A SHORT GUIDE TO AIRTIGHT PASSIVHAUS DESIGN

 

The aim of this guide is to provide you as a UK builder, building owner or specifier with clear information on Passivhaus (Passive House) design, to provide links to key industry sources and current relevant UK Building Regulations and Standards.

What is airtight Passivhaus design?

Dr Wolfgang Feist, Director of the Passivhaus Institute (PHI) explains that passive heat sources in including the sun, people, appliances and heat from extract air can cover a large part of the heating demand of a building.

Airtight Passivhaus designs aim to reduce carbon footprint and keep energy costs to a bare minimum by containing heat inside a building through zero energy construction, including insulation and airtight design. Passivhaus designs allow thermal comfort to be achieved by only post-heating or post-cooling the fresh air flow required for good indoor air quality reducing the need for additional air supply or recirculation to heat or cool a building.

It is important not to confuse airtight Passivhaus design with passive solar design, which was common in the 1970s, when many passive solar buildings incorporated a significant amount of south-facing glass, sometimes coupled with high thermal mass and suffered from overheating.

The Passivhaus standard is now the fastest growing energy performance standard globally.

When did airtight Passivhaus design originate?

The Passivhaus standard was developed in the early 1990s by Dr Bo Adamson of Sweden and Dr Feist of Germany. The first buildings completed to the Passivhaus standard were dwellings constructed in Darmstadt, Germany in 1991 in a cool moderate climate. Today the standard is being developed worldwide for use in buildings beyond dwellings.

Who are airtight Passivhaus designs for?

The owners and users of notonly residential dwellings but also of commercial, industrial and public buildings such as schools can benefit from airtight Passivhaus design.  Although it is for mainly integrated into the architecture of new building construction, it can also be used for retrofit applications.

Why are airtight Passivhaus designs important and of benefit?

Forthcoming EU regulations on carbon footprints will necessitate a more focused effort by member states to reduce energy use. Airtight Passivhaus designs reduce the carbon footprint of a building. The strengths of Passivhaus standards lies in the simplicity of their approach.

In airtight Passivhaus design, building heat losses are reduced so much that they hardly require heating at all.  For example some airtight Passivhaus residential applications only have a heated towel rail as a means of conventional heating and this heat can then be recovered and circulated by a mechanical ventilation and heat recovery (MVHR) unit.

They have the following characteristics:

  • Exceptionally high levels of insulation
  • Well-insulated window frames and glazings
  • Thermal bridge free design and construction
  • An airtight building envelope
  • Excellent indoor air quality
  • Ventilation with highly efficient heat or energy recovery
  • More comfort, less energy

The myth that an airtight Passivhaus building ‘heats and cools itself’ is responsible for some misunderstanding.  With airtight Passivhaus design, careful planning and execution is essential and attention to detail required to achieve a minimal energy demand and to meet Passivhaus criteria.

For example, in terms of air tightness, a maximum of 0.6 air changes / hr at 50 Pascals pressure (ACH50) is required, verified via an onsite pressure test (in both pressurized and depressurized states).

The extra outlay involved for the integration of Passivhaus design is paid off through reduction of energy costs through time during the lifecycle of a building.

Where can airtight Passivhaus designs be built?

The vast majority of passive structures to date have been built in central Europe. However, Passivhaus designs can be built in a variety of climates. The PHI is currently developing guidelines for specific climate zones. Depending on a local climate, individual component properties vary. In hotter climates special attention should be paid to passive cooling measures, such as shading and window ventilation, to ensure comfort during the warmest months.

How can airtight PassivHaus designs be built?

Airtight Passivhaus designscan be builtwith meticulous attention to detail in their design and construction according to principles developed by the PHI and then certified to their Passivhaus standard.

Airtight Passivhaus building designs should be adapted to suit the prevailing climate.  To achieve this methods can remain the same but details have to be adapted.

The more compact a building envelope, the more cost-efficient and easier it is to reach the Passivhaus standard. Building components should be well insulated and edges, corners, connections and penetrations carefully detailed to avoid thermal bridges.

In order to reach the standard, all issues, elements and systems listed below should be optimised and checked for compatibility including:

  • Airtightness
  • Window glazing and frames
  • Window orientation and shading
  • Ventilation with heat recovery for efficiency
  • Ventilation with heat recovery for comfort
  • Protection against mould
  • Ground heat exchangers
  • Domestic hot water
  • Efficient appliances and lighting

It is incorrect to claim that a building is to the Passivhaus standard unless it can be shown to be designed and constructed according to all the certification criteria, not just for airtightness for example.

The UK passive house organisation, the Passivhaus Trust.  recommends that the best way to achieve quality assurance for a Passivhaus project is via certification.  A Building Research Establishment (BRE) Registered Passivhaus European Passivhaus Designer can act as a Passivhaus Certifier for you.

More information can be found on Passipedia, the online Passivhaus resource of the International Passivhaus Association (iPHA) as well as the Passivhaus Trust.  The related part of the current UK Building Regulations applicable to either, new, existing, dwellings or other buildings is the Part L Approved Document series.

                                                                                                 - Many thanks to Mary Bon RIBA - AirtightTAPES.co.uk