Deep retrofit to AECB retrofit standard
A complex retrofit and extension of a terrace of 6 houses used for postgraduate student accommodation in Cambridge has just achieved AECB retrofit standard certification. St John’s College, Cambridge was keen to upgrade and improve the amenity and facilities of the 43-bed student accommodation while also improving thermal performance and moving away from fossil fuel heating.
The retrofit aimed to futureproof the student accommodation in terms of overheating risk, thermal performance, water consumption, energy generation, decarbonisation, biodiversity, while making sure that the embodied carbon impact of the intervention was considered as much as possible. The historic importance of the existing properties was also a factor. While not being Listed Buildings, 1-11 Portugal Street are Buildings of Local Interest.
Greengauge delivered building physics, moisture risk, building services and AECB certification to the project.
Strategy
The retrofit of the six dwellings within the terrace posed a number of challenges, including space constraints (affecting building services options), moisture risk (affecting the internal wall insulation strategy), as well as proximity of the project to a conservation area.
The brick outer leaf of the building was retained, and internal wall insulation was the only option for improving thermal performance. Newbuild timber extensions at the rear of the properties were required to enhance amenities and deliver the space needed for all the building services. Internal layouts and designs were improved for accessibility and comfort.
Greengauge worked closely with the project team to manage moisture risks for the internal wall insulation strategy and advised on the suitability and thickness of internal wall insulation options. The IWI strategy predominantly entailed the use of wood fibre internal wall insulation.
In retrofits there are often challenges in terms of fitting building services into small existing spaces. The building services strategy focused on finding fossil free alternatives that would fit within the constrained site. The use of space-saving ASHPs (air source heat pump) were found to be the only viable option. The MVHR ventilation strategy was also carefully designed around the constrained site and consideration of where there was space for ventilation pipes.
AECB retrofit certification was an aim for the project team from the outset. All thermal improvements to the project were modelled carefully in PHPP and updated as changes in design were made. The moisture risk that was identified through WUFI modelling limited the thickness of the internal wall insulation that was safe. This meant that Greengauge needed to achieve AECB Retrofit certification through its ‘exemption’ route. The AECB Carbonlite Retrofit exemption route understands that a space heating target of under 50 kWh/m2/year may not be possible for retrofits that require internal wall insulation, because moisture risk limits the thickness of insulation that is allowable.
Project team
Client: St John’s College, Cambridge
Architect: MCW Architects
Contractor: PML
Structural Engineer: Smith and Wallwork
Building physics: Greengauge
Moisture risk: Greengauge
M&E Design: Greengauge
AECB Certifier: Greengauge
Key data
“Deep retrofits like the Portugal Street project require complex problem-solving. It has been great to be able to offer integrated solutions, combining both building physics and mechanical services design for the project. Congratulations to the client and project team for achieving AECB Retrofit certification!”
Mitch Finn, Building Physicist, Greengauge
Key project aims
- To retain the existing number of rooms (43) let as part of an House of Multiple Occupation HMO arrangement
- Improving the current limited amenity space in the building
- Increase number of bathroom accommodation above the minimum HMO requirements
- Improve accessibility to the site in general
- Make the social areas of the buildings accessible to visitors with disabilities
- Improve external amenity space
- Decarbonise the buildings – remove reliance on fossil fuel
- Improve thermal performance
Key features
Management of moisture risk
Portugal Street’s location in a high-water table area meant the lower parts of the building were suffering from damp. Greengauge completed material testing and hygrothermal risk assessments of the internal wall insulation strategies being proposed and worked closely with product suppliers to design a system which managed risks and met the client’s preferences.
The insulation materials that were chosen for the wall and most roof areas (woodfibre and mineral wool) are vapour open to reduce moisture risk. Internal wall insulation buildups were analysed for hygrothermal risk assessment using WUFI and were found to be in acceptable parameters. Buildups contained a vapour control layer on the warm side of the insulation.
Construction elements were protected from moisture damage through robust design and construction. The project’s MVHR systems help manage the moisture levels within the building. Surface moisture has been managed by addressing thermal bridging. Cracks and faults in the existing walls have been repaired to mitigate excess moisture ingress.
