Cost model: Part L

01 / Introduction

Part L 2010 is the latest stage on the road to zero carbon construction that was started by the European Performance of Buildings Directive (EPBD) in 2006. Subsequent initiatives, including energy performance certificates (EPCs), the carbon reduction commitment and the feed-in tariff for renewables adopt a carrot-and-stick approach to low carbon design. In particular, EPC targets have been adopted by many developers and owners as an additional benchmark to drive low energy design.

Part L 2006 proved easier to comply with than had been expected, partly because manufacturers responded by developing more energy-efficient products. With designers, contractors and suppliers facing a further 25% cut in aggregate emissions, the contribution of technology will continue to be important. However, given the scale of the reduction, a wider range of steps will need to be taken to reduce emissions to acceptable levels. The cost models in this article describe some of the options for a commercial office building, and how performance in one element can be used to offset others.

Owing to the complexity of the change and the extended consultation period, significant elements of the Part L jigsaw are still in development, including accreditation schemes and commercial software.

The timing of the transition is also difficult, as it coincides with reduced funding in the public sector and a fragile recovery in the private. This means there has to be a focus on total efficiency, based on holistic design, and Part L 2010 should encourage that. Meeting the revised targets will require an even better understanding by designers of the interplay between elements of the design, and how constraints caused by one discipline can be mitigated or avoided by the work of others.

Clients with projects that have obtained planning consent, and which could be submitted for Building Regulations approval by October, have the option of proceeding on the basis of the existing Part L, as long as work starts on site by October 2011 - subject to negotiation with building control.

However, with the New Part L leading to the recalibration of EPCs and emissions calculations, clients that choose not to implement the new standard may end up with a prematurely obsolescent asset.

This article focuses on the requirements of Part L 2A, dealing with non-residential, new-build construction. Section 6 of the Scottish Building Regulations has been revised this year and also uses the simplified building energy model (SBEM).

02 / Main changes to the regulations

Most attention has been focused on changes to the Approved Documents, which set out the criteria by which the functional requirement of the Building Regulations is to be met.

The detail that governs the implementation of the regulations can be found in supporting guidance such as the national calculation methodology (NCM) modelling guide or the guide for non-domestic building services compliance.

In practice, the effect of the regulations on design and construction will become clearer as 2010-compliant assessment software becomes available and is used to assess design. The only 2010-compliant software currently available for calculating target emissions rates (TER) and building emissions rates (BER) is the simplified building energy model (SBEM). Sophisticated software that can deal with more complex buildings will not be available until September, meaning that for owners of buildings with more complex features such as atriums or moveable solar shading, assessment is more challenging than it should be.

The principal changes to be introduced in 2010 are as follows:

• Aggregate reductions in regulated carbon emissions The 25% cut for non-residential buildings is the most eye-catching change, and one that requires a change in calculation methodology.

The final impact assessment published in April 2010 included a table of indicative emissions reductions, showing that a hotel, for example, would have to make a 16% cut, compared with a shallow plan air-conditioned office, which would have to hit 40%. In practice, the actual reduction calculated will be determined by a number of interrelated factors including size, shape, building services and use; the effects of combining these elements may be difficult to predict.

The objective behind the aggregate approach is to avoid penalising the owners of projects such as hotels, where carbon reductions are more difficult and costly to deliver. Whether the approach is easy for designers to work within, and whether it delivers consistent results, will only become clear once the software is in widespread use.

The change to an aggregate approach has resulted in a change in calculation methodology. From now on, the TER will be calculated with reference to a 2010-compliant reference building, rather than the notional 2002 building currently in use. This will result in a learning curve for all involved. The design criteria used to determine the performance of the notional building have been tightened significantly - by 26% in the case of thermal transmittance through walls. However, the limiting fabric parameters for the actual building are the area-weighted average U-values used in Part L 2006, so designers have some degree of freedom in meeting the TER. Other standards have also increased, such as minimum efficiency standards for services plant; most of these requirements can readily be met by current equipment specifications.

Exploiting this freedom will require a more holistic approach to design than has been the case. Much of the focus will be on trade-offs between different systems and design disciplines, and some of the options are illustrated in the cost models.

Another effect of Part L 2010 is that EPCs issued after 1 October will be recalibrated to use the revised carbon emissions rates, which in the case of grid-supplied electricity is increased 23%. This will mean that when buildings are reassessed, they are likely to get a lower EPC rating. This could help to accelerate measures to improve the energy efficiency of existing buildings - or could accelerate the obsolescence of unimproved buildings.

