Shopping is now the biggest leisure activity in the UK, but the introduction of the revised Part L of the Building Regulations means the design of retail complexes has radically altered. We discuss the changes and what it will cost.
The new Part L of the Building Regulations focuses attention on overall savings in carbon emissions rather than simply energy efficiency and apply to all new and refurbished buildings. They are relevant to where material alterations or material changes of use are undertaken, typically a refurbishment which may affect structural stability, fire safety, or access for the disabled or simply involve replacing ceilings or wall linings. The definition of building work is further expanded by including fittings and services addressed by Part L as 'controlled services and fittings"– glazing is now a controlled service.

The main changes to the document and the resulting impacts on construction are:

  • Lower U-values for all elements – requiring more thermal insulation.
  • New restrictions on solar heat gains – greater requirement for shading or solar control glass.
  • Tighter and broader reaching efficiency requirements on services – new controls on energy efficiency via approved calculation methods and required to be submitted for approval of carbon emissions.
  • Improved building construction in terms of air leakage and cold bridging – better sealing of facades and pressure testing in certain cases.
  • New requirements to improve energy management via log books, commissioning data, and sub-metering.

Three different methods are provided within Part L2 to demonstrate compliance: the elemental method, the whole building method and the carbon emissions method. Although each method is different, they all aim to achieve the same reductions in carbon emissions.

The starting point will usually be the elemental method, but difficulties with its inherent lack of design flexibility may quickly force progression onto the whole building method or the carbon emissions method. But with increasing flexibility comes increasing sophistication of the calculations involved.

Complying with the regulations
The elemental method is a prescriptive and therefore relatively inflexible method, but the process will be appropriate on straightforward projects. If the item in question does not comply then, in theory, an overall pass cannot be achieved. However, there is some scope within this method for trade-offs between the more and less effective facade elements, and highly efficient heat generating plant which can be used to reduce fabric insulation requirements.

The whole building method simply considers the services loads resulting from a building design, rather than considering all elements of the facade. If the facade and services are efficient overall, then the emissions will also be within the required limits. Currently, the method is not seen as an acceptable route by the Building Control department for retail buildings. This is because benchmark energy/carbon emission data is not currently available. This is unfortunate, as this method offers a relatively quick calculation process, which would probably be the easiest to comply with, without having to undertake an extensive analysis. This method would also provide more scope for flexibility in design than the elemental method. Expansion of the list of accepted benchmark documents may eventually allow the use of this method on mixed-use projects.

The carbon emissions method again looks at the whole building, but can be applied to any building. Although this can be calculated manually, in reality, it will normally be preferable to employ an 'approved' dynamic computer based 3D thermal model. The method analyses the building performance and thus carbon emissions for a typical weather year. The resulting total carbon emission is then compared with a benchmark emission rate. A recognised benchmark performance does not yet exist for retail schemes, thus, for this method, it will be necessary to calculate a benchmark 'notional building' performance. This is done by first undertaking an assessment of the same building form, but with a scheme which is fully compliant with the elemental method or which adopts certain reference values. In effect, this means that for buildings other than offices, schools or hospitals, this method will involve doing two analyses.

However, the carbon emissions method is worth considering for larger schemes; it provides greater power to value engineer the optimum level of insulation, glazing specifications, comfort and provisions for occupiers, and m&e systems design, than the elemental method.

The mixed use challenge
Mixed-use/retail is a sector of the construction industry with a varied reputation for energy conservation. The new regulations will certainly have an effect on current practice in terms of design, construction and operation of these buildings. Large retail schemes, in particular, are complex, and Part L2 does not make specific provisions for any of the typical configurations.

Many aspects of retail design are in direct conflict with the principal intentions of Part L2:

  • They have high air leakage rates, and they have numerous corridors and services penetrations, which makes sealing of the overall facade hard to achieve.
  • The design team has to try to pre-empt how shell and core areas will be fitted-out in order to achieve a pass for the overall building.
  • They have a very high proportion of display lighting – where low efficiency lighting is used.
  • They have large areas of (usually single) glazed rooflights/entrances.

