Passivhaus standard: comparison with traditional designs

10 October 2016

Paul Mallion explains how the German Passivhaus standard can result in fuel savings and greater comfort compared to more traditional designs

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Studying the links, on the relationship between indoor air quality and asthma, will concern surveyors involved with energy-efficient buildings. How do we create efficient, comfortable buildings without creating air-quality problems?

Studies into completed housing projects show that most new dwellings fail to live up to the energy standards to which they were designed, creating a so-called ‘performance gap’ that increases the risk of homes being inadequately heated and ventilated, according to the Joseph Rowntree Foundation.

The gap is caused by a multitude of small failings, such as thermal bridges, gaps in insulation, air leakage caused by poor design and construction, over-optimistic U-value calculations and inaccurate energy assessment methods, such as the Standard Assessment Procedure and the resulting energy performance certificates (EPCs).

Passivhaus is only an energy and comfort standard, setting no restrictions on the construction method that can be used

In the 1980s, Professors Wolfgang Feist and Bo Adamson considered why low-energy dwellings failed to meet expectations in Germany and Sweden, and their research led to the construction of four trial houses in 1991. They called their approach Passivhaus, later establishing the Passivhaus Institute, and defining a true Passivhaus as:

'A building for which thermal comfort can be achieved solely by post-heating the fresh air mass required to achieve sufficient indoor air quality conditions without the need for recirculation of air.'

Therefore, to tackle the failings that affect air quality and energy efficiency, could the UK adopt the Passivhaus standard as an alternative to Building Regulations Part L (Conservation of fuel and power) and Part F (Means of ventilation) compliance?

Passivhaus principles in practice

I incorporated Passivhaus principles into a new conference centre in 2005, the Pines Calyx, built under the guidance of David Olivier of the consultancy Energy Advisory Associates. Initially, I was sceptical about achieving U-values of 0.15W/m2K or better for all the thermal elements, using triple-glazed windows and creating an airtight envelope. Olivier’s attention to thermal bridges and airtight details required a total rethink of common junctions such as window jambs, thresholds and eaves. However, the result was a 390m2 building that could be heated by the equivalent of a single radiator.

Key principles of the Passivhaus standard Maximum U-value for walls, floors and roofs is 0.15W/m2K, though it is less if the building has an inefficient surface-area-to-floor-area ratio. Windows and doors usually need to be triple-glazed with a U-value of 0.8W/m2K, averaged between the glass, glazing spacer and frame. All thermal bridges must be less than 0.01W/m2K, known as a psi value; if greater, they must be included in the calculation. Thermal bridges are weaknesses in the insulation where it is not continuous, such as wall ties and junctions. They often occur at internal corners, resulting in condensation and mould growth. The Building Regulations allow bridges 15 times worse than Passivhaus levels. An efficient mechanical ventilation system with heat recovery is needed, supplying fresh outdoor air to each habitable room, extracting damp from wet rooms; minimum heat-recovery efficiency is 75% and fresh air supply at 30m3 per person per hour, or a whole-house ventilation rate of 0.3 air changes per hour. Airtight construction should be tested to a maximum of 0.6 air changes per hour using a blower door at 50 pascals, under both positive and negative pressure. Under current Building Regulations, a dwelling can be completed without testing for air leakage. The regulations allow a leakage rate equivalent to around 15 air changes per hour, which is 25 times worse than Passivhaus levels. Overheating is prevented, with temperatures above 25°C occurring for no more than 10% of the year. Heating may be provided simply by warming the ventilated air or using any efficient system. Following the Passivhaus principles will result in a building with a maximum heating demand of 15kWh/m2 annually, or a heating load of 10W/m2. To put this into perspective, a typical three-bedroom UK house uses around 180kWh/m2 a year and still fails to provide comfort and sufficient fresh air.

I still believe Passivhaus offers the best voluntary energy and comfort standard available, and the only one that accurately calculates the fresh air supply, extraction rates and thermal bridges to combat condensation and mould growth by maintaining the temperatures of internal services above 12.6°C.

Unlike the Building Regulations, EPCs and the Code for Sustainable Homes (the code), there is no way of cheating the software. Calculations are thorough and can be time-consuming, but the rigorous approach has proved to be statistically accurate for thousands of completed buildings around the world.

Passivhaus is only an energy and comfort standard, setting no restrictions on the construction method that can be used: the Pines Calyx conference centre was built of rammed chalk, yet meets the thermal insulation requirements thanks to its external insulation system.

Constructing to higher standards can have cost implications, although experienced Passivhaus designers in mainland Europe argue that these will become neutral in time. Commercial buildings and schools can even cost less than conventional buildings, because the need for complex heating and cooling systems and advanced building controls can be avoided.

Savings and comfort

Cost savings on fuel are significant and last for the lifetime of the building. A 160m2 timber-framed, detached, three-bedroom Passivhaus that I designed in Dorset has a peak heating demand of 1.9kW at an external temperature of –1°C, the equivalent of heating the whole house with a hairdryer. A similar house built to Building Regulations might need a 20kW boiler.

We have low expectations of comfort from our homes due to the UK’s ageing housing stock and lack of awareness of the alternatives. A Passivhaus avoids cold surfaces and draughts and provides perfect levels of fresh air 24 hours a day.

Building to the Passivhaus standard requires a greater level of competence from the designer, more collaboration with suppliers and knowledge of windows, ventilation, insulation and airtightness products. There are a number of training courses, numerous certified products and building systems, and established building certifiers.

Figure 1: Pines Calyx Conference Centre

Designing to the Passivhaus standard can reduce energy consumption by 80–90% compared with conventional housing, providing continuous fresh air and ensuring that condensation cannot occur. Passivhaus does not cover water usage or sustainable materials, a void left with the end of the code.

However, guidance is available from the Association for Environment Conscious Building, the Alliance for Sustainable Building Products and the Passivhaus Trust, which was established by the AECB to promote the standard in the UK. These bodies offer training, guidance, open days and a register of members.

BRE’s Home Quality Mark may address some of the problems with indoor air quality and the performance gap, but must be totally open to scrutiny, with no commercially sensitive hidden detail – a criticism that is often levelled at BRE’s Green Guide.

Paul Mallion FRICS is Director of Conker Conservation Ltd and a certified Passivhaus designer

Further information

• Related competencies include Environmental assessment, Housing management and policy, Housing strategy and provision
• This feature is taken from the RICS Property journal (July/August 2016)