An historic Listed Georgian townhouse in London’s Bloomsbury has been transformed into a comfortable, peaceful and energy efficient home that approaches Passivhaus EnerPHit standards. This was achieved by using modern insulation techniques including an advanced secondary glazing system from Selectaglaze that markedly improves the performance of the traditional, single glazed sash windows. Robert Prewett of Prewett Bizley architects took on the challenge of convincing conservation officers that a Grade II Listed building could be successfully fitted with a host of energy saving measures whilst respecting all the historic features. The large single glazed sash windows were a particular challenge as they represented a significant proportion of the wall area and could not be replaced. Therefore the answer was secondary glazing; but the extremely high levels of insulation required, the aesthetics and the need for working shutters in some rooms demanded a bespoke solution. Series 25 window proved perfect for the project as it offered high sealing efficiency, a very low U-value and a discrete frame with a specially developed slide catch that allowed shutters to fully close. The house now has air tightness close to 1 air change per hour at test pressure which demonstrates that the secondary glazing is performing very well. Monitoring tests will now assess the efficiency of the scheme over time but meanwhile there has been a transformation that provides a modern comfortable home environment within an historic building.