Cost model: Part L for residential

01 / Introduction

Part L 2010 marks the latest stepping stone in the housebuilding industry’s journey to the zero-carbon home. In the private sector, it has been able to comply with Part L 2006 through incremental improvements to fabric insulation and airtightness. Most of the improvements that will be necessary to meet the revised Part L 1A 2010 are already being specified by registered social landlords (RSLs) as they deliver houses that comply with level three of the Code for Sustainable Homes, as required by the Homes and Communities Agency. Applications for the next round of grant funding allocation are expected to require develpments to meet code level four, requiring a 44% reduction in emissions.

Some commentators argue that it is better to invest in improvements to the basic building fabric, so that long-term, low-carbon performance is assured, no matter how the building is used by its occupiers. With new measures being introduced in Part L 2010 to assure actual, as opposed to planned, performance, this approach will be based on much better assurance of build quality. However, changes to assumptions concerning the performance of party walls introduce some interesting design challenges related to achieving high levels of thermal and acoustic insulation.

The introduction of Part L 2010 has been timetabled since the introduction of the Energy Performance of Buildings Directive in 2006, but comes at a particularly difficult time for the housebuilding industry. Levels of production are low, and many developers are burdened with large, underused landbanks. Low levels of activity in house sales and land acquisition mean that developers do not have the opportunity to recover costs either through increased house prices or lower land costs. As a result, the cost-effective implementation of the new Part L will be essential to maintain affordability and to enable developers to retain some margin.

There has also been a consultation this year on revisions to the Code for Sustainable Homes as the 2016 zero-carbon target date approaches. The main changes proposed relate to aligning the code with zero-carbon policy and definitions, as well as bringing together other guidance affecting lifetime homes standards, site waste management and so on.

This article is a companion to the assessment of Part L 2A published (30 July, page 66). We assess the financial impact of options for complying with the regulations for a typical mid-terrace house and for a high-quality, mid-floor, city-centre apartment.

02 / Overview of the changes

The changes introduced in Part L 1A 2010 are relatively simple compared with those found in the non-residential variant. The five criteria that define energy-efficiency and building performance remain unchanged, although they are recalibrated to meet improved standards.

A simple overall carbon reduction target has been adopted for new dwellings, as opposed to the aggregate reduction methodology used in Part L 2A.

The assessment methodology, which is based on SAP 2009, is more sensitive than before. For example, monthly heating calculations are employed. The detail enables the modelling of summer cooling loads as well as the more accurate modelling of highly energy-efficient buildings that require very little heating, such as Passivhaus schemes.

The other big change in approach concerns a greater focus on assuring as-built performance, as opposed to modelling expected emissions. A submission to building control is required before construction commences, and there is more extensive air-pressure testing on completion.

Outside of the requirements of the regulations, SAP 2009 has undergone methodological changes. In practical terms, the most substantial is that all assessments are likely to be completed using SAP-compliant software rather than manual worksheets. As described above, calculations for many aspects of the design, including heating, hot water and thermal bridges must be carried out in detail. For example, the calculation must take into account the reduced water heating load that will follow revisions to the water rules in Building Regulations Part G.

SAP 2009 will also factor in fixed mechanical cooling, taking into account system efficiency, thermal mass and fuel. Despite the improved functionality, SAP is not a sophisticated building analysis tool, and for more complex schemes, further modelling work may be needed to optimise the design and avoid the risk of problems such as over-heating.

03 / Practical implications

The main changes to the regulations are as follows:

• Overall reduction of carbon emissions by 25%

This is the main mandatory change introduced in the 2010 revision. The assessment includes fixed building services including heating, ventilation, cooling, hot water and fixed internal and external lighting. Although calculated in a different way, the 25% reduction is broadly equivalent to the current target improvement required to meet level three of the Code for Sustainable Homes.

Important variables in the dwelling emission rate (DER) assessment include:

• The location and orientation of the notional dwelling. This may limit the options available to reduce carbon emissions through fabric improvement. For example, mid-floor apartments do not have the same range of options that houses or top-floor flats do to improve the U-value of a roof. The effect of orientation is to vary the contribution of solar heat gain in winter months, which can be significant in apartments.
• Changes to the treatment of party walls. In Part L 2006, no heat losses through party walls were included in the calculation. Recent research has demonstrated that in new dwellings, losses through cavity party walls can be much greater than a typical external wall. As a result, under the 2010 regulations, the calculation for the notional building continues to assume zero heat loss, whereas the DER includes losses through the party wall. The U-value of an unfilled, unsealed party wall is taken as 0.5W/m2K. This is a positive move for the long-term assurance of building performance, but will need careful detailing.

The wider implications of the change are discussed later

• Secondary heating devices such as gas fires or electric heaters are only included in the assessment if included or allowed for in the design
• Low-energy lighting is calculated on the basis of the proportion of fittings used, and a minimum of three out of four fittings should be low energy
• It is recommended that heat loss through non-repeating thermal breaks is worked out on the basis of detailed calculations rather than an assumed allowance. This assessment is combined with a new incentive system that will address the growing proportion of heat loss through thermal bridges in well-insulated dwellings.

