The philosophy behind Passivhaus design
Sustainability Report 2011
The philosophy behind Passivhaus is simple: build homes that require very little energy to run, and not only will this reduce the amount of CO2 that is pumped into the atmosphere but it will also create extremely comfortable places in which to live, all at a very low annual running cost. As the primary energy demand of a house in a European climate is heating, the Passivhaus design approach focuses on keeping heating energy demands to a minimum through good building design and high construction quality.
In addition, all other home energy demands are considered (including ventilation, domestic hot water, cooling, lighting and household activities such as clothes washing and drying, cooking and electronic devices) and low energy demands and rigorous comfort standards must be achieved in each area. Since its original conception, the Passivhaus approach has been adapted to suit the whole spectrum of building types and it is now possible to design almost any style of building to the Passivhaus standard.
Passivhaus is a design philosophy which encompasses both a benchmarking standard and a calculation methodology. A calculation of energy demands must be undertaken using the Passivhaus House Planning Package (PHPP), a spreadsheet based calculation tool which uses algorithms and design standards developed by the Passivhaus Institute in Germany. The PHPP has been developed from the on-going research into building physics which is carried out by the Passivhaus Institute and is continuously updated to reflect the most up-to-date level of understanding.
As a consequence, the PHPP incorporates design standards which are often more rigorous than British and European standards and predicts annual energy demands with an accuracy considered superior to other commonly used calculation methodologies, such as SAP and SBEM which are the required methodologies for demonstrating UK Building Regulations Part L compliance. Once energy demands have been calculated using the PHPP, the following standards must be achieved in order for a building to be certified by the Passivhaus Institute.
Annual Space Heating Demand:
≤15 kWh/m²/yr (@20°C internal design temperature)
Annual Primary Energy Demand:
≤120 kWh/m²/yr Inc. domestic electricity consumption
Air-tightness:
0.6ach@50Pa Verified through air pressure testing
Insulation:
0.1-0.15 W/m²K walls, floor and roof
0.5-0.9W/m²K windows (whole unit)
Extremely low thermal bridging
Servicing:
High efficiency MVHR (π>90%, SFP ≤0.31Wh/m³)
Solar thermal
Photovoltaics
Low NOx gas-fired condensing boilers
In addition, the certifying body must be confident that the finished construction has been built to the standards described in the certified design. To ensure this, documentation of the on-site practices and the materials used is required, in a process similar to that used to demonstrate BREEAM compliance.
Case study – Sulgrave Gardens
At Sulgrave Gardens, a 2,500m² site in West London, close to Shepherd’s Bush, Atelier Ten are working on a multi-dwelling Passivhaus project for Octavia Housing, in conjunction with Cartwright Pickard Architects. This is the first new-build Passivhaus project that Octavia have commissioned and their aim was to test-drive this approach as a route to delivering Code for Sustainable Homes level 4 as well as reduced fuel bills for tenants and higher occupant comfort.
The development consists of four blocks, two blocks of terraced houses and two blocks of flats, and is a mixture of private sale, shared ownership and social housing. At an early stage it was determined that achieving the Passivhaus standard on two of the blocks would be extremely challenging and the decision was taken that, although all dwellings would be built to the same extremely high standard, certification would only be sought for two blocks, one row of six terraced houses and an apartment building containing twelve dwellings.
This experience highlighted an essential consideration for Passivhaus design: the standard is so rigorous that Passivhaus principles on building form and orientation must be incorporated at the very earliest stage of the design if the standard is to be achieved and that for some sites in built-up urban areas achieving the standard may be extremely challenging. Passivhaus relies on beneficial winter solar gain and a simple building form which encompasses a small heat loss area to keep heating energy demands low. On a constrained London in-fill site with high overshading and planning restrictions due to site lines and adjacent buildings the requirements were judged to be too onerous.
A building envelope constructed from Structurally Insulated Panels (SIPs) was selected as a way of achieving extremely low heat loss (U-values of around 0.1 W/m²K for walls, floors and roofs) within an acceptable construction thickness. For the blocks of flats, a concrete frame was required to provide structural strength and the SIPs were clad externally to create a warm blanket of insulation around the frame.
Building to Passivhaus standard using this construction methodology was new for the SIPs manufacturer, so numerous bespoke details were developed in order to get as close as possible to ‘thermal bridge free’ design, another fundamental principle of Passivhaus construction.
Additional hurdles were encountered as some products used widely in Germany to reduce heat loss at construction junctions are not yet recognised by bodies such as the NHBC and could not be used. In order to achieve both the Passivhaus primary energy benchmark and Code level 4, highly efficient building services systems were required in addition to the highly efficient building envelope.
A client preference drove the strategy towards individual gas boilers, rather than a communal system, and these were coupled with solar thermal panels in the houses, where integration with the domestic hot water system was simple, and photovoltaics on all blocks. Mechanical ventilation heat recovery units (MVHR) have also been specified to ensure that minimum fresh air rates are achieved, an important consideration for a highly air-tight Passivhaus design where air leakage through the building fabric is extremely limited.
Design challenges included incorporating sufficient plant space to accommodate the ventilation units and duct runs and designing a heating system which can meet both the high instantaneous domestic hot water load and the very low space heating demand without boiler cycling.
Pursuing Passivhaus certification at Sulgrave Gardens has brought numerous benefits. It formed an integral part of the energy strategy, allowing Code level 4 to be pursued with a reduced renewables requirement, and it was looked on favourably by Hammersmith and Fulham planning department, contributing to the overall successful package. Looking beyond this project, however, we can see that there are many applicable lessons learned for our future housing design work within the UK when it is not being assessed under the rigours of Passivhaus.
The proposed changes to the domestic energy efficiency standards of the building regulations (Part L1) could potentially introduce an absolute limitation on energy consumption, alongside the conventional comparative calculations. As Passivhaus is an absolute standard of energy effacing (i.e. based on an absolute numeric quantum of energy consumption) rather than just an improvement over a building of the same form, means that the UK regulations could start to shadow some of the Passivhaus requirements. While the UK methodology is unlikely to involve the rigour and complexity of the Passivhaus standard, our experience of working to this standard will allow us to readily respond to these upcoming challenges in the wider housing market.