5A Castle Hill - Passivhaus
This Passivhaus EnerPHit project comprised an extension to, and comprehensive refurbishment and retrofit of, an existing detached house built in 1959.
The accommodation includes:
4 bedrooms, each with ensuite
Kitchen / diner
TV Room / snug
The usual Passivhaus features include mechanical ventilation with heat recovery providing constant pre heated filtered fresh air; tripled glazed windows; high levels of air tight construction; very high levels of insulation keeping the house warm in winter and cool in summer. South facing windows have integral blinds to mitigate the effects of summer solar gain and thereby preventing overheating.
Our client set the extremely challenging objective of achieving the full Passivhaus standard, rather than the slightly more relaxed EnerPHit standard.
Despite this, the energy required to heat the house is less than the full Passivhaus requirement of 15 kWh/m2a at just 14 kWh/m2a.
One of the key criteria for a Passivhaus is the level of airtightness. In a new build or full Passivhaus, this is required to be 0.6 air changes by volume per hour. Recognising the effects of existing construction on the airtightness, this is relaxed to 1.0 for an EnerPHit retrofit.
Sadly, when tested, the refurbished house came up just short at 0.7 – still a huge achievement for a retrofit and significantly better than required but not quite good enough to achieve the full Passivhaus standard. Despite this, the energy required to heat the house exceeds the full Passivhaus requirement of 15 kWh/m2a at just 14 kWh/m2a.
During the winter months, the client advises that the house was heated by just four towel rails, maintaining an internal temperature of 20 degrees C when the external temperature was just 0 degrees C.
In occupation monitoring shows that the house is using less energy than predicted by the thermal model prepared at design stage.
The project was a regional winner in the LABC 2020 design awards. It was also a recipient of a Berkhamsted Parish Council design award.