Guy Shackle, senior associate at Curl la Tourelle Architects, describes work on Crescent Primary School in Selhurst, Croydon.
Selhurst Boys School is a listed 1913-built Edwardian grammar school that’s undergoing a £5.5m refurbishment to transform it into a modern three form entry primary school.
Although Building Control only required that 10% of the contract sum be spent on improvements to energy efficiency, we decided to aim significantly higher than that and bring it in line with performance levels expected in a new build.
The combination of cavity wall insulation, roof insulation and a complete window replacement are expected to reduce thermal heat loss through the external fabric to a U-value of 0.3W/m²K and help us achieve a BREEAM rating of “very good”.
The external wall is an unusual early form of cavity wall with an outer leaf of brickwork tied to the inner leaf using brick stretchers, which can cause moisture bridging and is therefore unsuitable for most blown fibre insulation products. Walltite was one of the few foams that would work in this application.
For the thickness of material – a 3in filled cavity – Walltite has a very good thermal performance and low air permeability compared to other similar products and, in combination with new Velfac double-glazed windows, it will reduce noise transmitted from the school. Boosting the thermal performance of the walls also meant that during the design stage we could reduce the size and power of the radiators and boiler inside.
An alternative to blown fibre insulation would have been to line the external walls with insulated render, but this would have impacted the building’s listed status, and an internal layer of insulation and dry lining would have reduced the available floor area of classrooms, which were already close to the minimum size required for primary school teaching.
Pumping foam insulation was beneficial for the programme and main contractor Kier was able to insulate 3,000m2 of wall in just two weeks. Post-installation, the foam will not shrink or settle over time, which is great from a building maintenance point of view.
