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Passivhaus is an internationally recognised, sustainable building standard, which delivers resilient, low energy designs with the highest air quality and thermal comfort experience, and a performance gap-free design. Whilst Passivhaus offers multiple environmental and operational benefits to the users, it also creates a series of challenges, including an increased design and capital cost, longer programme and the need for skilled workforce.
Bryden Wood’s unique, 10-step design approach to Passivhaus, and our adoption of Platform Design for Manufacture and Assembly (P-DfMA), facilitates the achievement of stringent Passivhaus performance targets, creating the perfect response to reduce construction cost and programme, whilst also responding to the labour skills shortage. At the same time, our innovative approach facilitates a well-integrated design that addresses the complexities of Passivhaus via high-quality fabrication.
Passivhaus is a well-established, international building performance standard that delivers resilient, ultra-low energy consumption buildings, whilst maintaining the highest levels of occupant thermal comfort and air quality experience.
This is delivered through a keen focus on a fabric first approach that seeks to reduce space heating demand which can be met either through useful solar gains, internal gains or via modern, high efficiency and low carbon heating systems.
The facade performance in a Passivhaus building goes well beyond current UK Building Regulations, though a combination of highly insulated walls, high-performing windows and by ensuring “thermal bridging” around windows, doors and junctions is reduced to as close to zero as possible. This contrasts with traditional building construction, where these elements account for upwards of 10% of the building’s heat loss.
The design must achieve the following targets to gain Passivhaus certification:
The building design also needs to achieve low primary energy & renewable demand. This is a combined target and is based on the energy consumed by building systems and the renewable energy generated by building mounted wind, photovoltaics or solar thermal systems.
For the above reasons, Passivhaus is a suitable standard for clients and developers that seek a well-established, sustainability standard to deliver low energy buildings with the highest construction quality, aspiring to net zero carbon in operation.
Whilst there is no fixed formula to Passivhaus, Bryden Wood proposes the achievement of the above targets via the following 10 steps, which relate both to design decisions and construction specifications for improved sustainability:
Figure 1. Recommended design measures for the achievement of Passivhaus performance
These are the main benefits of adopting a Passivhaus standard:
However, there are four important challenges for the adoption of Passivhaus:
Bryden Wood is an industry leader in the adoption of Platforms-based Design for Manufacture and Assembly (P-DfMA) and has always shown a keen interest in the adoption of the newest and more advanced sustainability certification schemes, such as Passivhaus.
Our experience indicates that the standardisation process of P-DfMA, can be a suitable approach to counterbalance Passivhaus’ challenges described above, as follows:
Reduced design cost and complexity:
The standardisation process reduces the number of construction elements which in turn dramatically reduces the number of elements and potential thermal bridges, making the design simpler and more cost-effective. The repeated use of the same details significantly reduces the design cost and complexity, and facilitates the achievement of Passivhaus’ thermally bridge free design ethos.
Being able to deliver Passivhaus detailing onsite requires complex coordination, overlap of materials and components and accuracy in order to achieve the specified final performance and certification. A P-DfMA approach ensures the highest quality, working with reduced tolerances which align with Passivhaus requirements.
Reduced capital cost:
The standardisation process reduces the volume of materials and the number of construction elements which in turn reduces the capital cost of the project. The repetition of components at a large scale can reduce the cost of standardised elements.
A Platforms digital workflow delivers tremendous improvements to the procurement process. Having all data centralised in a BIM model provides instant access to both the cost and availability of a project’s components, and creates a more direct relationship with the supply chain cutting down transaction costs.
The design of each assembly and junction can be pre-tested and individually certified to Passivhaus standards before being included within any design. This approach facilitates the pre-certification of components, panelised systems, building systems for thermal/hygrothermal/airtightness performance, reducing design time and programme.
The repetition of detailing between projects means that a design library of pre-certified components is retained for future projects, making the design process more efficient and quicker.
Achieving Passivhaus performance can require an iterative design process to ensure performance during the final stage of construction inspection and testing is achieved. This iterative process can be shortened using BIM integration and digital twins which are inherently part of a DfMA approach.
Our experience has shown that the automation and design of P-DfMA processes simplifies the construction and the need for a skilled taskforce and their preparation. It also reduces the number of people onsite, increases safety as a result of reduced work at height, lowers capital costs and improves construction speed.
There is an increasing amount of pressure growing in the construction industry to design net zero carbon buildings, both in terms of operational and embodied carbon. In this context, bodies such as the London Energy Transformation Initiative (LETI), RIBA, GLA and UKGBC, have developed guidance documents on embodied carbon, which include specific targets and roadmaps to achieving net zero carbon prior to 2050.
