26th August 2020

Wall Insulation and Windows

To allow the old building to provide a comfortable internal environment and reduce its energy use the walls, floors and roofs are being insulated. Given the importance and quality of the facades this is internal, which is challenging to detail. A moisture-open but air-tight system of lime plaster and woodfibre boards, Pavatex, is used as this minimises the moisture and condensation risks to the historic fabric.

At the same time the performance of the windows needs to be improved. Following a long study of the respective options before the listed building consent application, Heritage England and Rugby Borough Council approved the replacement of the old windows with new ones with thermally broken frames and double glazing.

This lining begins in earnest in the Power Hall, with the initial application of a levelling coat on the south wall and taping of services sleeves for the external lights. Briggs and Forester, the MEP contractors, find they need longer drills given the thickness of the old walls.

The plaster parge coat and insulation is installed after the new window frames and sleeves for services penetrations, so that a specialist joining tape can be used to lap these with the wet plaster, creating a continuous and durable air tightness barrier around the whole envelope.

The installation is complicated by the Power Halls expressed steel frame and crane rails. These are carefully sealed and all gaps filled prior to application of the insulation boards, so that the air tightness is maintained and no voids are left that would form a potential condensation risk.

In the Power Hall the former steel windows are replaced with slim aluminium frames, using a system with similar sightlines and profiles. In the Transmission Block the windows were generally timber, and these are replaced with new timber frames using the same mouldings and profiles but made deeper to accommodate the double glazing.