Curo Enterprise is delivering the multi-phase Mulberry Park development in Combe Down, just outside Bath.
Darren Evans provided consultancy and calculation services to support Curo Enterprise in securing planning permission for and delivering phases 3 and 4 of the development.
National building regulations set a minimum energy efficiency standard, but some Local Authorities have more stringent sustainability targets. As part of granting planning approval, they can impose conditions requiring developments to achieve levels of energy efficiency and carbon dioxide emissions better than those required by the Building Regulations.
This is the case for Bath and North East Somerset Council (B&NES), who have continually refined their own sustainability guidelines over recent years. For a multi-phase development like Mulberry Park, in Combe Down, Bath, this means new phases must meet frequently shifting (and improving) targets, even though the baseline Part L 2013 regulations remain the same.
Mulberry Park will deliver nearly 700 homes by 2026 and is being developed by Curo, a housing association and housebuilder with operations across the West of England. Curo engaged Darren Evans’ expertise for the third and fourth phases of the development, which has been live since 2015.
The Code for Sustainable Homes was a method for assessing the environmental performance of new homes, but was withdrawn in 2015. Its benchmarks, however, are still sometimes used as the basis for setting performance targets. B&NES use the former ‘code level 4’ as their benchmark, which was required for the homes at Mulberry Park.
In practice this meant achieving carbon emissions 19% lower than the compliance target set by Part L 2013. An additional aspect of the requirement is that B&NES asks for a minimum of 10% of the reduction to come from renewable sources. There is no stipulation as to how the remaining 9% should be achieved.
Any dwellings built and certified to the Passivhaus standard - of which there were four in the third phase and eight currently being built in the fourth phase of Mulberry Park - are exempt from the 19% reduction requirement. This is because the Passivhaus standard is considered to reliably demonstrate sufficient energy savings to meet the requirements of the Local Authority’s sustainability checklist, which was introduced in 2018.
“Our ‘base specification’ for the development was identified in phase 1,” explained Bradley Bush, Technical Coordinator at Curo. “Since then, we have further developed this to meet changes in guidance. Flexibility is essential. We work with Darren Evans to make those adjustments, reviewing our U-values and SAP calculation inputs to ensure we meet the sustainability requirements.”
The requirement to include a certain amount of renewables to help achieve carbon reductions can potentially add a lot of cost to a development, especially one the size of Mulberry Park. An additional cost multiplied across nearly 700 homes has significant implications, so it’s important to look at all specification options to find the most cost-effective way of meeting performance.
Brandon Wipperfurth is Senior Sustainability and Energy Consultant at Darren Evans: “We value-engineered the site and set the reduction from PV at 12%, with 7% to find elsewhere. It can be very expensive to achieve the 19% with add-ons, so we really focused on adjusting the design of the build to find the best overall solution.
“The site wasn’t a simple set of buildings and featured all kinds of unique house types. The site layout was set before we got involved, but we were still able to value-engineer as needed. This project shows the benefit of not taking the same approach every time - we were able to ‘play off’ different specifications against each other, designing the solution to achieve the Local Authority’s targets.”
For Curo, maximising the cost effectiveness of the development while ensuring quality and comfort for occupants is a key part of their approach to development. Being a not-for-profit organisation, any profits made are invested into delivering more affordable housing and supporting communities.
“As a housing association and housebuilder, we are keen to maximise the provision of affordable housing on sites,” said Bradley Bush. “Phase 3 of Mulberry Park featured 38% affordable housing, and the development will deliver 31% of the homes as affordable housing overall. Essentially, the greater the profit achieved on a scheme, the more affordable housing we are then able to deliver through reinvesting on future schemes, so we’re keen to maximise the value of the specification while achieving the same performance.”
An example of how the specification has changed over time is that phase 1 featured medium dense concrete blocks in the external walls. Phase 4 saw these switched to aircrete blocks with some recycled content. “The aircrete blocks were more difficult to source,” said Bradley, “but they helped Brandon to adjust the calculations and meet the SAP target due to improved thermal performance and, being lightweight, could also be laid more quickly on site.”
For phase 4, the partial fill cavity wall insulation is being switched to full fill insulation as part of further value engineering. The airtightness value in the SAP calculations, meanwhile, is set at 4.65 rather than a more typical 5.
Bradley explained the relevance of that difference: “Thanks to the experience of building the Passivhaus homes at Mulberry Park, we’re now consistently achieving an airtightness result of around 4 on all of the homes across the development. Brandon showed us that changing the target to a value of 4.65 in SAP was enough to offset the amount of PV on site. We were able to carry that saving across the whole development.”
“By making the models as accurate as possible, it helps to make the PV calculations as accurate as possible,” added Brandon. “We’d estimate that we helped to save £30,000 to £40,000 in cost on phase 3, and we’re taking a similar approach on phase 4.”
The first four homes in phase 3 certified to the Passivhaus standard were a trial for Curo. As well as looking to further improve their sustainability credentials as a developer, there was a desire to investigate the benefits of building affordable homes with very low space heating demand.
“Passivhaus lends itself to affordable homes,” said Bradley Bush. “It helps the occupants by giving them lower heating bills, which can be savings of as much as a 90% when the home is used efficiently.”
Understanding the difference in use between a Passivhaus home and a more traditional home was a key part of the learning exercise. Bradley continued: “Darren Evans helped us with the Passivhaus-certified homes by updating U-values and producing bespoke junction details and psi value calculations. We also worked with them to produce a post-occupancy evaluation (POE) questionnaire.
“We are benchmarking four Passivhaus homes against four ‘typical’ homes of a comparable size on the same site, to understand how the average costs differ. More importantly, we want to ask residents how their lifestyle is different due to the passive construction. We want to investigate developing more Passivhaus schemes in the future, and the feedback from this exercise will help us to advise future residents and market the benefits of constructing to this standard.”
In addition to value engineering the specification, producing SAP calculations, carrying out thermal bridge detailing and helping to initiate POE, Darren Evans also provided Part G water calculations and Energy Performance Certificates (EPCs).
A further aid to Building Regulation compliance was being on hand to address a condition of the Building Regulation approval. NHBC Building Standards raised concerns about condensation risk in an external balcony over a bay window. A condensation risk analysis was carried out to demonstrate that the proposed construction was acceptable and to get the condition removed.
“Darren Evans are versatile in the services they offer, and Brandon is a really approachable person,” concluded Bradley. “Although they’re dealing with complex technical subjects, they explain what they’re doing in a way that you can grasp and come away feeling that you understand it.”
To find out more about how Darren Evans can help you to achieve your sustainability goals and save money on your construction projects, view our complete range of services. The best time to get us involved is at the start of a project, when we can have the most impact. However, you can start a conversation with us at any time - contact us to discuss your needs.
Met the Local Authority target of 19% reduction in regulated carbon compared to Part L 2013. Building fabric improvements achieved the first 7%, with the remaining 12% from solar PV.
“The site wasn’t a simple set of buildings, and featured all kinds of unique house types. The site layout was set before we got involved, but we were still able to value engineer as needed. This project shows the benefit of not taking the same approach every time - we were able to ‘play off’ different specifications against each other, designing the solution to achieve the Local Authority’s targets.” - Brandon Wippurfurth, Senior Sustainability and Energy Consultant at Darren Evans.
“Darren Evans are versatile in the services they offer. Although they’re dealing with complex technical subjects, they explain what they’re doing in a way that you can grasp and come away feeling that you understand it.”
Bradley Bush, Technical Coordinator
