Surrey, Low-Rise Educational Building Retrofit

Brief

The building was built to a ‘Maycrete’ archetype. This form of construction comprises a prefabricated structure of reinforced concrete posts that support a pitched roof. The walls between the posts are constructed using Maycrete sawdust-concrete panels. (The name comes from the designer of the buildings – architect Bernard Maybeck.) Maycrete huts were used widely on wartime airfields but they also found popularity in other sectors. These included schools, domestic garages and, to a more limited extent, post-war housing both in the UK and overseas.

The original design enabled buildings to be erected quickly and cheaply but the resulting thermal performance would be considered very poor by modern standards. Consequently, the school appointed Nye Saunders Architects to plan a refurbishment project that would simultaneously enhance the appearance of the building and improve its ability to retain heat.

An important principle governing the project was to demonstrate an effective and responsible use of resources. This included repurposing existing parts of the structure wherever possible – a decision that helped to reduce overall waste and demand for new, virgin materials.

Design and Specification:

The featured building is a single-storey, rectangular structure. One of its long walls was constructed as a wide cavity wall, comprising an outer leaf of bricks with an inner leaf of slag-concrete blockwork. The other three walls were built to the Maycrete system design, with an inner leaf of the same slag-concrete blockwork. The Maycrete panels are tray shaped, with thick (ca. 160 mm) flanges, but a wall thickness of only 50 mm.

The plan was to provide a new insulated roof and to apply a Permarock external wall insulation system to all the existing walls.

Permarock technical staff worked with the architect to design a viable solution. However, before they could propose a specific EWI system for the Maycrete walls, they had to undertake a series of in-situ pull-out tests. This was essential for proving that contractors would be able to achieve a satisfactory fix of the insulation boards into the sawdust-concrete panels. Neither the installers nor the design team had previously encountered the material, and, at only 50 mm thick, it could potentially have proven challenging to fix the insulation without causing damage. In the event, however, it created no problems.

Permarock systems technicians liaised with the architect and the school to arrange the testing, and were duly able to establish that good mechanical anchorage could be achieved. Having done so, they then worked with the architect to develop the thermal design and detailing.

The project called for all of the walls to be treated with the Permarock Mineral Fibre system, a non-combustible external wall insulation system that would be adhesively bonded and mechanically fixed. A number of different insulation thicknesses would be used to infill between – and overlay the front of – the projecting concrete posts and concrete sills. This would create a flat, flush external appearance and overcome the risks of thermal bridging presented by the projecting posts and concrete window surrounds and sills. The area surrounding the main doors to the building would be treated with thicker insulation to create an obvious focal point into the entrance.

The insulation would then be protected with a reinforced basecoat layer and Permarock’s Silicone^Ultra K-Finish – a high performance through-colour decorative render – here applied in two colours.