It is a flexible approach that does not prescribe how to meet the baseline targets, allowing creativity and a variety of solutions.  It is dependent on careful detailing and rigorous attention to getting things right on site.

A key aspect of a passive house is a reduction in the energy used to heat it.  This can be reduced to such a level that the heating demand can be met by passive means – solar gains from the windows and the heat produced by the people and equipment inside it.  By using high levels of insulation and carefully controlling air leakage, the building can be kept warm during cooler months and have reduced heat gains inside the building in the summer. 

Passivhaus buildings can typically achieve a 75% reduction in space heating requirements, compared to standard practice for UK new build.  This means that it is a great way to achieve the kind of energy savings that we should be aiming for to achieve zero carbon targets.

As well as energy efficiency, a key driver of the standard is occupant comfort.  By maintaining a stable temperature, both of the air and also internal surfaces, drafts are reduced as well as the feeling of cold given by cooler surfaces such as windows and uninsulated walls.  Maintaining surface temperatures also eliminates condensation and mould growth.

There are a number of different standards to choose from, as well as retrofit standards for upgrading existing buildings.  Full Passivhaus certification (or EnerPHit for retrofitting existing buildings) doesn’t suit every project or budget but it makes so much sense that we aim to adopt the principles when we can.

There are very specific technical requirements that need to be achieved that define the Passivhaus standard:

• The Space Heating Energy Demand is not to exceed 15 kWh per square meter of net living space (treated floor area) per year or 10 W per square meter peak demand.
• The Renewable Primary Energy Demand (PER, according to PHI method), the total energy to be used for all domestic applications (heating, hot water and domestic electricity) must not exceed 60 kWh per square meter of treated floor area per year
• In terms of Airtightness, a maximum of 0.6 air changes per hour at 50 Pascals pressure (ACH50), as verified with an onsite pressure test (in both pressurized and depressurized states).
• Thermal comfort must be met for all living areas during winter as well as in summer, with not more than 10 % of the hours in a given year over 25 °C.

For retrofits the standards are slightly relaxed, but the basic principles are the same.

In practice this requires the following basic principles to be followed:

• excellent thermal insulation of the building envelope.
• high performance windows, with triple glazing and insulated frames
• airtight construction to eliminate drafts and ventilation heat loss
• avoidance / minimising of thermal bridges through the insulation
• a mechanical ventilation system with highly efficient heat recovery

As well as these a key aspect is harnessing the passive energy opportunities of the site.  This means orienting your building carefully to maximise useful solar gain, creating an efficient form that minimises heat loss, and providing shading to prevent excessive solar gains and overheating in the summer months.