This chapter is mainly of interest to those wishing to commission a new-build or retrofit project. While some points made here relate specifically to new builds, retrofits or extensions, many are relevant to all Passivhaus projects. There are many variables to consider when setting up a building project, but we have restricted the discussion here to those that can have an impact on Passivhaus builds.

Choosing a plot

Acquiring a viable building plot at a sensible price in a half-decent location is a challenge in the UK. Any choice will involve uncomfortable compromises. That said, it is worth examining the factors that will impact on a future build, particularly a Passivhaus. Similarly, it can be a challenge to find a building that lends itself reasonably easily to being retrofitted.

There are many considerations involved in choosing a plot that apply to building projects in general and are outside the scope of this book; these are covered by other publications and online resources. However, one worth mentioning here is site access. Plots with difficult site access or steep inclines will cost more to build on, and this is just one of many factors affecting cost (other factors were discussed in Chapter 2). If you are able to choose a plot that is cheaper to build on, you will of course be freeing funds for the build. If there are concerns about site access, it’s a good idea to ask a builder for an opinion.

None of the issues discussed in this chapter will determine whether or not the Passivhaus standard is achieved, but they will have a bearing on the cost of the project. If you already have a plot, you may be constrained by the extent to which you can change these variables, depending on how much scope there is in determining the building’s location on the plot.

Shading

As we will see in the next chapter, during wintertime, solar gain – heat energy gained passively through the windows – makes a significant contribution to reducing the building’s heat demand. If little or no wintertime solar gain is available, it means that the roof, walls and floor U-values will need to be improved considerably to bring the overall net heat demand down. Figure 6.1 overleaf shows the energy losses and gains through different elements and from different sources in a typical Passivhaus. The amount of energy needed to balance the building’s heat losses and heat gains – the heat demand – is 15kWh/m².a in this example. Energy balance is a key Passivhaus concept.

Check whether the location of your proposed new build or retrofit building is in shadow due to buildings, trees or other structures adjacent to your plot. This is particularly important if the shading structure or tree belongs to a neighbour or if you would like to retain it yourself. Take care not to overlook any trees with tree preservation orders (TPOs) or other planning restrictions that might have an impact on shading, now or in the future. If in doubt, check with your local planning authority. Your solicitor should also highlight any TPOs on the plot, if you are in the process of buying it.

Some sites may have east or west shading, which can be beneficial in helping to reduce summertime overheating risk.

While some shading will no doubt be unavoidable, a judgement will need to be made about acceptable levels of shading. Chapter 7 explains how shading is assessed in the Passivhaus Planning Package (PHPP). If you have already chosen an architect with experience of Passivhaus (see page 83), he or she should be able to provide some advice on shading, as well as on other issues related to choosing a plot.

Figure 6.1 Energy losses and gains, and overall energy balance, in a typical Passivhaus.

Planning considerations

If you are considering a retrofit project and your proposed purchase is a listed building, or is in a conservation area or national park, making changes to the external appearance of the building will be much harder, if not impossible, and therefore likely to exclude the usual option of external wall insulation. If a new-build plot is located in a conservation area or national park, it will also be subject to more restrictions. However, even if this is the case, a new build does not have the constraints of working around an existing structure and so allows much more discretion in the internal make-up of the building assemblies, as long as the external appearance is deemed to be ‘in keeping’ with its neighbours.

Conservation areas and Passivhaus components

Although it is technically possible to reach the Passivhaus standard by insulating internally, doing so will involve more costly and invasive interventions inside the building, which can often compromise the internal layout and will nearly always reduce useful floor area. Internal insulation also requires much more careful attention to moisture management, to avoid damage from cumulative build-up of water and water vapour within the building structure. For most buildings, being located in a conservation area – especially if the building has listed status – will impose additional costs and restrictions that will, in many cases, make reducing its energy use down to Passivhaus or even EnerPHit standards economically unfeasible, and probably too risky from a moisture-management perspective. However, Passivhaus methodology can still be applied pragmatically to get the biggest improvement in energy performance within the practical constraints of the building and the financial constraints of the project. Finding a suitable ‘conservation-friendly’ window product that will perform adequately for an ultra-lowenergy building is difficult; until recently there were few, if any, such products on the market. In the Princedale Road retrofit project (pictured top right, highlighted in pink), a bespoke window solution had to be designed and developed by the contractor, EcoHaus Ltd, in order to meet the requirements of both the local conservation officer and the Passivhaus standard, as no suitable products had been brought on to the market at that time. A wider range of styles and Passivhaus-certified windows is now available, an example of which is pictured below right.

