Brief

The principal objective for the scheme was to improve the thermal performance of the properties by eliminating thermal bridging, improving fabric air tightness and reducing the U-value of the walls and roofs. Other important aims included adhering to the highest fire safety standards, enhancing the appearance of the homes, and delivering the improvements with minimal disruption to residents. The properties in question would all be occupied during the works.

The work was to be funded under the Social Housing Decarbonisation Fund so it had to be completed by a Trustmark registered installer, and in compliance with the requirements of the PAS 2030 / 2035 standards. Other works, delivered concurrently, would include the installation of new loft insulation, together with high-performance double-glazed windows and doors.

Design and Specification:

The specification called for the installation of PermaRock Mineral Fibre System with 1.5 mm Silicone Ultra K Finish. This would improve the thermal performance of the properties while simultaneously enhancing their appearance. The system also exhibits excellent ‘reaction to fire’ performance, and achieves an A2-s1,d0 classification in accordance with BS EN 13501.

Key Challenges:

A notable characteristic of the Bison design is the use of splayed reveals to the heads and jambs of the window and door openings. To maintain the properties’ character, the design team required that the same appearance be preserved after the installation of the external wall insulation system. Achieving this required the installers to adopt an unusual approach – recreating the chamfered reveals by shaping the insulation boards on site, and fitting specially produced metal flashings and sills.

The fire safety requirements were primarily met through the use of mineral fibre insulation. However, in order to prevent thermal bypass and to prevent fire propagation behind the insulation boards, the recesses between the fluted ribs of the concrete panels were flush-filled with PermaRock Dubbing-out Compound, a non-combustible cement-based filling and levelling compound.

Airtightness was another important consideration. The Bison design features full-height, open, panel-to-panel joints, as well as ‘letterbox’ vents and other elements. These had to be filled with mineral fibre insulation and finished with cementitious mortar in order to reduce air movement and to ensure a sound, flat substrate over which the EWI system could then be applied. In addition, air leakage was further reduced through the use of airtight tape and careful detailing around doors, windows and other features.

The other very important challenge was to prevent cold-bridging at floor level. PermaRock technicians and the project design team therefore devised special EWI detailing at the various junctions with the ground, including garden wall abutments and other features. The team used fRsi calculations to determine the necessary thickness and depth of insulation required at the base of the wall to reduce the risk of condensation and dampness at the internal wall-floor junction.

Performance:

The client required a high level of thermal performance, near to Passivhaus standards. In the event, this was achieved using 230 mm of PermaRock Mineral Fibre insulation. This resulted in a calculated U-value of less than 0.15 W/m2K.

As specified, the system delivered an A2-s1,d0 ‘reaction to fire’ classification.

The system also greatly improved resistance to air leakage. It was measured at < 0.3 m3/m2.h at 50 Pa, which is significantly better than the level demanded by the Passivhaus standard.

Result:

The project has improved the thermal performance of the properties, thereby preventing damp and enabling residents to enjoy warmer conditions and lower energy bills. In addition, the work has delivered a noticeable improvement to the local street scene.

Another important consequence of the retrofit works was an improvement in acoustic insulation. The estate is located under one of the main flight path approaches to Heathrow Airport, so noise had been a constant issue for residents. However, the extra insulation was found to reduce this noise considerably, helping to further improve living conditions for the occupants.

Leanne Edmond, a Project Manager at United Living, noted that “the properties are much warmer and people are overjoyed by the level of sound insulation they have from the overhead aircraft, which is a great unintended consequence of the works.”

The many improvements resulting from the works were remarked upon by the judging panel in the 2024 INCA Awards. One of the installers, PJ Mear, was named winner of two award categories:

•    Refurbishment: Low Rise – Render Finish
•    Judges Choice: Environmental Impact – Low Rise (Refurbishment)

The judges wrote that that they had “decided on the winner because of such an incredible transformation, making the project look so clean and modern, and with such an exceptional U-value and good future-proofing… Such a thoughtful, holistic approach to this stunning project and the near Passivhaus criteria, which ensured that every EWI benefit was considered and implemented, made this project a very worthy winner.”

The scheme was also shortlisted in the Best Refurbishment Project category of the 2024 Facades Awards UK. The nomination related to another section of the project, which was completed by Eden Facades and which featured the same PermaRock external wall insulation system and associated measures.