Design and build procurement routes have been particularly popular in the UK for large scale Passivhaus projects. Clients, often public bodies, want the benefits that Passivhaus brings, but are wary of the delivery risks in a relatively immature Passivhaus market. Using a design and build procurement route allows the client to pass the majority of the risks to the contractor. The client may pay a higher price for this, but theoretically, it offers more certainty of cost and time.
However, using a design and build procurement route can offer the greatest potential for an integrated design approach. And this is beneficial for a Passivhaus building project. When it is done right, the full design and construction teams can collaborate to develop a highly integrated solution. Unfortunately, when not done right, design and build procurement can conflict with an integrated design approach.
This blog post looks at three situations where design and build procurement can conflict with Passivhaus;
- Design scope mismatch
- Consultants timelines out of sync
- Tender design overconstrained
These situations can cause issues with any building project and may increase costs and delivery time. For a Passivhaus building project, these situations may additionally jeopardise achieving the standard.
Passivhaus design and build procurement: avoid situations that conflict with an integrated design approach.
1. Design scope mismatch
This situation arises where the design scope of the architect does not match the design scope of the building services engineers or the structural engineers.
This can happen in the pre-tender stage where the design team is working to deliver the design and documentation for the Employer’s Requirements / tender information. The typical situation is that the architect has the most comprehensive scope while the building services engineer and structural engineer both have a more limited scope.
The architect will be designing the building in as much detail as practical to ensure that the client brief can be achieved and is protected in the tender design. This is ideal where the architect is experienced with successfully delivering Passivhaus buildings. A comprehensively detailed tender package will provide reassuring certainty to the client and will reduce the risk for the tendering contractors.
The building services engineer, on the other hand, will largely be engaged in developing the key building service strategies, performance criteria and schematics. This can be a considerable mismatch with the level of detail the architect develops. It is highly problematic as it makes it very difficult for the architect to accommodate the building services and fully coordinate the design and details. Plant space, duct sizes, along with pipe and cable penetrations, will only be approximate and remain at risk until a fully detailed building services design is complete. Additionally, the Passivhaus Planning Package model will be missing important elements or will need to contain conservative assumptions to be useful. The PHPP will only be able to provide a limited reflection of the suitability of the design to achieve the international Passivhaus Standard.
Where a structural engineer has a limited scope, they will typically be focussed on developing key structural strategies and a small number of guiding details. This feeds into and exacerbates the issues highlighted above relating to the limited scope of the building services engineer.
The logic of these scope-mismatches is that the post-tender engineers are best placed to complete the detailed designs in conjunction with the contractor. The contractor can provide cost and component selection guidance based on their preferences, subcontractor relationships and cost information. There are certainly some benefits to this. However, they do not outweigh the disadvantages and risks caused by the inability of the design team to produce an integrated and coordinated Passivhaus design. This often leads to late, and thereby potentially expensive and risky, design changes.
This situation can also happen post-tender when the construction details and documentation are being developed. In this case, the scope of the building services engineer’s design is limited in anticipation of the installation subcontractor completing the design.
The logic, in this case, is that the subcontractor is best placed to complete the design incorporating the specific equipment and suppliers the subcontractor would choose. The disadvantages and risks are the same as above.
The exacting performance requirements of Passivhaus mean that it is more cost effective when all the consultant are working to the same scope. This way a coordinated integrated design can be developed to the appropriate level of detail at each stage. The PHPP can be a detailed and useful model at each stage and risks to achieving Passivhaus can be carefully managed.
The solution: the scope of all the design consultants should match so they develop the design to the same level of detail at each stage.
2. Consultants timelines out of sync
This situation arises when the design consultants don’t all start working on the design at the same time. The longer the delay before all consults are working together on the design, the great the issue.
This happens largely due to an outdated view that an architect can design a building and then pass the design over to the engineers for them to fit the structure and services into. This isn’t a successful way of designing most buildings, and it is a particular issue for a Passivhaus building. Other unintended project specific issues may lead to the same situation occurring.