Even if your proposed purchase is not listed and is outside a conservation area or national park, if it is part of an estate or a row of relatively homogeneous properties that has a group value (in terms of architectural merit), it will probably be subject to more constraints than a building that is not. There may also be other circumstances that affect the degree of planning scrutiny for a given location.

It is worth noting that many planning applications make claims about their sustainability credentials to help the application gain planning permission; this is particularly true if ‘sustainability’ is being used to argue a case to build in a location where planning permission would not otherwise be granted. Planners are aware of this tactic, so be sure to explain (briefly) what Passivhaus means and its wider significance. If your project is going to be certified, mention this and its significance.

Princedale Road, West London: the UK’s first retrofit project to achieve full Passivhaus Certification. Image: Paul Davis + Partners

Wooden Passivhaus-certified windows by ENERGATE^® in a period building in Germany. Image: ENERGATE^®

Seeking planning advice

There are many other factors that the planners will take into account, which are not Passivhausspecific but affect all developments. Further reading is recommended (see Resources), to allow you to assess potential plots through the planners’ eyes. If the project size and budget warrant it, or if planning is likely to be particularly challenging because of factors specific to your chosen location, it may be helpful to hire the services of a planning consultant. Planning is discussed in more detail in Chapter 14.

Retrofit considerations

In the case of a retrofit project, there are further issues to consider, as follows. Even if the building or plot adequately addresses all of the planning requirements, some buildings lend themselves better to retrofitting than others.

•  Original brick or stone façades These can be externally insulated, but there are two issues. First, board-based insulation will require that the original façade be rendered before application of the insulation, adding to the cost. Second, while it is possible to buy external insulation products with a false brick façade, they really do tend to look false(!), and your building is likely to look better if you chose not to attempt to recreate what was there before.

•  Cavity walls These create two problems. They are often thicker than their solid (single-skin) wall equivalents, so the addition of sufficient insulation to achieve the Passivhaus performance results in overly thick walls (Passivhaus discourages wall depths beyond 500mm). On many sites, such thick walls are not practical because of space constraints, nor desirable for aesthetic reasons. Also, the cavity itself is a problem if left unfilled, because air movement within the cavity causes heat transfer by convection (known as thermal bypass). Of course, many cavities are already filled, and in some of these the existing insulation performs poorly and may be poorly installed (see Chapter 8, page 112). However, this doesn’t mean cavity walls present an insurmountable problem – the Totnes Passivhaus, for example (pictured on page 53), is a cavity-wall retrofit.

•  Concrete slab floor combined with low ground-floor ceiling heights If your building sits on a concrete slab, it will probably not make economic sense to remove it, so floor insulation will need to go on top of the slab. If there is already limited ceiling height and the finished floor level therefore cannot be raised, then unless the first floor is to be removed (a huge intervention by any standards!), the space for floor insulation will be very restricted. While this problem might be addressed by using specialist ultra-low-thermal-conductivity insulation, such as vacuum-insulated panels (VIPs), aerogels or similar, this is particularly expensive.

•  Suspended wooden floors Many homes have suspended wooden floors in all parts of the ground floor except the kitchen, where there is often a concrete slab. In this case it can be fairly straightforward to remove the slab, as it is probably not a structural element supporting the building. Insulating suspended floors is relatively straightforward if there is enough space to access them from below. This is particularly helpful if you want to retain the existing finished floor – a parquet, for example. However, the majority of suspended floors have only a 350-500mm void, making insulating from below either very difficult or impossible, in which case insulation would need to be installed from above. But removing the existing floorboards also has its benefits, as it makes it much easier to properly address airtightness requirements and deal with joist ends. The insulation of suspended wooden floors has to be detailed carefully to ensure that moisture will not form and collect around the joist ends. Some would favour removing the suspended floor completely and replacing with a solid concrete slab, as it’s quicker (although messier) and makes it easier to detail for insulation and airtightness continuity. (For more on managing moisture risks when insulating, see Chapter 10.)

•  Roof overhangs A building with deep roof overhangs should make it simpler to insulate externally because the roof does not need to be extended outwards to accommodate the insulation. It is important to be able to remove existing soffits (the underside of the roof overhang), as this allows the new external insulation to extend upwards to meet roof insulation (warm roof construction) or loft insulation (cold roof construction), minimising the thermal bridge along the wall–roof junction. For the latter, this building junction needs to be detailed very carefully to ensure there are sufficient airflows in the loft space.

