Cost model: City of London offices, Part L

National calculation methodology

The new calculation methodology involves inputting all aspects of the building design into a national calculation tool, called iSBEM (Simple Building Evaluation Method), which is being developed by BRE. This calculation will need to be done fairly early on in the design process and then updated once the design is fixed. This will make it more difficult for the design team to handle changes and, given the resource requirements of recalculation, this could limit design iteration. However, guidelines could be set that would allow some variations without recalculation. For example, if a maximum perimeter heat gain is agreed, the facade can be developed with a number of options that meet the criteria.

Online cost briefing

Readers can now search our unrivalled four-year archive of Davis Langdon & Everest’s cost models and specific, cost-based project studies, by clicking on the Cost Briefing section of our website, www.building.co.uk. Cost briefing itemises all the articles by subject and includes data correct up to the date of publication.

Overview of the revised Part L

Changes in approach

In addition to the introduction of challenging targets for reducing carbon emissions, the new Part L involves some significant changes to current practice:

• The introduction of alternative assessment methodologies for new-build and existing buildings.
• A move away from prescriptive technical guidance. The new Part L only sets out the required benchmarks. Users will need to refer to technical details taken from a wide range of sources to identify how to meet the new standards.
• The adoption of a single national calculation methodology. This change, driven by the EPBD, has led to the adoption of a carbon emissions method for buildings other than dwellings. As a result, the simple elemental method will no longer be available.
• The regulations include a provision to include the contribution of low or zero carbon (LZC) technologies, such as solar hot water heating, in the assessment.

  Requirements for compliance – new buildings other than dwellings

  The key features of the revised Part L are:

   

• Buildings should be designed and constructed to conserve fuels and power so as to minimise CO2 emissions related to the building’s operation.
• Buildings should be designed and constructed to minimise heat loss and heat gain through the building fabric and building services distribution.
• Buildings should be designed and constructed to avoid excessive solar gain and to reduce requirements for artificial cooling.
• It must be demonstrated that the requirements have been met though approved calculation methodologies, pressure tests and results of testing and commissioning.
• Appropriate operation and maintenance manuals are required so that no more carbon emissions than are reasonable occur in use.
• Energy performance certificates are required.

  The assessment method

  Demonstrating that a new building complies with the regulations requires a seven-stage process that is summarised below:

   

• Designing, in outline, a notional building to establish the carbon benchmark. This must comply with the 2002 regulations plus some additional standards and be identical to the proposed scheme, in terms of shape and size and occupancy. Standards for thermal performance, extent of glazing and building services system efficiency are set out in the regulations.
• Setting the target carbon emission level for the project. This is calculated in a two-stage process:

   

• Calculating benchmark carbon emissions for the notional building, using the standard methodology
• Setting the emissions target by applying percentage reductions to the benchmark. The reductions factor in improvements to building performance and use of LZC sources of energy. In aggregate for air-conditioned or mechanically ventilated buildings, a 28% saving in carbon emissions is required. For naturally ventilated buildings, the overall saving is set at 23.5%.
• Demonstrating that the proposed scheme does not exceed the target emissions level, again using standard calculation methodology.
• Demonstrating that areas of the building that do not have cooling installed will not overheat, or calculating and adding the carbon emissions related to providing the cooling as a retrofit.
• Demonstrating that aspects of design and construction meet minimum acceptable standards covering fabric and glazing U-values; air permeability, and installation, operation and control of building services systems.
• Demonstrating compliance with the regulations through pressure testing and effective commissioning of the building.
• Demonstrating that provisions for energy management, such as metering and enhanced controls, are in place.

   

Impact of the regulations on design

The revised Part L will affect all aspects of the building design. It allows flexibility in design and there are no prescriptive requirements, only some minimum standards. So, in addition to insulation levels, proportion and type of glazing, other factors that may be improved are:

• shading, such as external shading, double wall facades
• boiler efficiencies and chiller co-efficients of performance
• building envelope airtightness
• lighting efficiency and control
• fan and pump efficiencies, air and water pressure drops
• heat recovery.

  Energy recovery is likely to be needed for all buildings with mechanical ventilation. Efficient heat recovery will be particularly important, favouring thermal wheels rather than run-around coils. For buildings with air-conditioning, one of the largest factors in energy use is the perimeter heat gain. This depends on the amount and type of glazing and the effectiveness of the shading.

  A key characteristic of the new regulations is the flexibility offered to designers. Although the carbon emissions target has been cut by some 28% for cooled or naturally ventilated buildings, and some of the minimum acceptable standards have improved, it will continue to be possible to specify, for example, highly glazed buildings – albeit that significant improvement in other aspects of building performance will be necessary. Luminaire and lamp efficiency is an example of an area where substantial reductions in both energy consumption and cooling loads can be made at a relatively low cost.

Part L for existing buildings

A major feature of the revised regulations is the development of provisions that will widen the range of existing buildings and the work within them, to which the regulations apply. The aspiration of the regulations is to encourage thermal improvements in projects beyond the original scope of work. Given the size of the existing building stock relative to new-build output, this is an essential step in tackling overall carbon emissions.

