Passivhaus: creating a roadmap to net zero carbon

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.

What is Passivhaus?

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:

• A heating energy demand of < 15 kWh/m²/yr or a maximum required heating power of 10 W/m2
• Avoid overheating or have a cooling system demanding < 15 kWh/m2/yr
• Achieve an air tightness of ≤ 0.6 air changes/ hr @ n50

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.

Bryden Wood’s 10-step approach to Passivhaus building design and construction

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:

1. A compact shape with low building envelope to volume ratios.
2. Building orientation to benefit from useful solar gains.
3. Optimised window sizes to capitalise on useful solar gains, reduce heat losses and deliver exemplary levels of natural light into the building.
4. Highly insulated walls, floors and roofs.
5. Site specific shading to mitigate summertime overheating, whilst allowing winter solar heat gains.
6. High performance triple glazing with thermally bridge- free heads, jams and sills.
7. A thermal bridge-free building via detailed design of junctions, and clear and easily buildable air seal lines, to deliver the specified air permeability.
8. Mechanical ventilation with heat recovery (MVHR) and low-pressure loss duct systems to minimise heating demand.
9. High-performance air source heat pumps for heating and DHW.
10. Renewable technology, including building mounted photovoltaics, solar thermal or small-scale generators and consider the use of battery storage for improving payback periods of building mounted renewables.

Figure 1. Recommended design measures for the achievement of Passivhaus performance

What are the benefits and challenges of adopting Passivhaus?

These are the main benefits of adopting a Passivhaus standard:

• A well-recognised quality mark: Passivhaus is a well-recognised, international, sustainable design standard which is seen by professionals and non-professionals alike as guaranteeing a level of environmental excellence that goes well beyond national standards.
• Performance gap reduced to minimum: Its all-encompassing energy assessment methodology and bespoke design software (Passive House Planning Package) offers a different approach to the compliance-focused UK energy modelling, which results in performance gap being largely avoided.
• Ultra-low energy/carbon and lower operational costs: Passivhaus designs achieve low-energy consumption. Rapid grid decarbonisation and the use of high efficiency heat pumps results in extremely low carbon emissions and gives clients the option of going net zero in operation through the use of additional PV or green electricity tariffs. When coupled with building mounted renewables, it can deliver low to non-existent energy bills to occupants, with some designs capable of being cost negative over the course of the year.
• Excellent Occupant Experience: Both Passivhaus and Bryden Wood believe that environmental performance should not come at the expense of occupant experience. This is reflected in the designs targeting both the Passivhaus Thermal Comfort criterion as well as CIBSE TM 59, which ensures adequate thermal performance.
• Resilient and adaptable to different climate conditions: Passivhaus also provides additional resilience by adopting a design based on passive design strategies coupled with technological solutions that can easily be upgraded, refurbished and replaced. Whilst Passivhaus was originally developed for residential buildings in Germany, the standard and its principles can be adopted in all kinds of building types, locations and climatic scenarios, and respond to the effects of global warming.

However, there are four important challenges for the adoption of Passivhaus:

• Increased design cost and complexity: It requires specific detailing of the envelope, high quality of construction and specific performance testing in order to get the building certified.
• Increased capital cost: The additional construction capital cost can vary from 8% in the most optimistic scenarios (Passivhaus Construction Costs – October 2019) up to 25%.
• Increased programme: Delivering a Passivhaus building can take longer. Quality control on site is extremely important, strategies for airtightness require careful sequencing and interim testing and workmanship skills need to be of the highest quality.
• Skill shortage and understanding. There is a lack of large-scale builders who can deliver Passivhaus buildings. These projects can be seen as bespoke and are more expensive.

Bryden Wood’ response to Passivhaus’ challenges

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.

Reduced programme:

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.

Increased skill/knowledge/preparation:

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.

Passivhaus and net zero carbon challenges

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:

• Adding triple glazing instead of double glazing would increase carbon around 6 kgCO2/m2 over its lifetime
• Adding a MVHR and ductwork would increase carbon by 6.5 kgCO2/m2
• Adding a heat pump would increase carbon by 3.5 kgCO2/m2
• Adding insultation (rockwool) thickness in walls from 150mm to 250mm of rockwool would increase carbon by 3.6 kgCO2/m2
• Adding insultation (rockwool) thickness in floor and ceiling from 190mm to 370mm of rockwool would increase carbon by 6 kgCO2/m2
• Reducing the boiler size from a 15kw electric boiler to a 3kw boiler would reduce carbon by 1.69 kgCO2/m2
• Reducing the number of radiators from 10 to just one would reduce carbon by 10.7 kgCO2/m2
• Reducing the size of the photovoltaic array from 24 to 12 units would reduce carbon by 12 kgCO2/m2

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. 

Conclusion

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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