• Limits of solar gain at the perimeter Part L 2010 has much stricter limits on solar gain, expressed by “reference cases” for side-lit and top-lit buildings. This is probably the greatest source of concern for designers, although it should not be assumed that the requirement means the end of highly glazed buildings.

Taking the Approved Document at face value, for a side-lit building, the permitted day-lighting is determined by an unshaded 1m high strip of glazing with a 10% framing factor and a reasonable solar energy transmittance. In practice, steps taken to ensure occupier comfort in 2006-compliant buildings are probably sufficient to meet this criterion, although control of solar gain on north facades now needs to be considered.

Viewed in isolation, requirements to limit solar gain will no doubt make it more challenging to deliver buildings that take advantage of daylighting. Intelligent facades with integrated building service controls may enable an effective balance of daylighting and solar gain control to be achieved. Otherwise, high levels of fixed solar shading or highly tinted glass could result in dark interiors, which paradoxically would require greater use of artificial lighting and produce higher carbon emissions.

• Shell-and-core compliance 

The regulation requires the demonstration that shell-and-core design will meet the TER in combination with a fit-out based on assumed occupancy and performance parameters. The construction assessment must also be used as the basis for the design of the fit-out - effectively establishing the emissions parameters available to subsequent occupiers, which in theory would discourage the back-loading of carbon reduction initiatives onto the fit-out works.

• Focus on construction and building performance The regulations extend the accredited details scheme into the non-residential sector. The aim here is to reduce heat loss by linear thermal transmission through frames and interfaces between different constructions.

Part L 2010 proposes a system of accredited details and/or approved calculation methods that demonstrate that expected targets for linear thermal transmittance will be met. Where compliance cannot be demonstrated at the design stage assessment then penalty increases in the value of linear thermal transmittance of up to 50% are applied, potentially making overall compliance more difficult than with accredited details.

Whether this change has a significant effect on design and procurement will depend on the importance of linear thermal transmittance as a component of overall carbon emissions. This will not be a problem for a warehouse, but for a complex design with lots of framing in the facade, opening lights and junctions between systems, it could be a material consideration.

It is unlikely that specialist facade contractors will publish their proprietary design details as part of any accredited detail scheme, which will result in penalty values being applied to transmittance calculations, and possibly increased building control fees. The irony is, of course, that these details are likely to perform well. Penalties will be higher for organisations that cannot demonstrate competence in an approved calculation process, which could create opportunities for specialist engineers in support of smaller specialist contractors.

Other measures to assure building performance include an increased standard for air pressure testing for infiltration (5m3/m2/sec), as well as enhanced requirements for building services testing and commissioning. The infiltration standard is not a problem for system-based facades, but will require good quality detailing of insitu construction and interfaces between systems, potentially encouraging a more holistic approach to procuring building envelopes.

• Design-stage and actual building assessment Building control submissions based on the NCM need to be made before construction starts and upon completion. The assessment of the completed building has to take into account design variations introduced during the construction stage. One interesting feature of the new regulations is an explicit focus on key features, that is, building components that make a greater contribution to the BER than might have been expected. The requirement for the design-stage submission introduces some interesting procurement considerations. Under previous iterations, designers would always take informal steps to ensure that designs were compliant. However, under the 2010 rules, in order to avoid penalties, it will be necessary to ensure that specialist contractors can comply with technical requirements to support assumptions made in the design-stage assessment. This is not necessarily difficult, but it does require that competence is accounted for in the selection process.

Taken in the round, the two main challenges for the project team are the absolute scale of carbon reduction, which will necessitate a holistic approach to design, and the learning curve associated with the revised NCM. In their favour, designers will continue to benefit from innovation by manufacturers, who, in common with sectors such as automotive, have responded effectively to the low carbon challenge. Air-conditioning manufacturers have led the charge since 2006, and lighting firms appear to be picking up the baton with LED-based luminaires. The performance of facades continues to improve, and building-integrated renewables such as PVs may provide opportunities to offset carbon emissions elsewhere in a scheme.

03 / Design challenges

The devil is very much in the detail of the supporting documentation of the 2010 regulations. How project teams collaborate to deliver buildings that meet reduced TERs without a disproportionate loss of performance or value will be an important measure of success and a pointer towards how even lower carbon design solutions will be developed.

