Where do the principles for retrofit come from? The most thorough experience in low energy design comes from Passivhaus, a world wide energy standard that has been around for 25 years. Tim Martel has been applying his training as a Passivhaus Designer for the last 5 years. He is uniquely fortunate in having taken the course and exam at BRE (using the Passivhaus Institute course) but has also taken the AECB course and was the training manager for a year.

Passivhaus principles can be applied to new build but they can also be applied to retrofit. Passivhaus training gives detailed guidance on the technical issues. There are also further moisture issues specific to retrofit which are much more advanced, this section will focus on the Passivhaus principles.

You may have a 1950’s house but you don’t have to have 1950’s comfort.  It’s far easier to build right first time, but existing buildings can be converted and most of the principles are the same. Passivhaus buildings:

  • reduces heating costs by about 80% compared to current Building Regulations, heating bills of around £200 a year are typical
  • are remarkably constant in temperature, they stay at 20⁰C through the heating season and don’t cool down very fast when the heating is off
  • make environmental sense for the world

The standard is not just for houses, commercial buildings can be Passivhauses too. To achieve this they have a very special design approach which needs planning and calculation. When built new they typically only cost about 10% extra to build, using a variety of different construction methods  . . .

There are 4 main Passivhaus principles.

1. Efficient shape

The building should be reasonably compact, a complex shape increases the surface area and makes the design more expensive. Good, compact shapes are, for example, flats and terraced houses. Semi-detached houses are less compact and the least efficient shape is detached houses. On the other hand, detached houses tend to be simpler from the point of view of planning and some of the calculations.

2. Insulation

Insulation needs to be thick and continuous all the way round. Usually there are lots of breaks in the insulation called thermal bridges for example at the eaves or wall to floor junction. Typical houses have over a dozen different types of thermal bridge, Passivhauses are usually built with none.

Passivhauses are better insulated, as shown by U-values. A typical detached house would need an average U-value of around 0.1 W/(m2.K) for wall, roof and floor elements. Windows almost always need to be triple glazed in this country to get anywhere near this. The best triple glazed windows have a U-value of around 0.5 W/(m2.K), a typical double glazed window would be 2.8 and a single glazed 5.8.

This all-round insulation ensures that internal surface temperatures are no less than around 17C, preventing temperature related draughts.

3. Airtightness and Mechanical Ventilation & Heat Recovery (MVHR)

Passivhauses use mechanical ventilation because it allows the heat to be recovered from the ventilation air. They are almost 20 times less leaky than normal houses. Reduced leakiness (airtightness) is needed for 3 reasons:

  • to ensure that most of the ventilation air goes through the heat recovery unit (example shown right)
  • reduce draughts both for comfort and energy reasons and
  • prevent condensation


4. Orientation and Shading

New buildings are optimised to minimise the heating and cooling needs and a large part of that is the orientation and shading of the building. Trees, buildings and hills to the south should be avoided if possible to allow winter sun to warm the building through the windows. Passivhauses are so efficient that they get a lot of their heat this way, the rest is from a smaller heating system. Both winter sun and shading work better if the main windows are within 30 degrees of South.

During summer shading devices above the windows may be needed to avoid overheating.  Windows can be opened, as in a normal house, if it is too warm.