Overall, the assessment of the DER is more detailed than it was for 2006, providing a good foundation for assured performance during construction and occupation. Under the 2010 regulations, details of the assessment must also be submitted to a building control body before work commences on site, including the highlighting of the measures being taken to reduce emissions so that a “risk-based” compliance process can be adopted.

• Revised standards of energy efficiency for building fabric and fixed building services

The lower U-values of limits for fabric performance have typically been improved by 0.05W/m2K, with walls being reduced to 0.3 and roofs to 0.2. Glazed openings and doors have been reduced from 2.2 to 2.0. In practice, much higher levels of performance can be readily achieved using conventional brick-and-block construction and rigid insulation. This is important; if emissions are to be reduced mainly through improvements to fabric, then much higher standards of fabric performance will be necessary.

• Reasonable provision to avoid over-heating resulting from solar gain

This is increasingly important as more developments adopt lightweight or air-tight construction methods to improve performance. Well-managed passive solar gain is also an important element of a low-energy winter heating strategy. A further consideration is related to mobile cooling units, because as they become cheaper, steps must be taken to reduce the risk of increased non-regulated energy consumption.

The revised risk assessment of overheating is designed to take into account thermal mass as well as the usual factors, such as window size, orientation, ventilation and shading.

The risk assessment ignores any contribution from fixed cooling installations, so even for air-conditioned developments, features such as solar shading, natural ventilation or night-time cooling may be necessary to achieve compliance.

• Ensuring actual performance is consistent with the design emissions rating Given the relative simplicity of most domestic construction, the areas of concern are continuity of insulation, the avoidance of thermal bridges and the reduction of air infiltration. The risk-based approach to monitoring installation quality will help reduce poor quality or compromised installation work. However, the main means of ensuring ready compliance is the promotion of a new accredited details scheme. Enhanced construction details, for example, are claimed to reduce losses by 50% compared with conventional detailing based on Part L 2006-compliant details. The typical cost premium is 10% of the work to the envelope.

Under the new regulations, the thermal loss values of non-quality assured details that are prepared following the BR497 methodology are increased by a minimum of 25%. Where no accredited details are provided, a default psi value of 0.15W/m2K will apply - a 100% increase in the factor used in calculating losses through thermal bridges. The net effect will be to encourage the take-up of the quality assured detail scheme.

Requirements for the number of air-tests have also been increased as part of this criterion. Dwellings that are not tested are required to add a tolerance allowance of 2m3/hr/m2 to the value used in the design air pressure test. As airtightness standards increase beyond 7m3/hr/m2, buildings will have to be designed to a very high standard if they are not to be individually air-tested to demonstrate compliance.

• Provisions for energy-efficient operation

There are no major changes in the provisions for energy-efficient operation; the owner of the dwelling is simply to be provided with sufficient information to enable efficient operation and maintenance of the building. As systems become more sophisticated, this element of the regulation will become more significant. The owner also receives the recommendations report prepared for the environmental performance certificate.

04 / The potential impact

Carbon emissions reduction is relatively easy to achieve in most residential development, as the bulk of the regulated energy consumption relates to heat loss, which can be addressed through improved insulation levels and greater airtightness.

• Conventional methods Our cost assessment shows that in most instances, Part L 2010 can be achieved with naturally ventilated buildings, constructed using good quality but conventional construction methods. The experience of RSLs in the delivery of code level three and four dwellings prior to 2010 suggests that cost and complexity issues generally kick in only once the 25% emissions reduction threshold is exceeded. This means that for many buildings, it is still possible to meet the revised Part L requirements using fabric improvements only. This is the preferred approach as it assures emissions reductions at source, and does not rely on relatively expensive set-off technologies such as renewables.

Some RSLs have opted to use a range of renewable energy sources as well as fabric improvements to close the carbon emissions gap when meeting code level three. The new Part L will certainly provide further growth opportunities for the renewables sector. The introduction of the feed-in tariff (FIT) for photovoltaics (PVs) in 2010 should also provide a secure, long-term cashable benefit to homeowners that can be sold-on, making renewables a much more attractive option. Furthermore, the option to assign FIT revenues could secure alternative sources of investment.

For RSLs, where tenants could potentially secure the benefit of the FIT without incurring either capital or maintenance costs, other options may be more cost effective. The levels of FIT payments (index linked for 25 years) are currently set until 31 March 2012, when this incentive is due for review.

• Party walls New solutions are needed for party walls. Designers will have to enhance the specification of the exposed building fabric as well as tackling the substantial losses from a cavity party wall. But many designers and developers may not want to specify a fully filled party wall cavity - especially if higher levels of acoustic performance are needed as part of a Code for Sustainable Homes assessment.

If cavity sealing alone is used - preserving the acoustic properties of the party wall, but resulting in a U-value of 0.2W/m2K, then the improvements required elsewhere in a mid-terrace dwelling are significant. In our assessment below, they include:

• The use of details that provide a performance that is equivalent to enhanced construction details
• Air infiltration of 3m3/m2/hr or less
• The use of an MVHR system.