The main issues that need to be considered include:

A tougher baseline 

Aspects of the baseline calculation that make the carbon emissions cut much harder to achieve include:

• No gain from design fundamentals. The location, orientation and massing of the design can have a critical effect on actual energy use and a building’s EPC rating, but as the notional building and actual building are the same shape and size, benefits are not accounted for in the calculation
• The increased carbon intensity of energy sources. As part of the change in the NCM, the baseline values of energy have increased by more than 20%; electricity for example has increased from 0.422kgC/kW to 0.517
• Changes in the fuel types are assumed in the notional model. Low carbon fuel sources such as biomass have been included as a notional fuel in the 2010 NCM, and therefore cannot be used to contribute to a reduction in the TER. This is intended to stop poorly performing buildings being able to comply through the use of low carbon technologies. But low carbon technologies support better EPC ratings, so there are still benefits from these technologies
• Substantial improvements in design criteria such as U-values will require efficient envelopes, additional reductions to services emissions or the adoption of a holistic approach to intelligent design
• Limiting factors on aspects of building services design, including ventilation and lighting will have an effect on other aspects of design - such as increased duct sizes.

Implications of the baseline 

Some of the baseline standards are difficult to deliver without affecting other aspects of the design. Project teams must be careful when assuming that assumed performance is deliverable, or relying on offsets from other aspects of the design.

• The notional building assumes a solid wall with 40% glazing. A comparable area-weighted U-value for such a construction is difficult to deliver using a framed curtain wall. This is likely to result in a greater take-up of solid, insulated substrates, the adoption of thermally efficient composite framing sections, or the use of intelligent facade solutions on higher value buildings.

Implications for efficiency 

Aspects of the limiting factors in the regulations could have a significant impact on the development efficiency and operational effectiveness of future buildings, and this will require an even greater consideration of set-off within a holistic design solution. Examples include:

• Specific fan power calculations. One of the limiting factors in the calculation is a reduction in specific fan power from 2.2W/l/s to 1.8W/l/s. This cut is typically achieved by increasing the cross-sectional area of air handling units and ducts. Bigger air handling units and ducts are more expensive in themselves and also need larger plant rooms and ducts, which reduce building efficiency. Given occupier expectations for increased occupation densities, requiring even greater air volumes, the affordability challenges that result from low carbon design are plain to see. The cost models in this article illustrate the cost implications of meeting a lower rating of 1.4W/l/s, which is necessary to compensate for other aspects of design
• Limits on solar gain. The measures on limiting the effects of solar gain are aimed at future-proofing buildings against climate change. There are a number of means of meeting the criteria that could have an impact on the quality of internal spaces, or energy consumption related to artificial lighting. In the cost models, variant one complies with 34% glazing, whereas iteration three, with 60% glazing, has a much darker tinted glass. Both will result in quite dark interiors.

Where budgets allow for the active control of solar gain through moving shading or automated blinds, then a better holistic solution, involving less reliance on artificial lighting, may be achievable, which should in turn result in buildings that are able to respond more effectively to year round conditions. However, these buildings have operational and maintenance issues associated with the upkeep of complex systems.

Unintended outcomes 

As total allowable carbon emissions are reduced, a holistic approach is needed to ensure a balance between sources of energy use and carbon emissions. Examples include:

• Low U-values and cooling loads.

As energy efficiency increases, the design drivers for cooled and heated buildings diverge. Whereas heated buildings benefit from improved U-values, air-conditioned buildings tend to find it harder to get rid of waste heat from lighting and occupants. Any solution to this conundrum will have to be implemented across all design disciplines. It also shows that the direct application of the Part L limiting factors to the design may not deliver an optimum outcome

• Lighting loads and ceiling layouts. Unlike building services systems such as chillers, which achieve high levels of energy efficiency, lighting is relatively inefficient and remains a major source of cooling load. Lighting efficiency is measured by power density for a given lighting level. One option to increase efficiency is to specify more efficient fittings, and it is likely that “accent” lighting will be phased out as a result. Allowances for controls also help to reduce the power density. A more fundamental approach would be to optimise the spacing of luminaires, which for many buildings is determined by the planning grid or a suspended ceiling layout rather than optimum illumination. This is an example of how fundamental principles of building design - standardisation and modularisation - could be re-evaluated to deliver low carbon design, but also illustrates the potential losses in buildability and efficiency that might result.

