Architecture for London are leading Passivhaus architects in London. All of our designs employ a ‘fabric first’ approach to sustainability and director Ben Ridley is a Certified Passivhaus Designer. Visit our projects portfolio to see our recent work, or contact us on 020 3637 4236.
This article discusses 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.
So can Passivhaus principles be applied to our existing city homes? Can a historic, or even a Grade II Listed house be refurbished and upgraded to meet the strict Passivhaus energy performance criteria?
Passivhaus 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 homes in London
A new build Passivhaus design should be defined by several factors, including form factor, orientation and solar shading, rather than by purely aesthetic considerations of, 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, with a peak demand of 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.
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.
Architecture for London’s proposed new build Passivhaus housing scheme in Hounslow, west London
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. There are also many more fixed parameters that act as limitations in achieving the full Passivhaus criteria. These include issues of conservation, building orientation and window sizes.
Airtightness is only viable in parallel with a suitable ventilation strategy, to ensure fresh, oxygenated air is introduced into the building. A mechanical ventilation heat recovery (MVHR) system manages airflow to rooms throughout the house, removing stale, moist air from kitchens and bathrooms, whilst supplying fresh warmed air to living spaces and bedrooms.
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.
Existing homes in London
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.
Work in progress: A low energy refurbishment and extension of an Edwardian house with triple glazing, airtightness and MVHR.
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 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 Institute sets 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 as issues of thermal bridging (joist penetrations etc.) are 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.
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: thermal bridge free design, superior windows, ventilation with heat recovery, high-performance insulation and airtight construction.
Choosing Passivhaus architects in London
In some Passivhaus buildings, aesthetics play second fiddle to performance requirements.
Our aim is to create outstanding, elegant homes that also happen to 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. There are many ‘low hanging fruit’ design factors that are cost-effective and still have 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 Passivhaus standard and then later add external insulation, airtightness strategies and MVHR for phase two.
A proposed new home to Passivhaus standards in Muswell Hill
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.
Architecture for London also work with a number of 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 out 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, 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’ which will protect it better in the long term, by not locking in moisture and allowing damp and rot to damage the building.
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.
Local authority engagement
Engagement with the local authority is key, to ensure 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 slowly starting to change, but it is worth bearing in mind that if you want to ensure your house is future-proofed and sustainable in the long term, consider buying a property that is not a listed building, or in a conservation area. These have fewer restrictions and will proceed more smoothly.
We aim to be the leading Passivhaus architects in London. To view our recent projects and learn about the low-energy strategies employed, please visit our project portfolio.
Further reading – a map of Passivhaus projects: https://database.passivehouse.com/en/buildings/map/
Passivhaus 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.
Due to the difficulties when dealing with retrofit projects, the Passivhaus Institute sets a slightly less demanding set of criteria for the refurbishment of existing buildings, this is the EnerPHit standard for refurbishment.
Both the Domestic Renewable Heat Incentive (RHI) and the Green Homes Grant may be available. These can reduce the cost of insulation, triple glazing, an air source heat pump or solar panels.