•  Roof ridge line If tied into a fixed ridge height (i.e. it’s not possible to raise the ridge owing to adjacent buildings, for example in a terrace), your roof insulation solution will be less simple and this might drive you to insulate across the top-floor ceiling instead (‘cold roof construction’). Normally, Passivhaus insulation follows the roof line.

•  Fireplaces These are not compatible with the Passivhaus approach, as they are designed to draw volumes of air (creating draughts) through your building in a very energy-inefficient manner. Blocked-up flues can become potential moisture traps and need to be ventilated, which again is counter to low-energy design. It is perfectly possible to retain external stacks supported on gallows brackets, but it is best to remove chimney breasts internally. There are ways to seal a chimney flue and provide a Passivhaus-suitable log burner, but this would need to have a specifically controlled air supply and exhaust, sealed to the room, which has cost implications.

A building with a generous roof overhang, such as the one pictured right, makes it easier to insulate externally than when there is virtually no roof overhang, as in the photo on the left.

• Windows Some window designs will be more problematic and expensive to upgrade, e.g. dormers and bays.

• Extra complexity The time and money needed for design and energy modelling is greater in a Passivhaus retrofit than in a new build. A structure that was not designed to be to a Passivhaus creates specific issues that need to be overcome, often by quite creative but time-consuming solutions.

• Form factor The more spread-out your building, the harder it will be to achieve the Passivhaus or EnerPHit standard (see Chapter 1, page 21), and in some cases it will be impossible.

• VAT As with all refurbishments in the UK, the full VAT rate is payable for retrofit build costs, whereas it can be claimed back in a new build. This anomaly does not apply in Ireland. With VAT currently at 20 per cent, the ex-VAT cost of the retrofit will have to be a lot less than that of an equivalent new build for it to add up financially.

• Thermal bridges Nearly all retrofits are going to have a thermal bridge at the floor– wall junction that cannot be eliminated practicably or economically. Further thermal bridges are likely at the wall–roof junction. Any thermal bridges in a building approaching the energy efficiency of a Passivhaus will add significantly to the overall heat load. The heat loss arising from each thermal bridge has to be quantified in separate thermal bridge heat loss calculations; it costs upwards of £250 per junction to calculate its specific heat loss or psi-value (the measurement of heat loss in a linear thermal bridge). In a new build, it is possible to design out all thermal bridges, making this additional work unnecessary. (See Chapter 8.)

• Ventilation systems Installing a visually discreet ventilation system will often be harder in a retrofit because of constraints imposed by the existing structure. There will probably not be enough room for ducts between internal walls or within internal floors (where ducts need to run perpendicular to the floor joists). Ducts in a Passivhaus are oversized to reduce air speeds; slower air speeds improve energy efficiency and reduce noise. (Chapter 12 covers ventilation system design in more detail.)

• The quality of the original building Although it is sometimes hard to judge accurately until after retrofit work has commenced, it is vital to take time and get professional advice on whether a building is worth saving before deciding to go down the retrofit route. It is easy to let a (quite reasonable) belief in the value of retrofitting unduly influence your decision. There are many buildings in the UK that were built very poorly and really need to be put out of their misery! If building materials, particularly those with high embodied energy, can be viably reclaimed, all the better. Building new makes it much easier to reach the full Passivhaus standard and much more practical to use low-embodied-energy materials. Space constraints in a retrofit very often force the designer to choose between higher-performance but usually high-embodied-energy materials, such as foam board insulation, or lower-energy-performance but more environmentally benign materials. In a new build it is possible to do both.

Phasing retrofit work

Homeowners and registered social landlords (RSLs) may not be in a position practically or financially to undertake a full Passivhaus (EnerPHit) retrofit in a single phase. Such work almost always requires that the building is vacated for the duration of the build, and committing to such a large capital spend can be too big a burden, especially in the era of tighter credit conditions. The alternative is to plan a retrofit in stages.

