provision, based on standardised occupancy conditions. This enables a like-for-like comparison of dwelling performance and the required 25% improvement to be calculated.

As a result, the elemental U-value targets in Table 1 overleaf have not been raised by 25% across the board. Instead, the majority of the 2010 changes take place within the black-box technology of the SAP software itself, so it is difficult to see clearly the effects of the changes. Nor will it always be possible to design down to the U-values given in Table 1 and still achieve a pass.

That said, early modelling by SAP assessor Greenhouse Calculations indicates that the following specification for a typical house of 128m² will still pass without the need for renewables:

• U-values: floor 0.13; walls 0.22; roof 0.13; windows 0.15; door 0.1

• Air-pressure testing: 3m³/hour/m² @ 50Pa

• Mechanical ventilation with heat recovery: 1W per litre per second and 85% efficiency

• Enhanced construction details

• 100% low-energy lighting

One of the reasons why U-values appear to have changed by less than 25%  is that building fabric has pretty much reached the limit of economic return. A study by Zero Carbon Hub entitled Defining a Fabric Energy Efficiency Standard produced in November 2009 showed that increasing wall thickness from 320mm to 385mm reduced the U-value by 0.07W/m²K, yet increasing from 385mm to 425mm only reduced the U-value by 0.03W/m²K.

What is significant, however, is that heat loss through party walls is now considered for the first time, with a limiting U-value of 0.2W/m²K. An unfilled and unsealed cavity wall would achieve 0.5W/m²K, and therefore fail, so it is clear that insulation to party walls must be specified (see Table 2).

Pressure testing

The increased requirements for air-pressure testing will concern many builders, with the testing requirement almost doubling. This means Building Control will now expect to see tests from up to three units of each dwelling type, or half the instances of that type (whichever is smaller).

In addition, a “confidence factor” must now be applied to dwellings that are not pressure tested. Where the contractor decides not to test a dwelling, a confidence factor of 2m³/h/m² @ 50 Pa is applied, so an untested dwelling must be designed to 8m³/h/m² to get the same rating as a tested one designed to 10m³/h/m². Where a value of 15m³/h/m² is adopted there is no requirement to test, however, it is very difficult to get a building designed at this rating through SAP in the first place.

The use of a design value as low as 8m³/h/m², however, in turn risks the possibility of “accidental brilliance”, where a contractor unintentionally seals the building tighter than 5m³/hr/m² @ 50 Pa, resulting in additional requirements for ventilation kicking in under both Parts F and L.

Approved Document L2A — Non-dwellings

Part L2A is also based upon a 25% improvement over the 2006 standards, yet the limiting U-values for commercial buildings are unchanged (Table 3).

This is because the way in which the 25% improvement is being implemented is completely different to the approach in L1A. Here, the National Calculation Method (SBEM) doesn’t simply apply a 25% improvement to the previous model, but recasts the notional building.

So the TER is no longer based on a 2002 notional building and an improvement factor. For 2010 it is based on a building of the same size and shape as the actual building, constructed to a concurrent specification. Developers are still given the freedom to vary the specification, if the same overall level of CO₂ emissions is achieved or bettered.

This approach to target setting has been adopted because the level of improvement that can reasonably be expected varies significantly from sector to sector, so a blanket improvement factor would be inequitable. The specification instead delivers an overall 25% reduction in CO₂ emissions across the entire new-build non-dwellings sector (the so-called “aggregate approach”). As a result, some building types will be required to improve by more than 25%, some by less (Table 4).

This will put more pressure on certain projects, particularly those with shallow plan mechanical air-conditioning.

Thermal bridging

Another issue for small contractors is that guidance on avoiding thermal bridging at construction joints has been revised. Approved Document L2A states that the design must be calculated in accordance with BR 497 (Thermal Performance of Buildings). Perhaps more important, developers must demonstrate how specific details will perform when assessed to BRE IP 1/06 (Assessing the Effects of Thermal Bridging at Junctions and Around Openings).

In particular, builders will need to show they have factory-tested accreditation for the detail and that they operate quality-assured site inspection systems. If either of these criteria is not met, a 25% penalty will be applied to the design assumptions in the building energy rating (BER). If there is no accredited detail at all (likely to be the norm) a 50% penalty must be applied. The small contractor who doesn’t have an ISO quality assurance system will be operating at a significant disadvantage.

Other changes

Guidance on shell and core development has also been revised, and it will now be necessary to specify at the design stage how the shell and services will meet the requirements. It will not be possible to simply design the shell to base levels and leave it to the fit-out contractor to install efficient services to compensate.

On practical completion, it will be necessary to provide as-built calculations with the fit-out areas conditioned to the previously assumed temperatures. A predicted energy performance certificate (EPC) rating is also required to show what level of performance is possible. However, there is no requirement for an EPC to be formally lodged at that stage.

A revised procedure is now in place to limit the effects of solar gain, whether or not a building has air conditioning at the time of design. Even where a design for a naturally ventilated building achieves Part L approval, this does not automatically mean that internal environmental temperatures will be satisfactory as they can be affected by the user.

The 2006 requirement for a building log book was not regularly enforced, but under 2010 Building Control departments will be expected to demand to see that these are provided before they can issue a completion certificate.

Approved Documents L1B and L2B: refurbishment

Changes here are relatively simple and mostly concern alterations to minimum U-values to achieve the 25% improvement. The relaxation for listed buildings under Regulation 9 has been further clarified and now states that the exemption only occurs “where compliance would unacceptably alter the character or appearance of the building and it is: a listed building; in a conservation area; a scheduled monument”.

Even when exemption applies the aim should be to improve energy efficiency as far as practical.

Finally, there is also a change in the definition of renovation, which is now defined as 50% of the element or 25% of the total envelope in context. For example, when removing plaster from a room, then the element is the area of external wall to the room; when removing external render, it’s the area of that elevation. As a result, it will be necessary to consider upgrading the insulation or heating/cooling systems on almost any building project.

Geoff Wilkinson is managing director of Wilkinson Construction Consultants

geoff@thebuildinginspector.org