Insulation makers will be highly incentivised to develop insulation products that enable losses to be minimised while providing acoustic separation.

• The mix-and-match approach A contribution from renewables can be used to meet the 2010 regulations. They can reduce the need to maximise the performance of the building fabric, help to manage costs and give designers and manufacturers the time and flexibility they need to develop solutions for well insulated and airtight dwellings suitable for the UK market.
• Responding to incentives for best in class detailing Revised assessment methods for calculating heat loss through discontinuous thermal bridges provide an ideal opportunity to promote “fit and forget” methods of increasing building efficiency without affecting other aspects of performance, such the ventilation rate.

In a typical 2006-compliant dwelling, losses through thermal bridges are about 10-15% of the total. This proportion will increase unless better detailing is adopted. Tricky problems that can be found when detailing residential construction include the detailing of balconies and solar shading.

Given the widespread acceptance of accredited details, progressive adoption of better standards should follow the introduction of the new regulations. Adoption of details equivalent to the enhanced construction details will deliver a substantial reduction in carbon emissions.

• Long-term implications of renewables Renewables are an important component of low and zero-carbon housing and the FIT and renewable heat incentive will go a long way to transforming the economics of many systems. However, minimisation of landlords’ maintenance costs can potentially be an issue, and investment may tie up scarce capital. With regards to maintenance, roof access is always expensive and building in ready, safe access could be made in the initial design to reduce long-term costs.
• Living with more highly tuned buildings

Like most high-performance products, lower-energy houses are a little more complex to operate and there is a possibility that the below par performance of one element might affect another; for example, heat recovery may affect internal temperature levels. Issues that may need to be addressed to ensure effectiveness include:

• Manuals to support the use of more complex systems of domestic heating, such as zoned control or external weather sensing
• Providing for the regular maintenance and tuning of relatively complex systems such as MVHR, together with the potential for requirements for more reactive support in response to system performance
• The avoidance of inadvertent overheating using bypass systems or boosting ventilation in kitchens and bathrooms for dwellings equipped with MVHR
• Developing the skills base in the plumbing and heating trades so that they can maintain these systems.

05 / Cost assessment: The house

In this cost model, we examine a number of options for meeting the Part L requirements in a typical terraced house and an upmarket apartment.The calculations have been carried out using a preview version of the NHER SAP software. It is likely that there will be some detailed changes to the assessment before launch. As an example, at the time of writing, the preview software cannot calculate either the overheating or the carbon emissions associated with mechanical cooling.

The domestic house model is based on a mid-terrace dwelling with a gross floor area of 76m2. The options for meeting the regulations include a fabric-only strategy, a mixed strategy balancing fabric improvements and renewables and a third option that examines the effect of reducing the level of insulation in the party walls.

The results of the analysis show that fabric improvements are broadly sufficient to meet the revised Part L, even with the inclusion of the assessment of losses through the party wall. The costs of the PV system are high, but will of course benefit from the FIT incentive. Small-scale installations are also much less cost effective than larger arrays. The final option considers the effect of not filling the party wall, which might be adopted if there were concerns about acoustic breakthrough. The additional losses could be offset by a larger PV array at a cost of £4,600. Alternatively, fabric performance can be improved. which will require an MVHR system.

The assessment shows how a fabric-first strategy is th emost cost-effective from the point of view of minimal expenditure.

Costs for both models are at third quarter 2010 prices based on a south-east England location. The base building costs include preliminaries and contingencies. Costs of site preparation, external works, professional fees and VAT are excluded.

06 / Cost assessment: The apartment

The apartment has a gross floor area of 73m2 per unit and a wall-to-floor ratio of 0.6. Lobbies are not heated and the walls that face corridors are designed as external walls. Our modelling of the apartment excludes air-conditioning, which cannot be modelled in the preview version of SAP. In our assessment, winter solar gain makes a big difference in setting the target value, but there is no assessment of the corresponding cooling load. We also assume windows can be opened to deal with overheating.

Three options are considered. The first is a scheme based on typical design solutions where 60% of the external walls are solid. Here, compliance can be achieved with standard services. In option two we have increased the glazing to 65%, which can be delivered using a MVHR system.

A higher level of glazing is also possible, in this case 80%, and triple glazing is specified. Orientation has an effect on the assessment, as winter solar gain helps reduce emissions. Further enhancements may be needed for an “average” assessment, taking into account all sides of a building, or if mechanical cooling were specified.

The costs in the detailed breakdown below only describe the extra over costs required to meet Part L 2010 for each variant. For example, where the area of glazing is increased, only the extra over cost of the Part L enhancement is included, not the increased cost of glazing compared with solid construction.

The assessment shows that the cost of compliance on conventional dwellings is likely to be proportionately higher than for apartments - partly owing to the greater area of the external envelope affected, and partly because many of the modern methods of construction associated with flats have a better thermal performance in the first place. The costs of compliance in flats will increase once requirements for cooling are added to the assessment.