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The Entopia Building has been awarded the prestigious EnerPHit Classic certification by the Passivhaus Institute, a significant moment for a project that has transformed a 1930s telephone exchange into an ultra-efficient office-space meeting the highest possible sustainability standards. But what is EnerPHit?

Read the full announcement by The Cambridge Institute for Sustainability Leadership here.


EnerPHit is a standard specific to retrofit of existing buildings and is awarded by the Passivhaus Institute.  The Institute was set up by a German physicist – Dr Wolfgang Feist - in the 1990s.  Feist asked a seemingly simple question – can we go back to first principles and use a scientific approach to stopping our buildings wasting so much energy?

The institute came up with a solution that is so effective that the need for central heating is almost eliminated; the heat from the sun, occupants and appliances is enough to keep a building warm during much of the winter.  

The standard has been described as accountancy. A giant spreadsheet called the Passive House Planning Package (PHPP) is used to calculate everything:  heat flow through walls, roof, windows and floors; heat loss through ventilation; heat gains from the sun on windows; and energy used by every appliance and system in the building.  

To receive EnerPHit certification the designers and builders had to demonstrate the following:

  • That a continuous wrap of insulation had been added to the inside of every single external surface (computer modelled, photographed during installation, and thermally imaged to prove there were zero gaps).
  • All windows were triple glazed and calculations showing that the window arrangement would minimise risk of overheating, despite only a small amount of mechanical cooling being allowed.
  • An extraordinary level of air-tightness, around 80% less leaky than recent typical new builds (and this for a 1930s building!).  This has been tested by pressurising the whole building to 50 Pascals (a unit of pressure and stress).
  • A guarantee of fresh air using a very efficient mechanical ventilation system that recovers the vast majority of heat from the exhaust air in winter.  You can still open the windows, but shouldn’t need to.
  • That the above measures will mean the building requires less than 25kWh/m2 per year of energy to heat (around 80% less than a typical building on the University Estate).
  • That the efficiency of all energy consuming systems and appliances is such that when everything is added up the demand for non-renewable primary energy from the UK grid is less than 74kWh/m2 per year.

Congratulations to all that worked on the project!

Written by: Alexander Reeve, Sustainable Construction Manager