Passivhaus architects in London

Architecture for London are leading Passivhaus architects and builders in London. All of our designs employ a ‘fabric first’ approach to sustainability, and director Ben Ridley is a Passivhaus Designer.

This article outlines the challenges of meeting the Passivhaus standard in London homes, both for new builds and for existing housing stock. When we think of an architect-designed Passivhaus, we might imagine a modern, detached home in a rural setting with an unconventional appearance. Perhaps a timber-clad property with a ‘futuristic’ design that bears little resemblance to traditional architecture? This typical image of a Passivhaus could not be further from the reality of a Victorian terrace in London.

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So can Passivhaus principles be applied to our existing city homes? Can a historic, or even a Grade II listed Georgian House Refurbishment be refurbished and upgraded to meet the strict Passivhaus energy performance criteria?

Passivhaus (or Passive House) standards are defined by energy use principles rather than an approach to construction or a standard set of materials. The principles of Passivhaus can therefore be applied to houses in London in the same manner as rural properties.

The challenges lie in the details of how these principles are applied to meet the performance criteria. The criteria for a Passivhaus are detailed below:

New build Passivhaus homes in London

The design of a new build Passivhaus should be informed by form factor, orientation and solar shading. These are more important than purely aesthetic considerations, for example, symmetry or proportion. Form factor relates to the ratio of the volume to the surface area, so it affects the heat loss.

• Space Heating Energy Demand is not to exceed 15 kWh per square meter of internal floor area per year. Peak demand should be 10W per square metre or less.
• The total energy used for heating, hot water and domestic electricity must not exceed 60 kWh per square meter of internal floor area per year.
• Airtightness, a maximum of 0.6 air changes per hour at 50 Pascals pressure.
• Thermal comfort must be met year-round for all living areas. The internal temperature should be over 25 °C for no more than 10% of the hours in a year.

A proposed New build Passivhaus in Muswell Hill

The above criteria are difficult to meet. They require a significant investment of time from the architect, consultants, and contractor during the design and construction of the building.

The challenges are increased further when dealing with existing buildings. It can be more difficult to achieve high levels of airtightness and insulation in an existing building. Many more fixed parameters act as limitations in achieving the full Passivhaus criteria. These include issues of conservation, building orientation and window sizes.

Passivhaus ventilation strategies

Airtightness is only viable in parallel with a suitable ventilation strategy to ensure fresh air is introduced into the building. A mechanical ventilation heat recovery (MVHR) system manages airflow throughout the house, removing stale air from kitchens and bathrooms, whilst supplying fresh air to living spaces and bedrooms.

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A very efficient heat exchanger transfers most of the heat from the stale air to the fresh incoming air without mixing the two. This approach is more efficient than simply opening windows for ventilation, as much less heat is lost. Windows can, of course, still be opened in a Passivhaus if desired.

Passivhaus architects in London – existing homes

Historically, in British homes, the primary heat sources were coal, wood and later gas fires. The cost of fuel was low, and there was little concern about pollution or climate change. Consequently, airtightness levels were low and significant amounts of heat was lost through cracks in structures and around windows. Comfort strategies to deal with greater air movement included making use of window shutters, additional layers of clothing and sitting in high wingback chairs.

Our low-energy refurbishment of a Grade II listed Georgian house in Bow with triple glazing, airtightness, MVHR and air source heat pump.

Now we have central heating systems, and we expect to be warm throughout the year regardless of what we wear. The energy use of our ageing housing stock has therefore increased substantially. Even the current UK Building Regulations for residential projects are still fairly lenient in terms of airtightness requirements. They allow significant levels of air movement through the building fabric.

The Passivhaus EnerPHit standard

The Passivhaus Institute has created a demanding set of criteria for the refurbishment of existing buildings, this is the EnerPHit standard for refurbishment.

Enerphit is, however, slightly more relaxed than the full Passivhaus standard due to the difficulties in achieving very low energy refurbishment projects. There is, for example, a relaxed maximum heating demand of 25 kWh/m²/yr for existing building retrofits. This Enerphit standard is required as existing homes in London often face the following constraints when compared to new build projects:

• Planning rules may prevent external insulation at the front of homes in conservation areas or on listed buildings. Internal insulation usually performs less well than external insulation, as thermal bridging (via joist penetrations, etc.) is more difficult to resolve.
• Traditionally constructed homes often have a number of complex junctions that need to be made airtight.
• Orientation and window openings are, of course, fixed, and these may be far from ideal.

