A record-breaking Passivhaus school could change building design

The driver is not about claiming titles, though – it’s all about improving energy efficiency and environmental performance. Done right, using the Passivhaus concept means buildings need 90% less heating energy compared to conventional designs. The concept is also renowned for delivering clear improvements in air quality and user comfort.

Scotland’s take on Passivhaus is not just about residential buildings, although the initial focus started out that way. The review of current energy standards was launched in 2022 and the scope has since been broadened to include public sector non-domestic buildings.

Passivhaus concept

While Scotland’s standard may be new, the Passivhaus concept itself has been in practice for 35 years. The model was developed in Germany by Wolfgang Feist and the Passivhaus Institute in Darmstadt, with the first house constructed to the standard built in 1991.

The Passivhaus standard aims to significantly lower a building’s environmental footprint, mainly through reductions in operational carbon emissions. Implementing Passivhaus design principles promotes resilience and adaptability in the face of changing climate conditions and rising energy costs. 

The scientific design standard has gained huge popularity and success in northern Europe, especially Germany, Austria and Scandinavia. Although intended to be adaptable to any climate, the standard has proved itself in colder climate conditions.

The growth of Passivhaus in the UK has been slower than in the rest of northern Europe. However, it is becoming more popular across social housing, schools and public sector projects, aside from self-build architect-led residential projects. Kier’s Passivhaus leisure centre for Exeter City Council won the 2022 Construction Manager of the Year award.

Its adoption by devolved governments will drive higher implementation rates.

Designing for Passivhaus

The fundamental design tool used in pursuing a Passivhaus building is the Passivhaus Planning Package (PHPP). The PHPP is a specialised design and calculation tool developed by the Passivhaus Institute to help architects, engineers and consultants design buildings that meet the standard. It allows for precise energy and comfort modelling from the earliest stages of design, which supports a project’s ability to meet Passivhaus certification. 

The PHPP promotes an integrated approach to design and construction, which calls for collaborative input from the project delivery team to achieve the desired building performance. One of the biggest learnings and benefits realised through Passivhaus is the value of early contractor involvement and cross-disciplinary collaboration. Agreeing on a concept design before RIBA Stage 2 is seen as critical to avoiding downstream issues and improving outcomes. 

Early engagement of a delivery team with Passivhaus expertise helps to embed performance goals across the project lifecycle, enhancing assurance and alignment between client, architect and contractor. Additionally, this supports buy-in across all levels of the design and contractor teams, enabling engagement and ownership at all levels of design, installation and commissioning. 

Compared to other tools, the PHPP is primarily used for early-stage design. Tools such as Standard Assessment Procedure energy model (domestic) and the Simplified Building Energy Model (non-domestic) are used primarily to demonstrate compliance with UK building regulations. 

Design stages

While the Scottish government’s take on the Passivhaus standard is yet to be published, understanding what needs to happen at each stage of design and construction on other projects indicates what might be involved.

A systematic approach throughout the design and construction is essential for achieving and maintaining the required performance standards.

The design phase should begin with preliminary energy modelling, using software to simulate building performance in terms of energy efficiency, thermal comfort and air quality. Key passive design strategies, such as orientation, thermal mass, insulation levels and shading, should be incorporated to enhance energy performance. High-performance, triple-glazed windows and airtight, thermal bridge-free door systems are selected, and a ventilation system with heat recovery is designed to ensure optimal indoor air quality while minimising energy consumption.

The PHPP needs to be prepared and continually updated to document all design decisions and ensure they align or balance with Passivhaus criteria. Collaboration with stakeholders, including architects, engineers and contractors, needs to be initiated early to ensure understanding of rigorous Passivhaus requirements. 

Attention to the details

A detailed thermal bridge analysis should be conducted to minimise energy loss through structural connections. Materials that meet Passivhaus performance criteria, focusing on sustainability and durability, need to be selected and specified at this stage. Detailed drawings for airtight construction practices and connections created during this stage ensure clarity for the construction team. 

During construction, robust quality control and site inspections should be implemented to ensure delivery matches the Passivhaus design intentions. Preliminary airtightness tests are carried out during construction as early warnings for potential issues at completion. 

Finally, once complete, a comprehensive review of building performance against all Passivhaus criteria is carried out using the PHPP results. All evidence, including airtightness test results, commissioning reports for systems and compliance certificates, is assembled for review by a Passivhaus certifier.  

Hurdles to be overcome

Scotland’s new standard will invite building designers to think differently over the next couple of years, before they are forced to with the mandating of the new standard. Nonetheless, the barriers that have prevented Passivhaus becoming as common in the UK as it is in northern Europe still persist and the Scottish building market will need to overcome these hurdles.

A major barrier to the success of Passivhaus is cost. The capex of projects can present a significant obstacle to achieving the required targets. In the UK, especially, this is further exacerbated by the increased operational costs due to decarbonised electrical systems. But the opportunity for long-term energy savings, reduced carbon footprint and enhanced building performance can offset the seemingly prohibitive additional costs.   

An interesting insight and comparison between the cost of a grid connection upgrade for the electrification of heat for decarbonisation and the increased capex that is realised through designing to Passivhaus standard, suggests that a grid upgrade could now be more costly. This cost factor raises the question about the prioritisation of pure decarbonisation over the energy efficiency and resilience that concepts like Passivhaus develop.

The skills gap

Other barriers identified include the skills gap and the supply chain, with a shortage of skilled workers, designers and, most importantly, contractors able to deliver projects to Passivhaus standards. Schemes will also be limited by the availability of materials and products to meet performance requirements.

The greatest perceived risk to the standard, however, is the one it intends to mitigate: climate change. With global temperatures rising and climate changing, the ability of Passivhaus buildings to deal with overheating is a fundamental design challenge that forms part of the standard’s requirements and design criteria.

The experience gained in the Scottish market as it contends with these challenges will be invaluable and, if successful, are likely to drive an acceleration towards Passivhaus standards. Greater uptake will be positive news for the environment, energy bills and the wellbeing of occupants.

Fraser Reid is principal net-zero carbon consultant at Mott MacDonald.