What is Passivhaus?

Passivhaus is a voluntary standard and proven approach to achieving genuinely low-energy, net-zero-ready buildings. The design methodology can be applied to both new construction and retrofit projects, and is not limited to residential design or ‘houses’, (the word ‘haus’ in German translates as ‘building’). Passivhaus certified projects can be almost any building type within domestic, residential, commercial, education and civic sectors.

  • Low-energy consumption - A Passivhaus building can achieve a 75% reduction in space heating demand compared to a standard UK new-build.

  • Free from draughts - High levels of airtightness virtually eliminate uncontrolled and undesirable air flow so that appropriate ventilation is strictly controlled, low-velocity and supplied at close to ambient room temperature.

  • Few to no periods of overheating - Careful consideration of window sizing, orientation, passive shading and specification of glazing, combined with control of ventilation systems seeks to substantially limit or entirely remove periods of overheating often experienced in UK buildings.

    Contrary to popular belief, you can open windows in a Passivhaus building and it is possible to employ active cooling (comfort cooling/air conditioning).

  • Substantially reduced risk of mould and condensation - Factors of triple glazed windows, extract of humid air, air tight and thermal bridge free construction combine to ensure there are no internal cold surfaces on which condensation can form.

  • Substantial reduction in air pollutants - Controlled mechanical ventilation with heat recovery continuously extracts pollutants from the air (such as carbon dioxide, excessive moisture, pollen and VOCs), while supplying fresh, filtered air that is pre-heated to ambient room temperature.

The Passivhaus standard not only ensures exceptional levels of energy-efficiency, it also delivers high levels of human comfort. The key benefits are:

Unlike buildings that comply only with UK Building Regulations, or with other voluntary sustainability standards, Passivhaus buildings have been demonstrated to perform in use as they were designed to perform. The substantial ‘performance gap’ between designed and measured performance evidenced in most UK buildings, is virtually eliminated in Passivhaus. This has been achieved through decades of refinement of calculations in PHPP (Passive House Planning Package), continued assessment of as-built projects, design research and development, and a comprehensive quality assurance regime throughout the design and construction processes.

‘[in typical new-build homes] fabric heat losses can be 50 - 60% more than design predictions, and space heating typically 100 - 150% greater’ ¹

What are the 5 principles of Passivhaus?

1. High levels of insulation (super-insulated building envelopes)

The most robust and long-term approach to reducing building energy consumption, is to follow a ‘fabric first’ design approach, that is, to reduce heat loss through the building envelope as far as practically and economically possible. In cool and temperate climates the majority of energy consumption in buildings is associated with space heating. By minimising heat loss through the building fabric (walls, floors, roofs, windows and doors), space heating demand may be substantially reduced. The fabric of a building is typically the most expensive and permanent element of construction, which can generates substantial CO₂ emissions during construction. It should therefore be as good as it can be first time around. Other technologies (such as electrical fittings, lighting, appliances, space heating, water heating, ventilation and renewable power technologies), may be more easily modified or replaced during the life of the building as technology evolves.

2. High-performance, Passivhaus certified windows

This is an emphasis of the first principle on those building elements responsible for the highest proportion of heat loss. Passivhaus windows are typically triple-glazed with very low U-values (typically of 0.8 W/m².K or less).

Excessively cold window surfaces absorb heat from our bodies and create cold draughts. Excessively hot window surfaces radiate heat adding to our discomfort in summer months. Passivhaus certified, high-quality, triple-glazed windows moderate the internal surface temperature of the glass and frame so that it never feels too cold or too hot in close proximity. In colder months, the surface temperature of the glass and frame remain warm enough to remove the risk of condensation.

3. Air-tight building fabric

The most robust and long-term approach to reducing building energy consumption, is to follow a ‘fabric first’ design approach, that is, to reduce heat loss through the building envelope as far as practically and economically possible. In cool and temperate climates the majority of energy consumption in buildings is associated with space heating. By minimising heat loss through the building fabric (walls, floors, roofs, windows and doors), space heating demand may be substantially reduced. The fabric of a building is typically the most expensive and permanent element of construction, which can generates substantial CO₂ emissions during construction. It should therefore be as good as it can be first time around. Other technologies (such as electrical fittings, lighting, appliances, space heating, water heating, ventilation and renewable power technologies), may be more easily modified or replaced during the life of the building as technology evolves.

4. Thermal bridge free

Thermal bridges are weak spots in the building fabric where normal insulation levels are bypassed and heat escapes more rapidly to the outside air and ground. These weak spots typically occur at junctions between building components, for example at the perimeter wall-floor junction, around window and door openings and at junctions with external elements such as balconies and canopies.

Through early consideration of building form and ongoing careful attention to construction detailing, thermal bridges may be all but eliminated from Passivhaus buildings.

5. Mechanical Ventilation with Heat Recovery (MVHR)

Passivhaus buildings are fitted with accredited, high-efficiency mechanical ventilation systems with heat recovery as standard. MVHR systems perform many critical functions relating to energy efficiency, health and human comfort including:

  • At least 75% of the heat energy contained in the air extracted from the building is passed to incoming fresh air through a heat exchanger, substantially reducing heat loss. New MVHR units are over 90% efficient in transferring heat energy.

  • Incoming air is passively heated to within a few degrees of ambient room temperature and delivered at low-velocity to eliminate draughts.

  • In many units the heat exchanger may be bypassed in summer so that cooler air, particularly at night and early mornings, may be used to cool the building fabric and occupants.

  • Incoming air is filtered removing incoming pollutants such as pollen.

  • Continuous air extract removes pollutants from the building including

    • excessive moisture generated by occupants, cooking, showering, clothes washing and drying, etc.

    • carbon dioxide generated by the building’s occupants

    • harmful volatile organic compounds (VOCs) released from building materials, packaging, carpets, fabrics, etc.

    • naturally occurring radon released from the ground

Why employ a Certified Passivhaus Designer/Consultant?

Delivering a building to achieve such a high-standard, requires detailed knowledge and a rigorous, methodical approach to design and documentation.  Employing a Certified Passivhaus Designer ensures that your Building stands the best chance of performing to this standard. The figure below illustrates how early in the 7 RIBA work stages Passivhaus designers and Non-passivhaus designers consider in detail the key criteria of air tightness, thermal bridging, energy, comfort and overheating.

Please contact us to see how we can assist in planning for your Passivhaus project.