However, to achieve this objective, it has been necessary to adopt a flexible approach aimed at introducing incremental improvements as widely as possible, with their scope being determined on the basis of cost-effectiveness. Changes to the scope of qualifying works include bringing alteration and renovation works within the remit of the regulations, creating a new classification of “controlled elements” so that more work to walls, roofs and floors is captured, together with a broadening of the definition of material change of use.

Initially, buildings with a floor area of more than 1000 m2 will fall under the scope of the regulations. Smaller schemes may also be required to comply with the standards under the provisions of the Sustainable and Secure Buildings Act 2004.

In assessing the scope of work required, the carbon emissions approach can be used to demonstrate that the modified building meets its target. However, in many instances, detailed guidance on what constitutes best practice on an element-by-element basis will be used to demonstrate compliance. In the absence of an emissions assessment, tests of reasonableness based on technical risk, practical constraints and cost-effectiveness based on a seven-year project period will be used to determine an appropriate scope of work.

Assessing improvement works

The assessment of the appropriate package of improvement works is likely to become a balancing act between ease of implementation, effectiveness in carbon reduction and capital cost. The energy survey – carried out as part of the project to identify and assess opportunities – will be a key aspect of this process.

Isolating the costs of energy improvement initiatives and calculating marginal costs for cost-effectiveness assessments at an early stage of design is likely to be challenging and will be open to interpretation. On the other hand, the risk of cost overrun associated with energy improvement measures will rest with the developer.At this stage, it is difficult to predict the impact of the provisions of the regulations on the alterations and refurbishment market. The proposed 10% cost ceiling on improvement measures may prove to be a disincentive for further investment in existing fabric – potentially affecting the timing and viability of schemes and accelerating the obsolescence of some buildings. There is a substantial learning curve for both designers and inspectors in determining an effective balance between project feasibility and emissions control, which may lead to the adoption of less-than-perfect solutions during the early phases of the implementation of Part L.

Future trends in lower carbon emission design

The indications set out in the proposed document on future thinking are that the following changes will be introduced at the next review in 2010:

• The emissions benchmark will fall to a further 25% below the 2005 standard
• Solar control will need to be more effective, to allow natural ventilation as well as reducing cooling loads
• The renewable energy benchmark will be 20-30% of total carbon emissions.

  In design, low-carbon features that will become more common are:

   

• Effective, adjustable external shading (motorised or manual), possibly integrated into a double-wall facade solution
• More extensive use of narrow-plan spaces – for example, with a maximum centre to glass distance of 7.5 m. This will allow for optimal use of daylight with effective lighting and shading control, enabling lights off, blinds up operation when required
• Use of exposed thermal mass and night-time ventilation for passive cooling, to allow design solutions featuring natural ventilation or mechanical ventilation with heat recovery only
• Combined heat and power systems for mixed-use developments or small communities, including with bio-fuel such as wood burning
• Use of heat pumps for heating and cooling, particularly in areas with no gas supply
• Greater use of solar thermal collectors and other building-integrated renewable technology.

   

Cost model

This cost model is an evaluation of the extra over-cost of achieving 2005 Part L compliance based on different combinations of facade solution and building services installations.

The cost model is based on a generic air-conditioned office scheme in London. The gross internal floor area is 9300 m2, net-to-gross efficiency on office floors is 85% and the wall-to-floor ratio is 0.35. The notional building has 40% glazing in clear double-glazed units with a low-e coating. Solid insulated panels are located behind back painted glass units. All other design criteria and standards are taken from appendix B of the regulations.

Four options show how the 2005 regulations can be met using either 40% glazing or different combinations of 100% glazing and solar shading. All options are constructed from unitised curtain wall, featuring a combination of double glazing and solid panel. The double wall facade is an externally ventilated narrow-cavity unit. The key performance criteria table sets out the main differences between the options.

The results of the model show that for buildings with a low proportion of glazing, improvements to the glass specification, services plant efficiency and the introduction of simple heat recovery measures will actually result in a small reduction in capital costs resulting from the reduction in heating and cooling loads and plant sizing.For options with 100% glazing, solar shading to the affected facades and more efficient cooling, based on a water cooled chiller, are the key measures to reduce emissions. The combination of high-performance glass and fixed solar shading provides the most cost effective solution. However, in this option, natural light on office floors may be limited as a result of the heavy shading. The most expensive option uses motorised external louvres, a solution that is not used widely in the UK. Option four, the double wall facade – in this case a narrow profile system without access walkways – emerges as a cost-effective, practical solution, occupying a relatively narrow cladding zone and having buildability benefits related to unitised construction. In use, the cleaning and maintenance of both glazing and blinds is easier with the double wall facade, albeit that regular access for cleaning from lettable space has to be provided, which may have some impact on the fit-out and use of office space.

The costs in the model are at third quarter 2005 prices based on a central London location. The base building costs include the category A fit-out, preliminaries and contingencies. Costs of demolitions, external works, tenant fit-out, professional fees and VAT are excluded.