An ambitious net-zero residential retrofit

Andres Hernandez sets out the steps taken to achieve the net-zero goal.

This Mayfair residential retrofit project for Grosvenor is targeting net-zero carbon ‘in use’ by prioritising energy efficiency through fabric improvements, low carbon heating and onsite renewables.

Claridge House is next door to the luxury Claridge’s hotel on Davies Street and within the Mayfair Conservation Area. The building is not listed and dates from the 1920s, it originally comprised purpose-built apartments above retail units in a neo-Georgian red brick composition. Partly converted from residential use to office use after WWII, the project brief is to reinstate residential use to the first and second floors, creating eight apartments. The project forms part of a clear net-zero strategy from Grosvenor.

Grosvenor plan to reduce total carbon impact by 50% by 2030 and to meet a net zero carbon portfolio by 2050. The products, processes and construction methods learned from this project will help to inform best practices going forward to mitigate climate change. Broadly the strategy is to prioritise energy efficiency first and to offset as a last resort.

A key challenge for this project is that its scope is to retrofit a part of a building that will remain occupied and operational by retail tenants on the ground floor and residents above. The design approach uses LETI retrofit guidelines and involves providing energy efficiency by improvements to the building fabric, low carbon heating and onsite renewables.

As the historic façade cannot be modified externally, for energy efficiency the design utilises internal wall insulation and thermal window improvements. Existing windows to the principal elevations on Davies Street and Brook’s Mews are traditional timber sash, these were repaired, draft proofed and fitted with secondary vacuum glazing internally. Metal Crittall windows to a rear lightwell were replaced with thermal broken frames and double glazing. Elsewhere, plant for the ASHP is located in a cluster of individual units at roof level with any remaining zones of roof area converted to green roof and PVs. The basement will be reconfigured to provide a cycle store, commercial and residential bin stores with facilities for the building management relocated to accommodate these requirements.  

With sustainability as a primary consideration for this project, the team are using science-based targets across all aspects of the design. Interventions are needed across the building fabric and services as well as onsite renewables to reach the carbon target. The operational carbon of the scheme is calculated to be 68kWh, bringing its performance more in line with the LETI standard of 61kWh.

Illustrative graph to show how a 100% reduction can be achieved

The estimated EUI improvements exceed the client’s target

Internal Wall Insulation

Several internal wall insulation types were researched and tested using WUFI specialist building fabric software. To allow the original brickwork to ‘breathe’ as it would normally with historic fabric, a cork-based insulating plaster was chosen. The product Diathonite was selected as it is a non-combustible, breathable, low embodied carbon material which can be used directly onto the bare faced brick wall.

The thickness of the cork-based insulating plaster made it unfeasible for use on the internal window reveals, therefore a slimmer option was needed. In order to achieve thermal continuity, an ultra thin aerogel insulation was selected. The product was bonded directly to magnesium boards to allow for protection of the otherwise fragile product and easier build-ability of the interface with the secondary glazing.

Despite the use of internal wall insulation, efficient layouts have improved the overall GIA by 33sqm. This is partly due to the innovative use of insulating plaster for the wall lining specification.

Diathonite was sprayed on the walls in two layers of 25mm each.

An ultra thin aerogel insulation was chosen for the window reveals.

Glazing

Secondary vacuum glazing has been added to the historic sash windows, with the rear Crittall windows replaced with new double glazed Crittall units. The interventions will improve the thermal and acoustic performance of the primary façades whilst maintaining the external appearance of the building and provide additional performance in the areas where new units are fitted.

Partitions

As with other building elements in the project, the products used have been chosen with the aim of reducing the embodied carbon of the project. All partitions have been specified with timber studs and Fermacell boards to reduce their carbon footprint compared to more common, lightweight metal studs with plasterboard.

Fossil-fuel free

Different heating methods were considered for the project. Thanks to the improvements to the building fabric, it was possible to choose individual air source heat pumps with MVHR (Mechanical Ventilation with Heat Recovery). These and 45 PVs at roof level will reduce the carbon emissions as the entire operation of the apartments is now via electricity.

User guides

Because the MVHR heating system might be new or unfamiliar for users, tenants will receive a clear user guide. Graphically illustrated descriptions of how to use the equipment will ensure users can create the most comfortable environment. Maintenance aspects such as understanding when and how often to service the equipment and when to change the carbon filters will ensure the systems can be run at their optimal performance.

Innovations such as a ‘main off’ switch for all lights near the entrance will mean that residents can easily make savings on their own energy use by making it easy to control.

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