This Kingspan sponsored CPD takes a close-up look at the requirements of Passivhaus and how offsite approaches are making it increasingly viable and cost-effectivei
As we move towards net-zero building standards in the next few years, one of the most significant issues for the industry to address is closing the gap between the predicted energy performance of buildings and what they achieve once completed and occupied. Research has shown that the actual energy demand from buildings can be more than double that predicted at the design stage, undermining efforts to reduce carbon emissions and the cost savings expected by owners.
This performance gap can be caused by a variety of factors at different project stages, including inaccurate detailing, changes in specification, poor workmanship, and a lack of adequate building management information on handover. Occupier behaviour also has a big impact on building energy use. To overcome these challenges, it is vital to have greater communication between all stakeholders and a more rigorous approach to how work is considered, carried out and assessed. This has led to increased focus on the Passivhaus Standard – a voluntary energy performance standard with exacting requirements for fabric performance, airtightness and the elimination of thermal bridges to deliver buildings with extremely low measured energy demand.
Once considered a niche approach restricted to energy conscious self-builds, the standard is increasingly being adopted on larger-scale projects and housing developments, most notably the Stirling prize-winning Goldsmith Street in Norwich.
In this CPD, we will take a closer look at the requirements of Passivhaus and how offsite approaches are making it an increasingly viable and cost-effective solution.
Brief history of Passivhaus
While passive building approaches have been an essential component of construction since the earliest structures, their rise in the modern context came as a result of rising energy costs following the oil crisis in the 1970s. Throughout the 1970s and 1980s a number of research projects were established to consider building methods that combined highly insulated and airtight constructions and mechanical ventilation. The Passivhaus Standard itself was developed during the late 1980s by two scientists – Dr Wolfgang Feist and Professor Bo Adamson. Feist highlighted that concerns around the performance gap were a key driver in his own involvement. He said: “I read that the construction industry had experimented with adding insulation to new buildings and that energy consumption had failed to reduce. This offended me – it was counter to the basic laws of physics […] So I made it my mission to find out what [they were doing wrong] and to establish what was needed to do it right.”
The pair defined the core principles of the standard – setting clear limits for aspects such as overall energy demand and air leakage, to create properties that are comfortable and require very little energy to heat or cool. The first Passivhaus was completed in 1991. Since then more than 65,000 buildings have been designed, built and certified to this standard worldwide.
Performance requirements and certification process
To achieve certification under the Passivhaus standard, a typical European property must meet the criteria as shown in table 1. To put these figures in context, the maximum space heating demand for a Passivhaus building is about 10% that of the average UK home, which is estimated by the Passivhaus Trust to be around 140kWh/m2/yr. It is worth highlighting that, as Passivhaus is focused on reducing demand primarily through improvements to the building fabric, there is no requirement for renewable generation technologies such as photovoltaic panels.
In addition to the full Passivhaus standard, the Passivhaus EnerPHit standard has also been developed to provide a more practical and cost-effective approach for energy retrofit projects. The parameters within this standard are somewhat relaxed in recognition of the numerous additional challenges these projects pose. These include fixed building orientation, unusual architectural features and potential planning restrictions. European retrofits completed to this standard must reach the performance targets as shown in table 2.
|Primary energy demand||≤ 120kWh/m2 yr|
|Space heating demand||≤ 15kWh/m2 yr|
|Space cooling demand||≤ 15kWh/m2 yr|
|Specific cooling load||≤ 10kWh/m2 yr|
|Airtightness (n50)||≤ 0.6 air changes per hour|
|Primary energy demand||≤ 120kWh/m2 yr + heat load factor|
|Space heating demand||≤ 25kWh/m2 yr|
|Space cooling demand||≤ 25kWh/m2 yr|
|Specific cooling load||≤ 10kWh/m2 yr|
|Airtightness (n50)||≤ 1.0 air changes per hour @ 50 pascals|
Key design features
In order to meet these requirements, Passivhaus buildings feature a number of typical design elements. All construction elements will typically need to be insulated to a high level with U-values of at least 0.15W/m2 K and typically much lower. This means that the thermal conductivity (lambda value) of insulation materials is a key criterion during the specification process, as products with lower thermal conductivities will be able to reach the demanding U-value targets with a slimmer thickness. This can reduce construction depths and help to maximise internal space.
It is also a requirement that the building is fundamentally free from thermal bridging. To achieve this, close attention to detailing is crucial when designing the building and installing the insulation to ensure that potential thermal bridges around openings and at junctions, especially those between the wall and floor, are properly addressed.
As well as ensuring excellent insulation continuity at the junctions around windows and doors, these units themselves must deliver excellent thermal performance. Typically, this means installing triple-glazed windows and specially designed exterior doors. Care should be taken during the design phase to consider potential solar gains and to manage this either through adjustments to the orientation of the building, the size and location of glazing or through effective shading measures.
To maintain more consistent internal temperature and comfort levels, buildings must achieve air leakage rates no higher than 0.6 air changes per hour (ach) at 50Pa (Pascals) with a slightly more relaxed standard of 1.0 ach @ 50Pa for EnerPHit. According to BRE this is the equivalent of a hole the size of a five-pence piece for every 5m2 of building envelope. It is important to note that the testing methodology and expression of the airtightness for Passivhaus buildings, the n50 standard, is different from the q50 air permeability test used for Building Regulations compliance. Results from one method cannot be converted to the other. The required airtightness is typically achieved by installing an air barrier layer, such as oriented strand board (OSB), and airtight tape, which is applied to seal all junctions.