04 / Procurement implications

Part L 2010 will have a greater focus on ensuring that the design is delivered on the ground, and that the effect of variations is recorded and accounted for as part of the as-built assessment. In practice, the effect of these requirements will be determined by the ability of Building Control to enforce. Although these changes will not have a fundamental effect on procurement, there are some areas where consideration of current practice may yield some benefit.

• Building services design. Requirements for a more holistic approach to design will need earlier input from building services engineers to determine the optimum balance between fabric and system performance. This in turn will require the early fixing of building plans and layouts to facilitate thermal modelling.
• Performance specifications for building envelopes. As well as setting out physical performance requirements, procurement will need to ensure that contractors have the calculation competences and accredited details needed to secure the linear thermal transmittance credits assumed in design calculations.
• Specification risk. Designs that rely on a disproportionate contribution from one or two elements could be subject to greater scrutiny from building control and could in turn represent a greater risk to the contractor team charged with delivery - low-risk solutions are likely to be more attractive to bidders in the long run, as competitive conditions resume.
• Early involvement of specialist contractors. Advanced solutions necessary to meet the revised requirement such as an active facade will require the early engagement of specialist contractors to provide specialist expertise and to ensure co-ordination with aspects of the building services and fit out such as lighting control, BMS control and blinds.
• Change management. The requirement to assess all material variations for their impact on the as-built BER should in itself discourage variations, but will also increase the importance of impact assessment ahead of issue.

05 / Cost models

The cost model is based on a notional commercial seven-storey office building designed for a regional city location with a gross internal floor area of 144,000ft2. The building is efficient, the on-floor net to gross is 85%, and the wall to floor ratio is 0.36.

The cost of the base 2006 compliant variant, including category A fit-out but excluding external works is £1,775/m2 GFA.

Four specifications have been modelled and evaluated for compliance with Part L 2010 using iSBEM software.

The four demonstrate design solutions that permit an increasing proportion of glazing in the facade, including enhanced fabric insulation, high-performance glazing, reduced duct pressures, high-performance luminaires, chilled beams or moveable solar shading. Given that the 2010-compliant iSBEM software has only been available for a short period, the solutions are not necessarily optimised, but represent a range of options.

The initial results of this analysis suggest that premium costs related to Part L 2010 compliance can be kept within the predicted bands of 1-3% for deep plan offices, so long as passive means are relied on to manage heating and cooling loads, and the proportion of glazing is kept low.

These solutions will not necessarily deliver optimal buildings - the 60% glazed variant in the study has deeply tinted glazing that will restrict internal daylighting levels. Also, for the 60% and 70% variants, active chilled beams are used, a departure from the standard office product specification.

The analysis demonstrates that building design is likely to change in response to the new regulations and that systems that are taken for granted, such as fan-coil units, may be more difficult to specify in all circumstances. Option four includes features that are associated with active facade strategies, such as BMS-controlled solar shading, demonstrating that there are likely to be premium costs associated with adopting a holistic, high-performance solution. In this example, the bulk of the premium cost is in the moveable shading, controls and power supplies. Technology-based solutions such as these come with operations and maintenance implications that future occupiers need to buy into.

In buildings with a higher quality facade solution than that used in our exemplar, intelligent facades might also be worthy of consideration. The cost of a double-wall facade is typically £850-1,050/m2, including an allowance for BMS interfaces and power supplies for motorised blinds, a premium of about 25% on the cost of a typical unitised system with fixed solar shading.

One of the benefits of Part L 2010 is that more owners will need to specify plant and equipment that qualifies for Enhanced Capital Allowances (ECAs) and could benefit from increased tax relief. ECAs are available for high-efficiency plant and equipment such as boilers, chillers, motors and lighting, so long as they are on the ECA approved technology list. The 100% tax allowance can be recovered in the year of expenditure and in our cost model contributes to an increase in the net present value of tax allowances of 12% a year to about £1.23m, when calculated using a 6% discount rate. Other features such as the motorised solar shading qualify for capital allowances as “integral features” but the rate of recovery is slower, 10% a year on a reducing balance basis and as a result the value of the benefit is lower. The net present value of the allowances for option four, for example, is only 2% higher than option one, although the capital cost is 7.5% higher.

The costs in the model are at third quarter 2010 prices. The base building costs include category A fit-out, preliminaries and contingencies. Costs of demolitions, external works, tenant fit-out, professional fees and VAT are excluded.

Simon Rawlinson, Davis Langdon and Roy James, Arup