In order to establish a reasonable approach to this type of scheme, a detailed impact assessment and close liaison with the local Building Control team is imperative at an early stage.

Due to the widening requirements of Part L2, which now encompasses the direct regulation of building services performance, it is increasingly important for an integrated design to be developed between the architect and services engineer at an early stage to maintain design flexibility – particularly relevant to retail projects.

Overall, Part L2 is only guidance, and provided broad equivalence in carbon emissions can be reasonably demonstrated by the designers and contractors, this should satisfy the legal requirements.

The building types looked at here been picked in order to cover the majority of retail schemes in England and Wales. The equivalent regulations in Scotland are contained within the new Part J, which came into force in March this year. This document has some significant differences from Part L2 – most notably: no limits on solar heat gains, and tighter U-values. But these are not considered further here.

This summary in table 1 has been prepared on the basis of a developer providing shell retail, with temporary shopfronts and main services supplies. The exception to this would be for department stores, where the landlord would provide the permanent feature shopfront. The remainder of the works will be by the tenant and hence many of the requirements of Part L2 will apply to them. However, there is now likely to be a greater onus on landlords to monitor the activities of shop fitters: with increased provision of metering and sub-metering and further guidance also required in tenant handbooks, which may now incorporate the new requirement for 'energy performance log books'.

The costs set out in table 2 compared the differences between a fully compliant 1995 Part L building and a fully compliant 2002 Part L2 scheme. They represent the typical provision by a client as developer ie shell retail (albeit with shop fronts provided for department stores and fully fitted malls).

The regulations in practice
Plotting a path through the elemental method is not easy for retail schemes. And so the carbon emissions method could prove to be the most flexible tool in demonstrating compliance, particularly in mixed-use developments. This method also offers the mechanism to value engineer the overall scheme. The main areas where this is likely to be useful is: the optimisation of insulation thickness; glazing performance, in terms of solar shading versus daylighting, and the benefits to be gained from energy saving features in the services design, such as daylight switching of lights or heat recovery from extract into supply air.

It will very much be down to the skills of the design team to demonstrate the overall goal of reduced carbon emissions by using relatively complex calculation techniques. Moreover, a thorough understanding of Part L2 and the principles behind it will be essential to minimise the impact on design and cost.

Fenestration design will be of increasing significance in achieving Building Regulations approval – with the functional needs of customers to view goods often in direct conflict with solar shading requirements. Malls also present challenges in terms of minimising glazing costs, as well as retaining good daylight levels; while controlling solar gains.

Air leakage could prove a difficult area to solve until general design and construction practice is improved. The pressure testing process could prove costly and will be disruptive to the closing stages of the construction programme. More attention and adequate time periods will also be needed for commissioning and testing the building services systems.

Changes in lamp efficiency targets will mainly affect smaller retail lighting schemes – larger schemes are more likely to already be employing high efficiency systems.

Overall construction costs are likely to increase in the order of 1·5-3% for retail/department store shell and shopping malls.

There is a real demand for a benchmark energy usage document for the retail sector. This would allow the much simpler whole building method to be used and would also reduce the analysis required to carry out a carbon emissions method calculation.

Meeting the provisions of Part L will better enable tenants to comply with the prospective EU directive on energy performance of buildings.

