Key risks to be managed when aiming for Passivhaus Certification on large-scale projects
It is possible to achieve Passivhaus certification on any type or size of development. However, the scale and complexity of a project can present additional challenges and risks that will need to be carefully considered.
To meet the Passivhaus standard, a space heating demand target (kWh/m2.year) must be met. This target is accompanied by a number of other requirements on primary energy, overheating and airtightness. Crucially, the standard does not define how these requirements are achieved, so there are no restrictions on construction types or products. As long as a building has high enough levels of insulation and airtightness with appropriate passive cooling strategies and building services design, there is no reason why certification can’t be achieved, regardless of scale or complexity.
For more complicated sites with more than one building, a key decision has to be made on the approach to certification. Discrete buildings are often certified separately, but certain areas within a building could be excluded if they have unusual heating or cooling requirements. A larger building could also be divided up for certification purposes, allowing linking areas to be excluded if they are partially open to the elements or heavily glazed. The best approach will depend on the specifics of the spaces and how they will be used. This should be discussed with the Passivhaus certifier as soon as possible so the correct strategy can be adopted from an early stage.
Five key areas of risk to manage throughout the process
1. Communication between separate workstreams
The Passivhaus standard takes a holistic view of a building, incorporating the building fabric specification, geometry, structure (thermal bridging) and building services. All of these areas have an impact on Passivhaus certification, so workstreams that would usually run independently must be brought together. The Passivhaus Consultants take on responsibility for doing this, by collecting relevant information and informing the rest of the design team of the implications.
However, on larger scale projects it is vital that all designers are aware of what is or is not important from a Passivhaus design and certification perspective. This will enable them to raise areas of change or concern quickly and efficiently with the Passivhaus Consultant. In order for this smooth process to be achieved, workshops could be arranged early on in the project to brief designers on Passivhaus requirements and areas of their work that will be relevant. An understanding of the principles from all who are involved is invaluable as a project progresses.
In addition to an initial workshop, regular design team meetings with all in attendance promote good communication and will ensure that potential issues are raised early. They can then be resolved efficiently and inexpensively while maintaining the high-performance levels required by Passivhaus.
2. Building services design
For larger commercial buildings, the main barrier to achieving Passivhaus certification often comes in building services design. A favourable form factor (ratio of external envelope to floor area) means that the space heating demand can often be met quite easily, but designing spatially acceptable and compliant building services is much more challenging.
Passivhaus ventilation requirements are particularly stringent. To achieve certification in the UK climate, it is almost certain that mechanical ventilation with heat recovery will be required. This means both supply and extract ducting, and a centralised air handing unit or units. Heating and cooling design can also require a non-standard approach due to low demand and the extra consideration required for wild heat gains.
It is essential that the building services designers work closely with the Passivhaus Consultant to produce compliant designs. Engaging a building services consultant with prior experience of designing services for a certified Passivhaus building is recommended.
3. Managing the Primary Energy demand
The Passivhaus standard focuses mainly on the building fabric and space heating demand. However, the primary energy demand takes all energy consumption into account. This includes electrical equipment such as lighting and computers. In a residential scheme this is inconsequential, as space heating represents a large proportion of total energy consumption. This is not necessarily the case in commercial buildings such as offices, schools or universities, which often have large lighting and IT demands.
In these cases, meeting the primary energy requirement can be extremely challenging. The specification of loads relating to office and other electrical equipment should be defined early, so suitable assumptions can be made and the building specification can be developed with this in mind. In certain cases, dispensation has been allowed by the Passivhaus Institute for buildings with an unusual and unavoidably high primary energy demand.
4. Airtightness & airtightness testing
The Passivhaus airtightness requirement of 0.6 ach during a 50Pa pressure test is the only metric that is quantified on-site rather than through modelling. Failure to meet this standard means that certification cannot be achieved. Careful planning at the design stage, diligence on site and robust quality assurance are all crucial to achieving the airtightness target. ‘Toolbox talks’ can be used to educate and inform the site team.
A suitable airtightness testing strategy should be developed as soon as possible. With larger buildings, it can be difficult to accurately test different areas without using multiple fans, which aren’t always available at short notice. Regular diagnostic airtightness testing is a great help during construction, so a testing strategy that allows this is important.
5. Site evidence gathering
Passivhaus certification requires evidence to be gathered on site to demonstrate that what was built matches what was modelled. The evidence includes photos, commissioning sheets, and delivery notes for materials. On a large and complicated site, it would be easy to lose track of the necessary evidence. It is therefore important to clearly define who is responsible for gathering the various bits of information. In the same way that an “airtightness champion” is often nominated to take responsibility for airtightness, an “evidence champion” could take responsibility for collation of evidence from different areas.
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Christopher Hughes, Project Engineer & Passivhaus Designer