Every year new buildings in the UK join the ever-growing list of Passivhaus certified projects. The majority are one-off, bespoke private homes, designed by architects alongside their eco-driven clients. The Passivhaus standard works brilliantly in this context, delivering comfortable, energy efficient homes with a small carbon footprint.
The clue’s in the name – Haus – as it’s usually applied to houses. But what about other building types?
As the nation strives to tackle the climate crisis, there is increasing uptake of the Passivhaus design methodology on more varied, often large-scale projects. These include high rise multi-residential schemes, university accommodation and even leisure centres. Clients demand the same results as their smaller scale counterparts, but the route to certification and challenges faced along the way can be quite different.
Form Factor is More Favourable for Larger Passivhaus Buildings
The ratio of floor area (TFA) to a building’s external envelope area is known as the form factor. A high form factor is good, because you have more useful floor space per square metre of heat loss area. As most Passivhaus requirements are normalised by floor area, it becomes a crucial metric for comparing projects.
Larger buildings, whether commercial or multi-residential, almost always have a far superior form factor to smaller buildings such as detached or semi-detached homes. This means building fabric U-values generally don’t have to be as good, allowing for reduced insulation thickness and less expensive glazing.
On a similar note, large buildings have a higher volume to surface area ratio. As Passivhaus measures airtightness in air changes per hour, rather than envelope permeability in m3/m2.hour, achieving the required 0.6 air changes during a 50 Pa air pressure test becomes significantly easier. Some Passivhaus certifiers are addressing this perceived loophole by asking for an improved air change score, but this currently seems to be done on a case-by-case basis.
The geometry therefore should make achieving Passivhaus easier in big buildings, but this certainly isn’t the whole story.
Mechanical Services Become More Complicated
In reality an improved form factor only provides minor respite from other challenges, including dealing with building services. A big building means more complicated services, and larger areas across which to distribute those services.
Heating, cooling, hot water and ventilation pipework and ducting has to be routed throughout any building. This can be a real test for design teams, often requiring innovative solutions compared to the more standard approach used for individual houses. The equipment itself is also more complex, with numerous air handling units and boilers laid out in convoluted arrangements. These difficulties aren’t specific to Passivhaus, but strategies and PHPP inputs nevertheless become much more involved.
More specifically to Passivhaus, long hot water distribution pipework increases “wild” heat gains, when heat escapes from pipes into a building rather than making it to a tap or shower. This not only increases total energy consumption (primary energy), but can cause overheating during summer, often in circulation spaces and corridors.
Ventilation Requirements Result in Higher Heat Losses
A cornerstone of Passivhaus is mechanical ventilation with heat recovery (MVHR). Design strategies and flow rates are well developed in a residential setting. Building Regulations Part F provides a good starting point, which can be easily combined with the Passivhaus requirement for average air change rate. Flow rates are generally low, meaning low overall heat loss when combined with an efficient MVHR unit.
None of this is the case when it comes to larger or more complicated buildings, especially those for non-residential use. Spaces such as lecture theatres, swimming pools and laboratories all have much more varied and demanding ventilation requirements, with high flow rates to deal with high occupancy or removal of harmful chemicals. This is required to maintain healthy indoor air quality, and inevitably means higher heat loss given that most MVHR units lose 10-20% of the heat that passes through them. There’s no relaxation in the Passivhaus space heating demand requirement to account for this, so any gains from an improved form factor are quickly eaten up.
Internal Heat Gains can be More Important
As with ventilation, the Passivhaus approach to internal gains is well established when it comes to single dwellings. A default value of 2.1 W/m2 is used across the board, providing a small contribution to space heating. Although this will vary in practice depending on occupants and their behaviour, it’s regarded as a sensible assumption that allows like to be compared with like. Residential internal heat gains are relatively small, meaning it doesn’t become an important driver of design when aiming to hit Passivhaus targets.
Once again, this is not the case when it comes to situations where internal heat gains deviate from what might be regarded as typical. This could occur when occupancy is extremely high, for example in student accommodation, or when heat gains are caused by large amounts of electrical equipment, for example in a laboratory or hospital.
The PHPP spreadsheet does allow for project-specific internal gains to be used, but assumptions must be agreed with the project certifier. It can become contentious if internal gains are the difference between pass and failure.
Fair attribution of internal heat gains is therefore a critical aspect of design for large-scale buildings, particularly non-residential, and is something that isn’t always well defined by Passivhaus.
Primary Energy Becomes Contentious
Primary energy is usually low down the list of priorities during your average Passivhaus design. Default assumptions can be used for lighting and domestic appliances, and if you hit the space heating demand there’s a high chance that you’ll pass on primary energy.
This is not the case in any context where energy consumption for things other than space heating, cooling and hot water are significant. This often occurs in university buildings, schools or hospitals, where lighting and electrical equipment make up a substantial proportion of total energy consumption. Even with a comfortable pass on space heating demand, this can make the primary energy target unachievable, especially when coupled with increased hot water distribution losses.
The Passivhaus Institut has given primary energy dispensation in some cases, but as with the airtightness target this appears to be ill-defined and happen on a case-by-case basis.
There have been instances where specification of washing machines is the difference between pass and failure, which surely isn’t in the spirit of Passivhaus.
So can Passivhaus be Massivhaus?
Passivhaus is founded on the principles of building physics, so there’s no doubt that it’s applicable to any building type, of any shape and of any size. The challenges when delivering on a large-scale are different, arguably bigger, but not insurmountable. This is proved by a number of successfully completed and Passivhaus certified projects, such as The House at Cornell Tech (shown in the banner photo of this post). In pursuit of a zero-carbon future Passivhaus certification of a wider variety of buildings is a good place to start.
However, it’s also clear that the Passivhaus standard was developed for single homes. Assumptions, methodologies and targets were all chosen with this in mind, adding complication when they aren’t applicable. The Passivhaus Institut has made some efforts to combat this, but more is required. Airtightness and primary energy targets must be clarified if widespread adoption is expected beyond single dwellings, and accepted assumptions must be agreed. The community of Passivhaus designers can also help, sharing knowledge and experience as large-scale certification becomes more commonplace.