Table 1: Summary for a retail shell unit with main services supplied

Maximum U-values of facade elements
Wall and roofs will generally require increases in insulation thickness or a change to higher thermal resistance insulation. Deep-plan floors are unlikely to be affected unless suspended construction (eg over unheated car parks). In terms of urban retail units and department stores there will be limited savings on heating plant loads. Loads on landlord areas such as malls will see some reductions in plant capacity. Highly glazed facades with poor U-values will force the use of the carbon emissions method to comply. Significant increase in thermal performance required on all external glazing except for display windows. Maximum area of all exposed walls as glass
For urban retail, even with a fully glazed shop front, the 40% limit on wall areas as glazing should not be a problem, except perhaps on shallow plan shops, or those units trading at both ends of the unit. Unlikely to be a major issue on most department stores or shopping malls. Limiting solar heat gains
To reduce solar gains to less than the limit of 25 W/m² of perimeter zone floor area will require increased shading provision on glazing exposed to direct sun, in the form of: shading canopies, opaque glazed or insulated panel sections, neutral heat absorbing solar control glass, ‘framing’ display windows down to the required glass area, overall reduction in glazed area, shading from surrounding buildings or landscaping. Heating system carbon emissions
Grid mains electric heating is effectively excluded from the elemental method. Likely options which exist for using electric power for heating include: dx cooling units replaced by reversible heat pumps. If electric heating is combined with heat recovery such as: air/air heat exchangers or packaged thermal wheel ahus, it may be possible to argue that, in effect, the carbon emissions would be significantly reduced below the target figure. Electric door heaters are likely to be difficult to use under the elemental method. Alternatives exist such as lthw fed heaters, but they are more expensive to install and are usually not as compact. Lighting efficiency
Lamp efficiencies will need to improve from current standards to achieve the target efficacies. In practice this will mean that in display areas low voltage dichroic downlighters (12 V with mains voltage transformers) may generally be changed for higher efficiency mains voltage discharge units – eg ceramic metal halides. Conventional incandescent lamps will virtually be excluded altogether. In non-display areas modern high efficiency lamps/luminaires providing background lighting should generally already conform. Thermal bridging and air leakage
Significant improvements required to seal external facades and limit thermal bridging. Any space over 1000 m² will need to be pressure tested to prove the leakage standard is met (10 m³/h per m² of facade at 50 Pa). Below this floor area and working to robust construction details should suffice. Open-fronted urban retail units, without doors, are now unlikely to be acceptable. Energy log books
An energy log book must be provided for each tenant for management of energy usage. It is likely that the basic document would be provided by the landlord (or his design/construction team). Metering of fuel and power supplies
For smaller tenancies, where meters are already provided for billing purposes, this should be sufficient. Tenancies above 2500 m² will now require additional provision for metering. The minimum would be to allow for additional sub-meters on chiller plant, main lighting circuits, any motor control centre above 10 kW input, any distribution board above 50 kW input. For a typical shopping centre this could mean 50 extra energy meters, logged at the main bems. Sub-metering of gas supplies to boiler plant is required if provided.

TABLE 2: Cost comparison for a 1995 Part L and a 2002 Part L2 compliant building

Urban retail shell
Typical two-storey retail unit trading from one end only, with a flat solid roof, primarily brick rear elevations and party walls to other units. It is envisaged that the changes would increase retail shell costs by 1·5-2% on the basis of additional insulation to the rear wall and roof and thermal bridging/air leakage requirements. In respect of other implications on solar heat gain, heating and lighting these will be down to the tenant as part of their shopfitting. Department store shell
Typical three-storey unit as anchor store, with highly glazed frontage to mall, part glazed side walls and large rooflight. It is envisaged that the changes would increase the department store shell costs by 2-3% on the basis of additional insulation to external walls and roof, upgrade of glazing to side walls, shopfront and rooflight to high performance units, thermal bridging/air leakage requirements and metering. Again, other implications on heating and lighting will be down to the tenant. Shopping mall
Two-storey conditioned environment with walkways overlooking void to ground floor, 30% glazing to roof and entrances to be full height glazed with canopies. It is envisaged that the changes would increase shopping mall costs by 2-3% on the basis of additional insulation to solid external walls and roof, upgrade of glazing to entrances and roof to high performance thermal units, thermal bridging/air leakage requirements and metering.

Source

Building Sustainable Design

Postscript

Glen Irwin is engineering development manager with Roberts & Partners. Tel: 0121 214 6500 or e-mail: girwin@robertsandpartners.com
Paul Wilcock is a partner with Davis Langdon Everest. Tel: 020 7497 9000 or e-mail: paul.wilcock@DavisLangdon-uk.com
Keith Carter is an associate with Mott Green and Wall. Tel 0207 836 0836, e-mail: keith.carter@mottgreenwall.co.uk