There are simple principles for good low energy design. The highest performing building standard is Passivhaus, which has been around for over 25 years. For eco-refurbishment (retrofit) a major consideration is also moisture issues, which is a much more complex topic.
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.
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.