What makes a passivhaus?

In recent years the term ‘Passivhaus’ has become a byword for an ‘eco-friendly’ dwelling. The evolution of the energy-saving home has taken leads from Scandinavian and mainland European high-insulation standards, through exemplar houses by Brenda and Robert Vale (among others), to the UK Code for Sustainable Homes Initiative, that was intended to enshrine improved energy performance in legislation but which was withdrawn in March 2015. Since then, the UK government has instead introduced new optional building regulations for access and water efficiency (see our recent Building Regulations update article), and the Building Research Establishment has launched its own voluntary scheme (the Home Quality Mark) in an attempt to fill the void left by the Code for Sustainable Homes, and time will tell whether it gains sufficient popularity to stand the test of time. Through all of these initiatives, however, Germany’s Passivhaus standard (or, literally, Passive House) has been a constant benchmark since its creation in the late 1980s in Germany and Sweden. This article explores the defining characteristics of the Passivhaus standard.

What is a Passivhaus?

Passivhaus is a voluntary standard for achieving high energy efficiency in a building. Its use isn’t restricted to houses, although it is most commonly associated with them. Rather than being a ‘bolt-on’ test that is applied to a design, rather it is the integration of passive environmental design techniques into the architectural design process.  The key features of building that have been designed and constructed to meet the standard are that they need minimal energy for space heating or cooling, and result in high levels of occupant thermal comfort.

Passivhaus isn’t just limited to new buildings, either – it can also be applied to refurbishments as (EnerPHit to refurb and retrofit projects)

What are the defining characteristics of the Passivhaus standard?

The requirements of the Passivhaus standard are as follows:

• Achieve an annual heating and cooling demand of no more than 15kWh/m² per year, or a maximum peak load of 10W/m².
• The total energy consumption (‘primary’ energy for heating, lighting, hot water, power) must not exceed 120kWh/m² per year.
• Air leakage must not exceed 0.6 air changes per hour at 50Pa test pressure.
• Thermal comfort must be achieved in all living areas throughout the year, with temperatures not exceeding 25oC for more than 10% of the hours in a given year.

In addition, it is recommended that the specific heat load (for the building’s heating) should be limited to no more than 10W/m², at the design temperature.

In achieving the above standards, Passivhaus-compliant buildings are capable of being heated or cooled by less conventional methods.  Rather than a LTHW system served by a heating boiler, for example, the minimal supplementary space heating required can be provided by for example a heat pump (ground or air source), assisted by means of a high-efficiency MVHR system.  This in turn can provide additional benefits, as discussed below.

What are the benefits of a Passivhaus?

There are a number of benefits that designing to Passivhaus standards can bring:

• Reduced energy bills – high levels of insulation and air tightness eliminate the need for traditional heating systems.In the UK, it has been calculated that a dwelling built to Passivhaus standards would use 23% of the energy of an equivalent dwelling constructed to comply with the 2006 UK Building Regulations for space heating, 41% of the energy for primary energy, and 50% for hot water (source: PEP Project ‘Energy Saving Potential’ report 2006 (.pdf)).
• Improved indoor air quality – the combination of air tightness and a mechanical ventilation system can keep internal humidity at a level between 30 and 60%.Moisture- and odour-rich air from kitchens and bathrooms is extracted, and fresh air introduced into habitable rooms such as living spaces and bedrooms.Filtering of the air ensures that pollen and other allergens are removed before input into the building.
• Improved levels of comfort – the Passivhaus Institut resource ‘Passipedia’  defines thermal comfort as being established when heat released by the human body is in equilibrium with its heat production.Fanger’s comfort equation is derived from this principle, and is documented in BS EN ISO 7730:2005 ‘Ergonomics of the thermal environment - Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria’. Broadly speaking, the four comfort factors of air temperature, radiant temperature, air speed/ turbulence and air humidity, in combination with one another can create a ‘comfort range’ where the level of comfort is considered to be very good. Passivhaus-compliant buildings can achieve these levels of comfort by virtue of the standards required and the manner in which they are achieved (i.e. mechanical ventilation, insulation, air tightness etc.).
• Build quality, durability – reduced mechanical heating plant means that there are reduced maintenance costs; and reduced air leakage can only be achieved by careful and diligent construction, with great attention to detail. The risk of latent building defects should therefore be reduced.
• Improved acoustic insulation – reduced air leakage, combined with high levels of insulation and efficient glazing have a knock-on effect to acoustic performance, which can exceed the standards required by the Building Regulations.
• Reduced energy footprint – while there is still likely to be the need for some residual heating and cooling, it will nevertheless mean a significant reduction in demand on the national grid.
• Financial return - costs in use of a Passivhaus are likely to offset the increased capital cost of construction. The Passivhaus Institut maintains a database of all certified schemes, for ongoing monitoring and assessment purposes. Furthermore, wider social implications such as reduced fuel poverty and improved health in social housing can reduce the demand on health services.