It is critical for successful Passivhaus design that the architecture, structure and building services are fully integrated. The best way to achieve this is through a collaborative integrated design approach with everyone working together from the start. There are many aspects of both structure and building services that impact significantly on the architectural design of a Passivhaus building and vice versa. Progressing the design significantly while one discipline lags behind introduces significant risk of redesign and may preclude cost-effective solutions.
Strategic decisions about the foundation type, for example, can have a big impact on the design of other areas of the thermal envelope.
Similarly, the approach to heating, cooling, domestic hot water and ventilation needs to be explored and developed early on as these can all influence the thermal envelope and the structural strategy.
Another example is that if fixed shading will be required, the structural approach to this needs to be explored early to avoid introducing structural thermal bridges later in the design process.
And design lag, like mismatched scopes, introduces unnecessary risk into the development of the PHPP model. Without a holistic understanding of the design at each stage, aspects of the PHPP can only be developed based on conservative assumptions. While this does manage the risk of not achieving the international Passivhaus Standard, it makes it harder to optimise the design for Passivhaus and cost effectiveness.
For example, if the ventilation system has to be based on conservative assumptions at the stage that insulation thickness is being set, the conservative assumption about the heat loss from the ventilation system may require an increased insulation thickness to balance the losses. If this isn’t resolved swiftly, the detailing of the building envelope may progress and lock the increased insulation thickness into the design. Conversely, if the ventilation system has been designed to an appropriate level so conservative assumptions aren’t required, an optimal and cost-effective thickness of insulation can be set.
The solution: all the design consultants should work to the same timeline so they develop an integrated design together.
3. Tender design overconstrained
This situation can arise where the previous two situations take place at a pre-tender stage. This results in a tender design with planning or other forms of consent based on what is effectively an uncoordinated design.
This situation also arises when the pre-tender design team does not include anyone with suitable experience of successful Passivhaus design and delivery. This can result in a tender design with planning or other forms of consent based on a design that is not suitable or will be very costly to achieve the international Passivhaus Standard.
Attempting to shoehorn Passivhaus into a project late in the design process may also result in the same situation arising.
In all these cases, the architecture can be overconstrained by the level of design detail that is already fixed, making it impossible to arrive at cost-effective and low-risk solutions. Often the external visual appearance, material choices and dimensions are fixed, despite not necessarily being suitable for Passivhaus.
Specific issues that can be very difficult and expensive to resolve include, for example, orientation, complex forms, thermal bridging, oversized windows, lack of fixed shading, heavy-weight cantilevers.
If it turns out that the only way to achieve Passivhaus is to revisit fixed aspects of the design, this becomes an expensive and time-consuming process renegotiating client and consent requirements.
Passivhaus requires an integrated design approach from the start, with a practical understanding of how the specific requirements of the standard will be achieved. A good understanding of how the design will be constructed is also needed. Ignoring this and taking a naive piecemeal approach to design, without adequate consideration of the construction, can mean that expensive and risky solutions need to be implemented. Or worse, the Passivhaus standard might not be achieved at all.
The solution: either the tender design needs to be fully and competently designed to meet the international Passivhaus Standard. Or the design needs to be tendered at an early stage before it is too constrained.
Successful Passivhaus procurement needs to support an integrated design approach.
Procurement methods are not an issue in themselves to successfully achieving the international Passivhaus Standard. There are many successful and cost-effective Passivhaus buildings that have been procured, in the UK, using a design and build form of contract.
If, however, aspects of the procurement obstruct or prevent an integrated design approach, this makes it much harder. The risk of successfully and cost-effectively achieving Passivhaus increase significantly when the design is not coordinated and detailed to the same level across all disciplines at each stage.
These same risks are very high if the design is overconstrained before robust and reliable solutions for Passivhaus are integrated into the design. Constraints that prevent design optimisation also prevent cost optimisation.
The solution is to ensure; that the scope of all the design consultants is aligned, that they are all working to the same timeline and that the design team includes people with suitable Passivhaus design experience.
Passivhaus procurement needs to support and enable an integrated design approach, not obstruct or prevent it.
Do you know someone procuring Passivhaus buildings? Please send this blog post to them so they can avoid these mistakes.
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