Will MacDonald of Aecom examines the challenges posed by the 2014 revisions to Part L of the Building Regulations, which form part of the government’s strategy to cut carbon emissions from buildings
01 / Introduction
Part L of the Building Regulations concerns the conservation of fuel and power in both new and existing buildings, covering both domestic and non-domestic structures.
As government policy and international treaties and regulations have increased demands to use less energy - and use it more efficiently - so the Building Regulations have developed to encourage reduced use of fuel in buildings.
These regulations can both pose challenges for builders in themselves, and also raise knock on issues that require consideration.
02 / Changes in Part L
Since the introduction of Part L in 2006, the regulation has changed to reflect the tighter standards and how those are based on the government’s Construction 2025 strategy.
The strategy seeks to develop an industry that leads the world in research and innovation, and which will become more sustainable through delivering low carbon assets more quickly and at a lower cost, underpinned by strong, integrated supply chains and productive long-term relationships.
It thus lays stress on wider environmental considerations in construction and its implementation will not only change the Building Regulations and the way we build, but also how we approach the whole construction process.
Achieving the strategy’s vision will depend on improving clients’ capabilities and procurement, creating a construction industry that is instinctively “low carbon” and consistently taking an approach that can be summarised as “lean, clean and green”.
The latest revised Part L was released in April 2014 and replaces the 2010 edition. Among the main changes are that Part L specifications have been strengthened to deliver to 6% additional carbon dioxide savings across the new home build mix relative to Part L 2010. For non-domestic buildings this figure is set at 9%.
The government’s impact assessment on the Part L changes stated: “Reducing carbon emissions from the built environment is essential for the UK to meet its Climate Change Act targets. Appropriately designed performance-based Building Regulations can help to achieve this if the market would not make these changes of its own accord.
“Action at the point of build can lock in efficient design, reducing energy/heat demand and future retrofit costs. Regulation also has a role in setting energy efficiency standards where owners decide to carry out building work to existing properties.”
The assessment conceded that the revised Part L presented the industry with “demanding targets to reduce carbon emissions and to help reduce energy costs”, but pointed out that these would help to achieve “cost effective abatement in the construction of new buildings and stimulate fabric focused learning and innovation as the basis for more demanding future policies”.
The industry has perhaps been warned that it must get to grips with this change to Part L, as future ones will seek even more stringent conservation of energy.
The good news is that the requirements are not too onerous to comply with, given an understanding of the Code for Sustainable Homes Level 4 demands, though “glass boxes” are difficult to design under the revised regulations.
03 / Part L in domestic buildings
The key changes to the 2013 Part L1, from those of 2010, are due to a new regulation (26A) that has been introduced.
The following requirements now need to be complied with to achieve the necessary 6% carbon saving.
- New dwellings must achieve a better fabric energy efficiency (FEE) target in addition to the carbon dioxide reduction target. This is done by defining the dwelling FEE against the target FEE defined by the fabric’s thermal performance and the air tightness of the dwelling.
- The notional dwelling used in the Standard Assessment Procedure (SAP) calculation to determine the CO2 FEE target is the same size and shape as the actual dwelling constructed. The Part L 2013 target emission rate has been strengthened to deliver a 6% CO2 saving across all new homes built to part L 2010.
- If the actual dwelling is constructed entirely to the notional dwelling specification it will meet the CO2 and FEE specifications. However, the developers are free to vary the specific values to provide the same overall levels of CO2 and FEE.
The five steps for compliance with Part L may be summarised as:
- Achieve the target emission rate (TER) and target fabric energy efficiency (TFEE). The emission rate must be lower than the target rate and the dwelling’s fabric efficiency.
- Limited design flexibility. Building Regulations say reasonable provision may be made to limit heat gain and losses though the fabrics of the building. One way of ensuring this is that the fabric elements and the fixed building services all meet the minimum energy efficiency standards specified.
- Limiting the effect of heat gain in summer and from other sources, as it is now not merely solar gain that must be considered. Other considerations for instance might include the fact that in apartment blocks poorly insulated pipes can contribute to overheating.
- Ensure good quality of construction and commissioning. Is the work well planned and executed?
- Provide information to users on the energy operations of the dwellings, for example including explanations of essential design principles and features, floor plans, explanations of how to operate, control and maintain building services and signpost other key information including manuals and EPC reports.
Updated SAP Calculations
The SAP calculations (SAP 2012) used for CO2 emissions have also been updated to include the following key changes:
- Updated fuel prices, primary energy and CO2 factors.
- Use of regional weather data for wind speed and solar radiation.
- Updated calculation for solar radiation.
- Controls for heating and hot water systems with boilers.
- Options for heat loss through party walls.
- Insulation of primary pipe work and others.
- Default seasonal performance of heat pumps.
Floors and facades
Another key change is that the area used is now calculated using floor area and not the facade area. This could make it difficult for certain developments with full height floor-to-ceiling glazing to achieve their target values.
For example, if a dwelling has a floor plate of 100m2 (say 10x10m2) there can only be a maximum of 25m2 of glazing on the facades at the FEE u-value (1.4W/m2K). If this amount of glazing is exceeded then the u-value would have to be improved and would require triple glazing. In the past the area could be closer to a 30m2 glazed area, based on a three-metre floor-to-floor height.
In the consulting period prior to the release of Part L, the government relaxed the FEE levels by 15%, mainly due to the fact that some developments, such as those that use lightweight wall construction, would not be able to achieve the required values.
For multiple dwelling developments it is still permissible to average out the target emissions rates over the entire facade.
04 / Part L2 in non-domestic buildings
The notional building used in the national calculation modelling (NCM) to determine carbon dioxide target must be the same size and shape as the actual building, if constructed to a concurrent specification.