Based on LETI Climate Emergency Design Guide, a typical medium size residential building embodied carbon, would be 33% of the total carbon, whilst the operational carbon would be around 67%. However, for an ultra-low energy building, like Passivhaus, the breakdown would be 77% embodied and 23% operational and this balance is likely to become more enhanced with the decarbonisation of the grid. This means that embodied carbon is becoming a more important focus for the sustainable design of buildings.
Figure 2. Typical operational and embodied carbon breakdown for medium scale residential for a standard building (left) and for an ultra-low energy building
Passivhaus standard has always been focused on operational energy, and it is only in recent years that the focus has grown to both operational carbon emissions and the embodied carbon within the building.
Operational carbon in sustainable building design
From an operational carbon perspective, Passivhaus’ low energy targets mean the dwellings are likely to achieve very low carbon emissions. As a result, it becomes technically and financially feasible to offset any carbon emission through the use of building mounted, renewable technologies. This means that for certain types of residential buildings, it is possible to achieve net zero operational carbon without the need for a PPA. For non-residential buildings however a PPA is still necessary. Due to low energy demand, any price increases associated with PPA or offsetting, achieving net zero carbon becomes affordable.
This is in contrast to a UK Part L1a compliant building, which in order to become net zero would require a PV array larger than the area available on its roof. The difficulty in installing adequate PV on multi residential developments will be even greater. The only immediate pathway for these dwellings to become net zero is to invest in either a zero carbon PPA or carbon offset scheme, both of which come with significant increases in the cost of purchased electricity.
Embodied carbon in sustainable building design
On one hand, a Passivhaus building will need triple glazing, additional insulation and airtight membranes. The heat pump may contain refrigerants with high global warming potential and the MVHR unit will require insulated ductwork. Much of this may be made out of materials with high embodied carbon such as aluminium or blown plastics. This additional material volume becomes additional embodied carbon.
On the other hand, a Passivhaus design tends to be a more compact shape, thus less materials used. Due to its more efficient envelope performance, a Passivhaus building needs a small heating system, and due to its reduced energy demand, it requires a smaller PV array. These characteristics, when coupled with a focus on procuring low embodied carbon materials and equipment, can deliver objectively low embodied carbon designs, despite the additional material volume.
Based on the above, it can be observed that some of the inherent characteristics of Passivhaus increase embodied carbon whilst others reduce it. Taking a 200 m2 house, (10 m x 10 m x 2 storeys, 40 % WWR) as an example, Bryden Wood has done a rough estimation of the impact that Passivhaus distinctive strategies have on embodied carbon:
All the above items added up together would mean just a reduction of around 4.7 kgCO2/m2, mainly due to the simplification of the heating and photovoltaic systems. Compared to a residential LETI 2020 (Band C) target building (A-C) with a total embodied carbon of 675 kgCO2/m2, that is equivalent to just 0.7% reduction in carbon.
Figure 3. Comparison of embodied carbon (A-C) between a baseline residential building based on LETI Band C and same building with Passivhaus characteristics
The adoption of the above Passivhaus standard does not have a substantial impact on the embodied carbon compared to a standard residential building. The adoption of Passivhaus does not prevent the incorporation of additional strategies to reduce embodied carbon and all designs retain the potential to achieve low embodied carbon performance if it is part of the design intent.
Further potential benefits from Passivhaus arise from the compact shape and the use of timber, although full life cycle analysis is required to quantify this. The compact shape is predicted to reduce the absolute quantity of materials whilst timber is a material with low embodied carbon which can be ultra-low depending on its end-of-life treatment.
Timber shows its maximum potential if it can be continuously reused at the end of a buildings’ lifecycle. If it is burnt or sent to landfill it will release CO2 and methane to the atmosphere, losing its properties as a heat sink. In order to enable timber to be continuously reused, the building should be designed for deconstruction. Most Passivhaus buildings have not been designed for deconstruction in part due the complexity of junctions and the need to achieve the required overlapping and airtightness. This is however possible with the implementation of DfMA which can design assemblies that meet the stringent envelope performance requirements and can also be disassembled.
Passivhaus is a sustainable building certification standard that reduces operational energy and carbon emissions with minimum performance gap and achieves high levels of thermal comfort and air quality.
Bryden Wood’s P-DfMA approach to building design offers multiple synergies with Passivhaus since it is able to reduce construction programme, cost and design/construction complexity, and labour skills which are some of the inherent challenges of adopting Passivhaus.
Whilst the Passivhaus approach is focused on operational energy/carbon, there has been a keen interest in the industry to understand if this standard favours or penalises embodied carbon. Bryden Wood’s analysis shows that the impact of the adoption of a Passivhaus system has a minimum impact in terms of embodied carbon.
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