News A Q&A with Certified Passivhaus Designers

A Q&A with Certified Passivhaus Designers Eve Russell and Domnica Chisca

Features, News

April 30, 2024

Off the back of our recent webinar, The Benefits and Costs of Passivhaus, we asked two of our speakers to answer a few questions sent in by our audience and here's what they had to say:


Is the cost of a Passivhaus project really only 10% more than a non-Passivhaus project? 

It can be difficult to assess what is extra cost due to Passivhaus requirements and what is simple extra cost of construction, especially with the cost of construction materials in the last few years. The values quoted by Passivhaus Trust come from a compilation of numerous Passivhaus projects built in the UK. The 10% figure is an average they have arrived at when compared to baseline, but it will depend on the size and typology of the units.

All of our panellist noted that the earlier a Passivhaus Designer is involved in the process, the better it is to avoid extra costs later down the line. As an example based on the presented case studies, the location of the MVHR had been problematic in all designs that had not been included PH at early stages, this could have been avoided otherwise, therefore saving on the cost of either moving it post Stage 3 or mitigating it with longer ducting and more insulation. This is the case for a lot of other design considerations that add up and can be converted into cost savings.

The other critical issue is the experience of the team, employing experienced designer and contractors will reduce the cost. We see a lot more contractors interested in getting experience with Passivhaus construction, so it is reasonable to assume that the experience in the field will increase further lowering the costs. 


Mixed use schemes - residential and primary healthcare - how do you work with Passivhaus and BREEAM - duplication of assessment costs?

The dual certifications have been achieved in projects before. Passivhaus principles align with a lot of the BREEAM principles, there may be some natural crossover. Choosing high-performing materials will have additional positive impacts which we may not intend them to have initially. There are challenges in achieving the assessment process, with dual certification bodies,, however Passivhaus principles can be used for most building types – hospitals, schools and fire stations have been certified already. BREEAM and Passivhaus promote health living conditions, high quality materials, good management of the process, and in-depth consideration of the energy source – there are crossovers. 


How have you relayed to residents how they use their new homes and how they may differ to what they may be used to?

This is a continual process and requires engagement for at least a year following building occupation with residents. Short ‘How To’ animations have been carried out to share with residents before and workshop sessions on how to use and make the most of the technologies. It lends itself to clients with longevity in their interaction and responsibilities with a building. POE (Post-Occupancy Evaluation) is carried out by PRP on a range of projects and part of this process can be to engage with residents to understand their concerns on how to use technologies and features which may be new to them.


How do Passivhaus standards relate to the Future Homes Standards? Why do you think the government felt the need to introduce new standards rather than enforcing the PH ones?

We cannot answer on behalf of the government, however, the industry is moving towards Passivhaus standards eventually. Becoming familiar with Passivhaus practises now will only equip clients, end users and designers to better future-proof our building stock and their ability to adapt. Within the industry there is not yet a widespread skillset to deliver Passivhaus buildings to that scale. Passivhaus is not just a design guidance, it is a delivery guidance with milestone checks up until the building is completed. There are definitely clear moves towards Passivhaus standards and often PHPP is requested as a method of measuring a building’s performance capabilities. Passivhaus principles are deeply embedded in good design decisions and only enhance methods that we already use in creating buildings. 


How wide spread is the usage of remote environmental monitoring in Passivhaus buildings and how effective it's been?

This is something the Passivhaus Trust and Passivhaus Institute have carried widespread research on. Nottingham University have also been involved in a scheme which was purpose built to monitor the lifespan of buildings post-construction. When inhabited correctly, with MVHR running continually and understanding of purge ventilation, Passivhaus buildings run incredibly well. 


What would you say is the cost/benefit difference between achieving PH certification and an equivalent, non-certified sustainable design?

The certification can only be obtained if the building meets the required standards, including having to prove the installation of all specified materials and the airtightness test. Essentially closing in the performance gap between design intent and actual construction. Therefore by comparison with the non-certified sustainable design methods, it does give the client and user an assurance of performance.  


Do you think Passivhaus lends itself to a particular form of contract (Trad or D&B)? Does tendering at RIBA 3 with contractor's design cause any issues?

It will depend on how prescriptive the contract is. If this expectation is clearly stated on the outset and recorded in the contract that the project is to achieve Passivhaus certification (and which of the standards of certification), then there is no reason for which a D&B cannot be used. Expect to encounter resistance however, and consider that an unexperienced contractor may miss the mark on the pricing for tender. 


What was the design journey for mitigating overheating risk and has any POE been carried out to identify potential performance gap?

Overheating is an important consideration and the issue is not as much the performance gap in the construction, as Passivhaus is as close as design considerations and construction can get, but the rapidly changing and the more extreme weather conditions we have been having, with the summer record temperature going over 40C for the first time on record in 2022. It is therefore crucial to futureproof our buildings in a measurable way.

Overheating is a major concern of Passivhaus, with a certification requirement that the building must have less than 10% of the year at or above 25C. Ideally in the design stages this percentage should be lowered as much as possible and to be achieved without purge ventilation (as this may not always be possible due to security, noise or other reasons)  or consideration for trees (as these can change/be cut through the year or the lifetime of the home).

In first instance consideration has to be given to passive design solutions that range from the size, position and orientation of the windows, the solar transmittance of the glazing, external shading, etc. With the external shading, particularly on the south and west elevations, being the most efficient out of the passive solutions (better to stop the heat from coming in, than having to find a way to get it out). For houses, external shading that has been successful in practice, even when added retrospectively, is something like a pergola to the back garden, or adding external blinds to the front elevations. For flats this can look like having stacking balconies that offer shading to the flat below, or recessed balconies, brise-soleil or external shutters.

In the UK there has been increasing need to provide shading or other overheating mitigation solutions since the original implementation and it has been found that one of the most efficient means of providing this is cross ventilation or stack ventilation. This can be either manual by the residents themselves or automated.

Here it is important to consider the type of project and the location: a house can make use of a high level openable rooflight or window such as over the staircase which can be mechanically actuated, whereas a flat is better suited for cross-ventilation, if the building is an office or a school it has been found that automatic overnight purge ventilation is very efficient, whilst for a medical environment it is more likely that a mechanical cooling solution would be employed to ensure the filtration of air.

This is why it is important to engage your Passivhaus Designer early, to see what solutions are available for your project and can be incorporated early in the design process to avoid having to do things retrospectively. 


Passivhaus does not consider embodied carbon as a stipulation for certification, is this true?

This is correct in that the Passivhaus Standard does not explicitly state it within the principles. However, through choosing high performing materials and technologies that will last a prolonged period of time, with considered design, the buildings being construction to Passivhaus standard will last longer and in turn be slower to produce waste carbon. It is a sustainable set of principles which aims to and tracks a building to perform better for longer.   

Passivhaus will therefore set itself apart when considering Whole Life Cycle Carbon. AECB have created a PHribbon to aid in this calculation. It is considered one of the more reliable ways of calculations and it does follow the RICS methodology recommendations. Additionally the embodied carbon can be reduced by early consideration and choice of materials.