If the actual building is constructed entirely to the notional building specification, it will meet the carbon dioxide targets.
Part L 2013 has been strengthened to deliver a 9% carbon saving for non-domestic buildings. The developer is allowed to change the various specification values as long as the overall level of carbon dioxide emissions is achieved.
The most important differences to the changes to the NCM notional modules are:
- U-values are improved in comparison to 2010
- Air permeability varies depending on the size of the building, so a building smaller than 250m2 floor area can have an air permeability of 5m3/m2/hour with larger buildings having an air permeability of 3m3/m2/hour
- Boiler heating efficiencies have been improved
- More ventilation controls have been included.
This table summarises the requirements:
The five steps to achieving compliance with Part L for non-domestic buildings are similar to those given above for domestic buildings, but the building emission rate must be less than target emission rate and, while some design flexibility is allowed, reasonable provision must be made to limit heat gains and losses though the fabric.
05 / Possible improvements needed for Part L
There are potential conflicts that could be identified between Part L and other legislation.
One such example is the utilisation of solar gains to heat north-facing dwellings. High-rise residential blocks may have some apartments with a single aspect and to meet the fuel efficiencies target one must allow higher solar gain.
Changing the g-values of the glazing will help achieve this, however in the summer months the dwelling will therefore most likely overheat. This means external shading devices will need to be included in the design.
Software used in calculations of thermal bridging may offer different results. There is a noted paradox that using the more sophisticated software is often a disadvantage, as the results it gives may be worse in terms of achieving compliance.
More complex software using 3D geometry to calculate the result is more accurate, though the regulations require only a 2D calculation.
Sliding doors used on many developments present a difficulty in meeting the required U-values. Due to the construction method used on sliding doors a larger profiled section is required to withstand the loads (of which wind load is the largest).
Sliding doors work on a rail design, which means carriages and rails are part of the system, resulting in triple glazing being specified to improve the average calculations for compliance.
06 / Working with Part L
To meet all the requirements of Part L, a greater emphasis is placed on the design team working more closely together on all elements of each project.
One approach is to have a fully co-ordinated design team, including all in-house engineers, M&E consultants, facade engineers, cost consultants and sustainability engineers.
In the past, the normal practice has been for M&E engineers to define the values of the envelope fabrics and for everyone else involved to have to align their specifications.
In the early design stages on projects, the reference data is typically used to make the Part L model work (normally based on previous projects). As the project develops these values become fixed, resulting possibly in expensive design solutions.
There should be early input from the specialists to get a coordinated design. As an example, a new dwelling may have to use triple glazing for the windows to achieve Part L compliance if the wall construction cannot accommodate the required thickness of insulation.
This decision can have an important impact on the procurement strategy and budget. For example, a developer wanted the entire project to be manufactured off-site to shorten the construction programme. However, this could not be done due to the specified performance requirements, particularly for the insulation.
Another issue is that certain developer procurement policies will not allow some insulation products to be used.
High-end developers may have proscribed lists of products they will not allow due to fire standards or sustainability issues.
Use of balconies
On multi-storey residential developments schemes that use protruding balconies, the design of the floor slabs is critical and has to be defined at an early stage. The linear thermal transmittance value used at the interface detail between the warm and cold areas can have a major impact on the construction method.
Thermal breaks must be built into the floor slabs or else floor-to-ceiling heights must be reduced to accommodate the additional thickness of insulation.
To achieve the correct balance between the glazing g-values and light transmission values, the type of glazing specification to be used must be carefully considered.
This is made more complex when the acoustic specifications are taken into consideration. The thicker glass required would have an effect on both the g-value and the light transmission value.
As daylight is not a legal requirement, this is often the amenity that has to be reduced. The high performance coatings on glass have been improved with triple silver layers being used on most large projects.
The glazing framing systems used to achieve the higher U-value often mean larger sections profiles are required, which in turn will change the ratio of framing to glass.
Early design use of the SAP calculation can avoid problems with reduced daylight and poor aesthetics caused when smaller windows are used.
07 / Part L and the market
Costs have increased under the revised Part L, as interfaces have to be detailed to show how the thermal bridging works. This generally includes additional material that drives costs up.
The main effect so far is an increase in demand for triple glazing on the glazing elements.
Overall, the requirements of Part L lean towards recognition in the industry that co-ordinated design involving the whole team is essential to meet the revised and amended requirements.
The following are the key cost elements to consider when working with Part L:
- Air tightness
- Enhanced glazing
- Improved insulation
- Accredited details
- Thermal bridge calculation
08 / Part L cost model
The following cost models have been based on the unitised facade of a typical office building in London.
The total area of the systems is approximately 8,000m2, with a vision-solid ratio around 75-25%.
All the curtain wall types panels have a typical size of 1,500mm wide by 4,000mm storey height.
The cost models show different configurations for glass and spandrels, but they all share a common concept design (thicker aluminium mullions and transoms profiles are assumed for the triple glazed options).
External Wall System (EWS)-1 is the major wall type, and it has a PPC finished aluminium profile frame with an intermediate transom, a partially fritted vision area 1,500mm wide and 3,000mm high, and a PPC aluminium spandrel 1,500mm wide by 1,000mm high.
EWS-2 represents the solid area of the building, with a 100% PPC aluminium panel to the outer side and an expected U-value of 0.6W/m2K for the base solution.
EWS-3 is similar to EWS-1, but with a full height vision area and a bulk head panel to the spandrel area.
The rates in the cost models are only indicative and have been prepared to give an estimate of the cost impact of the various steps to improve the thermal performances of the curtain wall. Expected indicative U-values are indicated.