Passivhaus - what is it?
Passivhaus buildings provide a high level of occupant comfort while using very little energy for heating and cooling. They are built with meticulous attention to detail and rigorous design and construction according to principles developed by the Passivhaus Institute in Germany, and can be certified through an exacting quality assurance process (Passivhaus Trust).
The basic principle of a Passivhaus is to reduce the heat losses of a building to the point that it hardly needs any heating at all. The sun, occupants, household applicants and the warmth from extracted air cover most of the heating demand. The standard relies on good levels of insulation with minimal thermal bridges and well-designed uses of solar and internal gains. The buildings are made as airtight as possible which
results in the installation of mechanical ventilation heat recovery (MVHR) systems which provide excellent air quality and heat recovery efficiencies in excess of 80%. All of this means that a traditional heating system is no longer required and the shortfall can be covered with a small unit, such as a wood burning stove.
Despite what you may have heard, you can open windows in a Passivhaus - in fact, they are often open more than in 'conventional' houses because they stay so much warmer! And they do not have to be box-shaped, just have a quick Google!
And despite the name, its not just houses that can be Passivhaus certified, there are a growing number of non-domestic Passivhaus buildings in the UK, particularly schools.
Elrond Burrell writes a brilliant blog about Passivhaus where he addresses the basics as well as listing links to a variety of resources for further information about Passivhaus and the certification process.
Passivhaus certification is also possible for very low energy retrofit projects. EnerPHit is a slightly relaxed standard for retrofit projects, where the existing architecture and conservation issues mean that meeting the Passivhaus standard is not feasible.
Performance targets for a European climate (Passivhaus):
Primary energy demand (electricity use): ≤ 120 kWh/m2. yr
Space heating demand (energy used for heating): ≤ 15 kWh/m2. yr
Specific heating load: ≤ 10 W/m2
Airtightness: ≤ 0.6 air changes/hr @ n50 (standard UK houses built to Building Regulation standard are around 10 air changes/hr!)
For comparison, the average UK house uses over 400kWh/m2.yr in primary energy - 4 times as much and around 200kwh/m2.yr in space heating - 13 times more!
To achieve the Passivhaus Standard in the UK typically involves:
accurate design modelling using the Passive House Planning Package (PHPP)
very high levels of insulation
extremely high performance windows with insulated frames
airtight building fabric
'thermal bridge free' construction
a mechanical ventilation system with highly efficient heat recovery (MVHR)