Insulation strategy
The existing solid brick walls were retained and insulated with either woodfibre or mineral wool. Mineral wool was specifically used in the lower ground floor areas, where the waterproofing eliminated the need for a vapour permeable insulation solution.
The new timber frame external walls of the extensions were insulated with mineral wool between and outside the frame. The project’s roofs were insulated with mineral wool (or PIR, only if there were space constraints). PIR and XPS insulation were specified for floor insulation improvements.
Triple-glazed windows were installed in the new build. Double-glazed sliding sash windows, designed to match the originals, were specified for the existing building, which is recognised as a Building of Local Interest within a conservation area.
Minimal thermal bridging
Thermal bridging has been mitigated through designing continuity of insulation. Thermal bridges where internal walls meet external walls have been addressed by returning the insulation along the internal wall. Thermal bridges at window and door installations have been reduced with use of insulated structural board. Greengauge undertook a number of thermal bridge models of steel connections to determine the fRsi value. These were shown the meet threshold fRsi value, with structural thermal-break pads being used in some areas
Airtightness strategy
A combination of airtight membranes, liquids and tapes were used to help achieve an average airtightness permeability rate of 1.8 m3/m2.h on the project.
Space-saving building services strategy
The client brief was to futureproof the project by switching away from fossil fuel heating. Use of the building as 43 student rooms represents an unusually intense energy demand for these small domestic buildings, a challenge intensified by only having a small amount of rear garden space available.
Space constraints at 1-11 Portugal Street meant finding compact building services was required. There was no room externally or internally for ground source heat pumps or biomass boilers. Once the design team established that ASHPs were the most viable option for the site as a whole, the design was progressed using both energy modelling and acoustic calculations to determine the size and placement of the pumps, as well as the required performance of the building fabric. Iterative energy modelling allowed assessment of how different fabric performances impacted the required output – and size – of the ASHPs. Emphasis was placed on keeping the units at a domestic scale as this was the only means of complying with acoustic restrictions, due to the quiet site and overlooking neighbours’ windows.
The solution that the Greengauge building services team came up with was the use of compact air-source heat pumps that provide domestic hot water, space heating, and cooling. It would have been difficult to accommodate a typical FCU (fan coil unit) air source heat pump in the constrained space, because of the difficulty of accommodating multiple vent pipes. Instead, the Greengauge engineers recommended condensate-free wall-mounted fan coils, requiring less pipework. The project’s hot water needs are met by the use of small cylinders rather than large ones, again in response to space constraints.
In four of the six houses, individual MVHR units are installed on each of the four floors, where space constraints prevent the installation of large ceiling ducts that would be needed if there were fewer MVHR units. In the other two houses, each dwelling is served by two MVHR units, with one serving the two lower floors and the other serving the two upper floors.
The newbuild extensions built at the rear of 1-11 Portugal Street added space and amenity to the accommodation, as well as allowing space for some of the building services.
To supplement the overall on-site energy strategy an array of PV panels was located discreetly on the upper hip of the roof facing Portugal St. A 10.005 kWp solar PV array on the roof delivers a predicted yield of 9,331 kWh/year.
AECB Retrofit certification
AECB Carbonlite Retrofit certification was targeted for the project from the outset. The exemption route was required for AECB certification because the thickness of IWI (internal wall insulation) had to be limited due to moisture risk. The AECB retrofit standard target for space heating is 50 kWh/m2/year, but exemptions can go up to 100 kWh/m2/year. In the case of 1-11 Portugal Street, the final certifiable space heating demand figure came in at 66 kWh/m2/year, which was permitted for certification through the exemption route.
The AECB retrofit standard was chosen because the approach centres around establishing a precise target for the building’s space heating demand and employs the Passivhaus planning package (PHPP) tool to achieve this goal. PHPP, a versatile spreadsheet design tool, enables a comprehensive assessment of the effects of proposed fabric improvements on energy demands and the feasibility of heat pump installation.
A noteworthy aspect of AECB is its alignment with the Passivhaus methodology, ensuring that assessment and modelling are carried out by certified Passivhaus consultants. This AECB methodology also incorporates a quality assurance component, necessitating the involvement of an AECB certifier. This holistic approach aims to minimise the performance gap associated with retrofit, striving to ensure that the final construction closely matches the originally designed building performance.