How do you achieve the Passivhaus certified standard?

A Passivhaus can be planned, designed and verified by use of the Passive House Planning Package (PHPP), which is a spreadsheet-based simulation/ calculation tool for setting and achieving the Passivhaus targets.  A combination of low-energy technology and construction techniques (including passive techniques) all contribute to compliance with the standards, and can include the following:

• Passive solar design – orientation of the building relative to sun path; positioning and size of windows to utilise/ combat solar gain (as appropriate to the climate or the season); shading of windows from the sun; integration of thermal mass to act as a thermal heat store.
• Energy efficient landscape design – vines or other climbing plants, or trees to provide shading; vegetation to absorb heat (as opposed to hard surfaces that can reflect heat onto surrounding buildings).
• Superinsulation – increased quantities of highly-efficient insulants to walls, floors and roofs, with U-values typically in the region of 0.10 to 0.15 W/m²K.Eliminate all thermal bridges.
• High-performance glazing – often triple-glazed, with low-emissivity and solar reflective coatings and argon- or krypton-filled cavities. U-values typically in the region of 0.70 to 0.85 W/m²K (including the frame).
• Air tightness – achieving low levels of air leakage requires diligence when sealing construction joints and service penetrations. However, this is required to ensure that air movement is carried out through a heat exchanger in order to minimize heat loss.
• Ventilation – employ passive natural ventilation (including stack effect with clerestorey glazing) where possible. Climactic conditions may not always make this possible, in which case high-efficiency MVHR is required.Earth warming tubes can act as a natural form of heat exchanger for intake air.
• Space heating – depending on location the need should be minimal, and will often include the use of a MVHR system to recover waste heat from occupants, appliances and light fittings etc.  Passive solar gain as described above will also contribute to the heating.
• Lighting and electrical appliances – passive solar design will again play a significant part in the natural light of the building. Avoiding deep-plan designs (or using skylights or suntubes) will also reduce the need for artificial light. Specify high-efficiency luminaires such as LED lamps, and electrical appliances that have been independently tested and certified to meet high energy-efficiency standards.

Examples and data

There are now many examples of buildings constructed to the Passivhaus standard.  According to the UK Passivhaus website, in excess of 30,000 buildings have been built around the world (source: www.passivhaus.org.uk/standard.jsp?id=122 ), of which some 250 had been completed in the UK by the end of 2013 (source: http://passivhausbuildings.org.uk/passivhaus.php ).  The Passivhaus Trust maintains a database of built examples in the UK, and the UK Passivhaus Awards 2015 included examples as diverse as social housing, university laboratories and offices. In the retrofit category, a 1960s-bulit flat above a shop was converted into an office, utilising triple-glazed windows and a self-balancing MVHR system. Common to all however is the concept of ‘super insulation’ i.e. extra-high levels of insulation to provide a thermally-efficient building envelope.