Our low-energy refurbishment of a Grade II listed Georgian house in Bow. This house now has triple glazing, airtightness, MVHR and an air source heat pump.

However, there is still an opportunity to make significant improvements to our existing building stock. The criteria for both Passivhaus and EnerPHit standards are achieved through intelligent design and implementation of the key Passivhaus principles. These are thermal bridge free design, superior windows, ventilation with heat recovery, high-performance insulation and airtight construction.

In some Passivhaus buildings, aesthetics play second fiddle to performance requirements. We aim to create outstanding, elegant homes that have high levels of thermal comfort and use very little energy.

We embrace the particular design requirements of passive houses and aim to celebrate them in the design. For example, thick walls, which are required to incorporate the high levels of insulation, when expressed through deep window reveals, can be a powerful architectural statement.

We also understand that other pressures exist, such as budget, and PH certification isn’t always the ultimate end goal. Many ‘low-hanging fruit’ design factors are cost-effective and still have a real impact.

Refurbishment in stages is also possible – this can be a great approach to managing your budget effectively. For example, phase one of a project may include an extension and new windows to the Passivhaus standard. A later phase two might be adding external insulation, airtightness strategies and MVHR.

A proposed New build Passivhaus in Muswell Hill

Grants and specialist Passivhaus architects services

We are also able to advise on the domestic Renewable Heat Incentive (RHI), as well as the Green Homes Grant. These can bring the relative cost of an air source heat pump or solar panels down (for RHI) and insulation, ASHP, double-glazing, etc. (for the GHG). These measures can significantly lower the energy demand of your property and reduce carbon emissions.

Our Passivhaus architects also work with several specialist consultants to provide a range of technical services in relation to Passivhaus and low-energy design:

• We are able to provide embodied carbon calculations for proposals at an early stage.
• We can review the merits of demolition versus new build, and the length of time required to offset the additional embodied energy of new build.
• 3D modelling of various tiers of options, such as more invasive versus more straightforward strategies.

Our in-house structural engineer also has specialist knowledge of low embodied energy and timber structures.

Operational energy versus embodied energy

Carbon emissions relate to the CO2 created in the process of constructing and running your building. Passivhaus standards do not consider the embodied energy of the structure, so they are not necessarily the only standards to consider for your project.

Increased levels of insulation save carbon during operation but have higher levels of embodied carbon initially. Working out how to balance these factors and the time frames involved can be difficult to define. Modelling the ‘payback period’ of the design options is therefore key. This allows us to see how long it takes to recoup the embodied CO2 of the building materials in the lifetime energy savings of the building.

Natural materials, such as timber, wood fibre insulation, and sheep’s wool, have much lower embodied CO2 than alternatives, such as steel and plastic-based insulation. Natural materials also have the advantage that they can allow the existing building fabric to ‘breathe’. This means that the materials do not lock in moisture, which can lead to damage to the building over time.

Energy sources

The government has set a target of removing gas heating from new build houses by 2025. Alternative heating and cooking supplies are already perfectly viable, and we consider removing the gas supply in all of our projects. Currently, gas sources are cheaper, but electricity is becoming more cost-effective, and this trend is set to continue.

Low Energy House, Muswell Hill: a retrofitted Edwardian home

Local authority engagement

Engagement with the local authority is key to ensuring that the planners understand the nature of a Passivhaus project. Since the National Planning Policy Framework states that there is a ‘presumption in favour of sustainable development’, it may be that an architect-designed Passivhaus proposal can secure planning consent more easily than a typical property designed to meet building regulations only.

Where there is a conflict of interest between sustainability considerations and protecting listed buildings and conservation areas, many London councils still prioritise protecting the historic fabric over longer-term environmental consequences. This is, however, slowly starting to change. In Kensington and Chelsea, it is now possible to add solar panels to a listed building without planning consent (although still advisable to obtain a certificate of lawfulness). If, however, you’d like your home to be refurbished to a low energy standard, consider buying a property that is not listed.

We are leading Passivhaus architects in London. To view our recent projects and learn about the low-energy strategies employed, please visit Architecture for London’s portfolio.

Further reading – a map of Passivhaus projects: https://database.passivehouse.com/en/buildings/map/

Passivhaus architects: summary FAQ