SUSTAINABILITY 

PASSIVE HOUSE (and off-grid)

As practicing architect, I had the opportunity to design several low energy and Passive House standard residential projects. 

Erwin Keck, a befriended architect and pioneer in Passive House design introduced me to the Passive House approach.

It is a often debated standard, as it is a technical approach focused on energy efficiency with the aim to achieve a heating demand of 15 kWh/m2 per year or lower, in comparison to typical new-built houses with an average of 80 – 120 kWh/m2 per year.  

Although there is criticism that user behavior changes are necessary to be truly efficient and that it is not a comprehensive overall solution, the focus on energy saving and preventing energy losses has many positive side effects:

1. Disciplined design:

As the ratio of exterior surface to the building volume is crucial particularly for smaller buildings.

In consequence small buildings must be compact, penetrations or cantilevering elements avoided, complicated forms prevented to achieve the certified standard.

2. Optimised detailing and improved built quality:

Avoiding draft and achieving a specified airtightness of the building envelope requires more thorough detailing as well as precise application and good craftsmanship.

A blower-door test is required to certify the airtightness test (50 Pa pressure difference). The rational behind this structural design measure is, most energy losses in residential buildings occurs through air leakage and draft.

3. Air-quality and heat recovery:

A central aspect, and what bestowed the name ‘Passive House' is the for most heating days, the heat recovery ventilation system is sufficient to keep the room temperatures stable. The primary heating is through heat recovery. 

The heat-recovery ventilation system makes use of the warm exhaust air and also the embodied heat in the ground through an under-surface heat exchanger. As the fresh air passes a pollen filter, this is also in favour of any preparatory illnesses or allergies. A well calibrated low-flow rate mechanical ventilation insures draft free permanent fresh air. 

4. Sustainable Construction:

The ambitious target requires excellent insulation, therefore favours timber frame construction, or platform framing where the entire frame space can be used to insulate the building exterior. A highly insulated exterior with 300 – 360mm insulation depending on climate zone easily outperforms any typical standard.

5. High performance glazing:

To achieve the required insulation standard, triple glazing with insulated frames is needed. This reduces not just the heat loss through windows and frames but provides a much-improved noise insulation both ways.

In my view, the Passive House was and still is a positive contribution and should not be considered as a finite solution. These positive side effects which clearly work in favour of this otherwise rather one-sided approach. In practice I combined elements of the Passive Hause with other techniques such as ground source or air source systems with extensive PV panelling. Without aiming for the exacting technical standard, it led to more flexible designs as well as excellent energy performances and design ecology.  Except one, all passive houses I designed were timer frame construction, often with locally sourced wood. 

CRITICAL REFLECTION

Michael Braungart, one of the founders of the Cradle-to-Cradle design approach, criticised the Passive House as insufficient as it aims just at minimising heat loss. Other aspects such as waste avoidance and recyclability are not actively considered. Braungart proposed houses with a positive energy balance (German: Energieplus Haus).

The title Passive House is indeed misleading. It still requires at least an electrical backup heating system in case there is not enough heat to recover. Another crucial aspect is the location and orientation, as passive houses must be sufficiently south oriented for passive heat gain through fenestration. Overshadowing, tree lines, dense developments can make it impossible to achieve the certified standard.

Despite valid criticism, the Passive House standard still outperforms the required legal standards by far is remarkable and is also a major concern.

However, in my view the approach leads in many ways the right direction, as it enacts other aspects such sustainably sourced renewable materials and high-quality insulation materials.

And it is technically possible to upgrade house aiming towards Passive House to an energy-positive house, or completely off-grid. One client, a four-person household, opted for off-grid which successfully was achieved, in essence an Energy+ house. The large ideally oriented roof allowed extensive and efficient PV panelling, combined with storage batteries and air source heat-pump, the house effectively achieve a positive energy balance.

PREFABRICATION

Timber frame construction is very suitable for prefabrication. In two cases, the client with support of a local carpenter business, prefabricated wall, and ceiling elements.

Designing the construction plan, lorry and crane specifications must be considered. Particularly for timber construction, prefabrication has many advantages:

independence of bad weather

shorter on-site construction period

protection of materials

preparation for installations

construction precision and quality. 

However, it requires thorough planning and logistics.

The success of the prefab industry in Germany is an impressive demonstration of these advantages. With standardised details and quality control, prefabricated timber frame achieves in average a more reliable quality and overall performance than an onsite erected building.    

In continental Europe, particaurly Germany, Austria, and Switzerland, prefabricate house companies enjoy a positive reputation. Companies such as HUF-house, or Baufritz, Zenker, Weber, Schwoerer, are well-known and respected brands. Whilst UK has a heritage with after World War Two prefab, today it appears still hesitant in accepting prefab (modular) in larger new developments. Most new built estates are still erected in traditional cavity-wall construction. However, the trend is positive.