The sequence of phased retrofit work may well be driven by the cycles of planned (or unplanned!) building fabric repair or replacement work. The common-sense approach is to plan strategically, taking into account how earlier work phases will impact on the cost of planned future work phases. This reduces the risk of having to undo earlier work. If the strategic goal is to get the building to perform to the EnerPHit standard, each phase will need to be approached as if it were part of a larger EnerPHit retrofit. For example, if working on the repair or replacement of a ground floor, you would not only install the required insulation; this work is also an opportunity to install an airtightness layer in the floor and to plan and execute the installation of insulation to minimise thermal bridging. These steps are vital in managing moisture risk properly as well as improving the overall energy performance (U-value) of the floor.

A similar approach could be taken when insulating or repairing a roof. Careful thought would be needed when planning the path of the airtightness layer, particularly where the roof meets the external walls.

Upgrading windows is more of a challenge. It is essential that the strategy for installing windows takes into account how the walls will be insulated. The Passivhaus retrofitting approach would by default involve external wall insulation. New windows would then be installed, mounted on the outside of the existing walls, with external wall insulation wrapping around the window frames to minimise thermal bridging. If the windows are to be installed months or even years before the external wall insulation, the windows will have to be installed in such a way that they work practically (no gaps around the edge, properly weather-proofed, etc.) in the interim.

To do cost-effective phased retrofit work, where the aim is to eventually achieve the EnerPHit standard, before any work starts on-site you will need professional design input by a Passivhaus Designer, PHPP energy modelling and possibly moisture modelling (see Chapter 10, page 158). In design terms, the project should be treated as if it were being undertaken in a single phase, with the caveat that the design needs to consider potential moisture risks between each phase of the work. An architectural detail of a building assembly or junction could cause moisture problems between interim construction phases, even though it may well perform correctly in energy and moisture management terms after the last phase is completed.

Extensions

It is also possible to adopt the Passivhaus building fabric standards for new extensions on existing properties. There is not a great deal of sense in losing the opportunity of building to this level (or near this level) if you are already expending financially on new work. Extra-over costs should be minimal.

This approach would bring particular benefits where the new extension is to be open to the existing house. Here you will generally be removing old, substandard external walls and, in effect, replacing them with new, high-performance ones. If old doors and windows are being removed, then the new extension replacements also offer the potential for an excellent upgrade. It is very common to create new, open-plan living/kitchen rooms when making such building alterations, and these modern spaces will often be where the majority of time is spent.

Janet Cotterell and Christine Harrison’s extension, built to Passivhaus standard, in north-west London.

A more comfortable indoor environment can then be achieved while reducing energy bills, plus there will be knock-on improvements to the rest of the house. Such an extension does not require any special ventilation strategy, as the existing house will be more than ‘leaky’ enough. In the case of a single-storey extension, you could then keep the first-floor sleeping zone at a cooler temperature – which many people prefer. If you are building a large two-storey extension with a great deal of new accommodation, the interlink between old and new will be more complex and the ventilation strategy will need more careful consideration.¹

You may consider using lower-performance windows (with a U_w of around 1.2W/m²K – see Chapter 11, page 171) if your budget is really stretched; however, there will be some risk of a comfort penalty. Also, if the extension is part of an EnerPHit retrofit that is being built in phases, you must specify Passivhaus-suitable or -certified windows to ensure that the building reaches the full EnerPHit standard after completion of the final phase.

Selecting an architect and builder

The decisions as to which architect and builder to appoint are those with the most far-reaching impact on any build project. In a Passivhaus project, these decisions are even more critical to its success. As discussed in Chapter 4, Passivhaus projects demand a much greater attention to detail during both design and construction than comparable non-Passivhaus builds.

Clearly, it will be a less stressful project for all if the architect and builder see Passivhaus as a goal they want to achieve for themselves; the team should have ‘common purpose’ (see box opposite). Also, there must be a genuinely cooperative and trusting relationship between architect, builder and client. The importance of cooperation and trust as success criteria cannot be underestimated on a Passivhaus project. It really is worth doing everything possible to make the right choice of people for these key roles, so do allow enough time to reach a well-considered decision.

Defining the brief

Having decided to go ahead with a build project, your starting point as the client is to write down your aims and requirements in an architect’s brief. The Royal Institute of British Architects (RIBA) provides good advice on how to brief architects in its booklet A Client’s Guide to Engaging an Architect (see Resources). It suggests providing information in the following categories:

•  functions of the building

•  (client’s) motivations and expectations

•  design direction

•  authority for decision-making

•  timetable and budget.

Kevin McCloud’s Grand Designs Handbook³ also has some very good tips about how to select and work with your architect.