This highly airtight construction necessitates the use of mechanical ventilation to maintain a constant supply of fresh air. In most cases, this is achieved via a mechanical ventilation with heat recovery (MVHR) system. MVHR systems extract the heat from outgoing stale air and transfer it to warm incoming fresh air, further reducing the heating demand and ensuring a fresh, comfortable environment within the home. It is essential that this is accompanied with a clear and comprehensive ventilation strategy to ensure that the system performs effectively in all areas of the property, maintaining high levels of indoor air quality.
The standard provides a clear and rigorous assessment framework to ensure all certified buildings meet performance expectations. This process is supported through the Passive House Planning Package (PHPP), a bespoke energy modelling tool that helps designers to assess and compare specification options and ensure the final design is expected to comply.
On a typical project, a basic assessment of the initial building concept will be carried out by a Passivhaus consultant or the architect using PHPP. This may, for example, include default U-values for the different elements without the final insulation specification being made. This will then be assessed by the Passivhaus certifier with feedback provided to allow for refinements of the initial concept. This process may be repeated more than once if substantial changes are needed.
Once the design is developed, including the full specification, it is more thoroughly assessed in PHPP to allow for further optimisation. Again, the PHPP output and any supporting literature should be submitted to the Passivhaus certifier for approval. Only once the design has been fully assessed and approved by the certifier should any construction work begin. There should also be clear agreement about how any queries or changes in the specification will be communicated to and approved by the certifier during the construction work to ensure smooth progress.
Both during construction and on completion, the building should be tested to prove it meets key assessment criteria. For example, in the case of air leakage, it is recommended to carry out an initial pressure test once the air barrier is fitted but before any appliances and services are installed, to allow any defects to be easily identified and addressed. Further assessments should then be carried out once service penetrations are made, prior to installing fixtures and appliances to allow easy access, and on completion. The results are then sent to the certifier to ensure they meet the standard’s requirements before the certificate is issued.
As airtight construction is an essential requirement, and relatively unusual in UK construction, the BRE recommends an “airtightness champion” is appointed to take a lead on all work related to maintaining the integrity of the airtight barrier. The champion should be site-based throughout the build but should not have existing administrative commitments, in other words not be the project manager or the clerk of works. The champion should have a clear understanding of the air-leakage strategy and be able to communicate this to members of the team and ensure that all measures are correctly installed. They can also act as the point of contact with the pressure-testing specialists and should oversee any remedial measures that may be required following tests.
This thorough process, supported with the post-completion assessment, should provide clear assurance that properties will perform as expected. A recent study of 97 UK Passivhaus certified properties showed that, on average, there was no statistically significant energy performance gap for these properties. The standard therefore offers a proven candidate for off-the-shelf adoption for compliance with the proposed Future Homes Standard as a method whose as-built performance can clearly be demonstrated at scale.
The issue of how scalable the exacting requirements of Passivhaus are has previously been seen as a barrier to adoption – particularly given the deepening skills shortage within the UK construction sector. In recent years, however, the increased adoption of offsite methods has begun to change attitudes. These methods offer several key characteristics that make them well suited to meeting the demands of Passivhaus consistently across large developments and complex buildings.
How offsite can support Passivhaus construction
By moving production offsite and into dedicated manufacturing facilities, offsite building methods can offer a more straightforward and precise solution for almost all aspects of construction, including the building structure and envelope.
Offsite elements are typically first created digitally using computer aided design (CAD). Once these designs are agreed, they can be input directly into the fabrication process, meaning the final construction exactly matches the design. This level of precision is particularly beneficial for Passivhaus projects as it means that services can be accurately pre-planned and features such as windows and doors will fit exactly within the envelope. Similarly, junctions between the different construction elements can be pre-designed to allow improved insulation continuity. This can help to minimise air leakage and thermal bridges, streamlining on-site processes while ensuring the demanding fabric performance requirements are met.
The factory-controlled manufacturing process also supports improved scalability on larger developments. Multiple units can be precisely recreated from a single CAD file and production timescales can be accurately predicted. This allows for improved planning and can facilitate just-in-time delivery of components during the construction work, thereby minimising the need for on-site storage.
Site labour requirements are also reduced as, in addition to simplifying detailing, large sections of the building structure and envelope can usually be fitted by a small team of workers using mechanical lifting equipment. This can considerably reduce the time taken to make structures weathertight – allowing internal trades to begin work much faster than would otherwise be possible.
For example, research has shown that on a typical two-storey detached property built using structural insulated panels (SIP), construction can be made weathertight as much as eight weeks faster than with a conventional masonry approach. This time saving can be carried through to completion, potentially reducing preliminary costs such as plant, scaffolding and security, and limiting potential disruption due to adverse weather.
How Passivhaus principles are influencing building regulations
The recent consultations for the updates to the energy performance requirements for new homes in England and Wales (contained within Part L of the Building Regulations in each country) suggest that the success of Passivhaus is starting to shape future building standards. In particular, the more ambitious option within the Welsh consultation, which targets a 56% reduction over current emission levels, includes stricter U-value targets and airtightness requirements with the requirement for MVHR units.
While the consultations lack the rigorous post-completion assessments that are a key part of Passivhaus, they demonstrate a clear direction of travel towards this fabric-first approach to construction. By understanding Passivhaus principles and how offsite solutions can allow these to be met, it should be possible to close the performance gap within new buildings, delivering properties that are comfortable and well constructed and which will contribute to meeting the net zero carbon emissions target by 2050.