All the above points apply to self-build projects in general. A brief for a Passivhaus build obviously needs to state clearly that achieving the Passivhaus standard is a key goal for the project, and the criteria for meeting the standard (see Chapter 1) should be stated in the brief. This is important, as it will help to identify which architects really understand what Passivhaus entails. If you are planning to get the formal Passivhaus Certification, you also need to state this in the brief.

Selecting an architect

Once you have your brief, the next job is to find an architect who is able and willing to design a Passivhaus. While RIBA’s website (www.architecture.com) will help you get a list of candidates, it is worth looking at two other sources of information to narrow down your list:

•  The Passivhaus Trust (www.passivhaustrust.org.uk) – the UK’s leading organisation working to promote and protect the Passivhaus standard. Its list of members includes architectural practices and design and build companies offering Passivhaus design services.

•  PassivhausPlaner.eu (www.passivhausplaner.eu/englisch/index_e.html) – a European register of Certified Passivhaus Designers and Consultants. Many Passivhaus architects, or someone from the architect’s practice, will have taken the Certified European Passive House (CEPH) course and should be listed on this register. This will give you more confidence that the practice understands the methodology and the challenges of a Passivhaus build.

Another obvious point is that there is a lot to be gained from examining the previous work of the architect and, if possible, arranging a visit; even better if it is possible to meet the occupants. An architect who has already built a Certified Passivhaus should be given serious consideration. This does not mean that architects without previous Certified Passivhaus experience are to be overlooked – there are many projects that have aimed for energy and airtightness standards that were ambitious by the standards at the time. The key thing to establish in this situation is what lessons the architect has learned from the experience.

Once a shortlist of candidates has been drawn up, the next step is to meet them. First, send them the brief. It should give them all the information they need to prepare for an interview. Before any meeting, write a list of fairly open questions that will allow you to test the following.

•  Can the architect explain what a Passivhaus is?

•  What problems and lessons has he or she learned from previous relevant projects?

•  Does the architect integrate low-energy goals into his or her design from day one of the process? In particular, does he or she focus first on optimising the building fabric, rather than on renewables and heating technologies?

•  How well does the architect understand the importance of the core Passivhaus concepts covered in this book (form factor, shading and solar gain, thermal comfort criteria, designing out or managing thermal bridging, achieving Passivhaus levels of airtightness, ventilation strategy)?

•  What are the principal pitfalls and challenges of a Passivhaus project?

•  What specific additional documents are needed from the architect in a Passivhaus project?

•  Does the architect know what the PHPP is for and understand its significance in Passivhaus design? Specifically, will the architect ensure that the PHPP is used by a competent person to model the proposed design early on and will it be used iteratively to inform the design process?

•  Does the architect understand the role of heat recovery ventilation in Passivhaus? How would he or she address mechanical and electrical (M&E) building services (i.e. heating, hot water, ventilation, electrics and plumbing)?

•  How committed is the architect to the goal of achieving a Certified Passivhaus?

•  How open is the architect to suggestions and input from the builder?

•  How does the architect see the role and selection of the builder in the project?

Finally, take a friend to the interviews so that you can compare impressions, and ask to speak to previous clients.

Selecting a builder

Unlike the architect, who does not have to be local to the project site, if at all possible it makes sense for the builder to be based in the same locality as the build. To do otherwise would not make financial sense in most cases. That said, in the Camden Passivhaus – London’s first Passivhaus – some of the members of the build team were ‘imported’ from Austria. However, the economics of a high-end home in one of London’s smarter districts does not reflect the economics of more typical builds. Also, pre-2008, demand for builders often outstripped supply in London. Being the first London Passivhaus, it was a big challenge to find a contractor team that could deliver the airtightness and thermal-bridge-free building standards required in a Passivhaus.

It will be a few years before it becomes realistic to find candidate builders with a track record in building ultra-low-energy and Passivhaus buildings. However, builders who have experience of managing conservation and renovation projects on listed buildings, and who can show that they have a genuine interest in and understanding of low-energy building, are quite likely to be good candidates for a Passivhaus build – as are builders who have previously worked on a project where they have had success in managing airtightness issues without excessive spending on remedial work. A background in conservation helps because such projects share some of the same challenges as builds aiming for good airtightness, because in a conservation project, building work can easily cause unintentional damage to the existing structure. Similarly, airtightness, once achieved in a partially complete building, can easily be compromised during subsequent phases of the build. Builders will need to demonstrate that they have a strategy to manage this. Chapter 9 explores this in more detail.

It is, of course, important to visit sites of builders’ previous work and, if possible, to talk to their clients. Try to visit both a complete build and a build in progress. The former should show the craftsmanship of the building team. The latter should, with a careful eye, reveal something of the building team’s ability to achieve the very low levels of air leakage needed for a Passivhaus, as well as the team’s attention to detail in installing insulation without any gaps, so that no unintended thermal bridges are created.

Visiting a site of a partially completed build – say, at electrical first fix (where the structure is still exposed) – allows you to check whether or not the build quality is just skin-deep. However, the relevance of a site visit will be limited if the builder is planning to use different personnel for your project. Find out whether the foreman or site manager on these previous projects will be available for your project. That person will have a key role in ensuring avoidance of both air leakiness and thermal bridging on-site.

Traditionally, construction contracts are formally tendered to three or four firms using Joint Contracts Tribunal (JCT) contracts (a standard format of contract and associated documents used in the construction sector). It may be that you prefer a particular contractor and there are alternative approaches, including ‘cost plus’. This essentially means that you pay the actual build costs plus a pre-agreed mark-up. If there is trust between client and builder, this can be an economical approach for the one-off self-builder. It is outside the scope of this book to provide advice on contracts, but an architect should be able to advise.

The role of the client

As has been discussed elsewhere in this book, a Passivhaus is not a bolt-on extra that can be simply tacked on to a non-Passivhaus design. For the client, this means being open-minded about how the build is approached and how the budget is apportioned. A Passivhaus will require money to be spent differently: more on windows and insulation products, without a doubt; plus a bit more for airtightness tape and membranes, and airtightness testing. Funds are also needed to pay for PHPP energy modelling and, optionally, Passivhaus Certification. However, energy modelling should save more in building materials than it costs in fees for the modelling. If the team is new to Passivhaus, certification could also save money by reducing the risk of errors, and it may also help to hold or boost the property’s value when selling. Money can be saved if you are flexible enough about the design to take on board advice from the professional team – architect, builder and Passivhaus Designer or Consultant – who should be able suggest ways to make the build cheaper. The form factor is probably the most important variable. This does not mean that inefficient form factors are ruled out; they just have a price tag attached to them (this is of course true for all buildings, not just Passivhaus). There are huge cost variables within a build: finishes (kitchens and bathrooms in particular) can vary by large margins and this does not always reflect quality.

You can help reduce the risk of cost overruns by allowing plenty of time for the design process. The more thought that is given to the project before it starts on-site, the fewer pressurised, last-minute (nearly always costlier) decisions have to be taken (see below). This means that you must be willing to spend a realistic percentage of your budget on design and energy modelling before starting the build. Where the budget is tight, it is tempting to spend the minimum needed on an architect to get a design through planning, then hand the plans over to the builder who has given the most optimistic view of the likely cost. But cutting corners in the design stage is unwise in any project; particularly so in a Passivhaus one. It has two main disadvantages.

First, while it is nice to hear a rose-tinted view of the build costs at the start of the project, this only puts off the day of reckoning, when the tenders or bills come in. For client and builder, not being realistic about the costs from day one will undermine the trust crucial to a Passivhaus project.

Second, insufficient time and resources spent on design before going on-site will inevitably result in ‘designing on the hoof’ during the build. Design decisions made during the build will be rushed and will increase costs and risks. It is not always clear what multiple effects will result from a design decision, and hasty decisions will therefore lead to unforeseen effects. Most design changes require building materials to be bought or necessitate returns of previously bought materials (if the supplier accepts returns). Last-minute purchases usually do not allow enough time to shop around for the best deals; sometimes the exact product may not be available and the substitute may not be appropriate or may be more expensive. Furthermore, unplanned changes in a Passivhaus build will add to the risk that corners are cut to make up for lost time. In a standard build, cut corners are often hidden beneath the finishes of the completed building without any apparent consequences, but in a Passivhaus build, those cut corners risk jeopardising the goal of creating a sufficiently airtight and thermal-bridge-free structure. Time on-site is also extended, which has a cost implication. This is not to say that changes on-site can or should always be avoided – sometimes they are necessary – but the temptation to change the design for marginal benefit should be resisted if at all possible. The full costs (financially and in terms of project delays) of making late changes need to be understood at the point the decision is made, or costs will build up over time and there will be a loss of financial control. Changes during the build will likely demotivate the site team, if these involve undoing work already started.