What is Passive House (Passivhaus)

Definition

"Passive House" is an energy standard for buildings. A "Passive House" requires very little energy for space heating or cooling yet it  maintains comfortable internal temperatures (20deg in winter) and good air quality. To meet the Passive House standard the yearly energy consumption for heating (or cooling) needs to be less than 15kWh per square meter of net living area; this is a reduction of 75% compared to ordinary new construction. The concept was developed in Germany in the 1990s and to date over 17,000 certified Passive House buildings have been built worldwide (not just homes but schools, offices and public buildings).

In a "Passive House" the heat losses through the building envelope and the ventilation losses are reduced to the minimum and are compensated by the free heat gains (solar radiations from the windows and heat radiated from appliances and people) and a small amount of heating. This “energy balance” forms the basis of Passive House design and is calculated in detail with the aid of a software package. 
 

Concept Design

Early on in the design process the building form and orientation are the key considerations. A compact, efficient shape with a small ratio of external surface to floor area will facilitate achieving the standard. Most windows should be South-facing, although this is not mandatory. The designer’s challenge is to balance these demands with the site specifics, the client's requirements and the overall architectural concept.

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Thermal modelling in PHPP (Passive House Planning Package) 

TimeThe Passive House process relies on a thermal model of the building to predict the yearly energy use. In the early stages a quick model can be used to evaluate different design options and optimise the massing and orientation. At detail design stage the thermal model is used to specify each building component (walls, roof, windows). The model is then updated throughout the design and construction process to make sure that later changes don't compromise the standard.

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Super-insulation

Passive House is a “fabric first” approach: the thermal performance of the building envelope is considerably higher than the current Building Regulations’ requirement (U-values need to be less than 0.15W/m2K). Walls typically require 300mm of insulation, and windows are triple glazed. The continuity of insulation needs to be maintained at the junction between different building elements (roof/walls, walls/windows, etc.) to minimise “cold bridging”. The Passive House standard specifies the performance, not the technology: the construction can be masonry, timber frame, SIP panels, insulated concrete formwork or even steel frame.
 

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Airtightness

Once the building is highly insulated it is necessary to limit infiltration losses, i.e. uncontrolled air leakage through the construction which typically occurs at the junction between building elements. The Passive House standard requires an airtightness of 0.6 air changes per hour which is about 16 times better than the minimum required under the Building Regulations (10 air changes per hour). This level of airtightness needs to be carefully planned on paper at detail design stage, and typically a PH designer will draw up large scale 1:5 details of all the building junctions showing how the airtight connection is to be realised in practice.

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MVHR (Mechanical Ventilation with Heat Recovery)

An airtight building requires controlled ventilation to ensure a constant supply of fresh air. In a Passive House this is achieved through "mechanical ventilation with heat recovery". The incoming fresh air goes through a highly efficient heat exchanger where it is pre-warmed by the exhaust air from kitchen and bathrooms. Only minimal amounts of heat are then required to bring the incoming air up to the required temperature.

Renewables

Perhaps counter-intuitively the Passive House principle doesn’t require renewable energy to achieve the standard, although solar hot water and photovoltaic panels are often included in the design. Renewable energy cannot offset a poor performing building envelope.

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Site supervision

A Passive House requires considerably more care and effort on site than standard construction, especially concerning insulation and airtightness. Quality control is crucial and the PH designer will liaise with the builders to make sure that things are built as drawn, and that any challenging detail is resolved satisfactorily. Everybody involved in the building site should fully understand and embrace the PH concept.

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In essence

Although any design can theoretically be built to Passive House standard, it can be difficult and costly to achieve PH if the building is not inherently suitable (large areas of North-facing glass or excessively articulated volumes present considerable challenges). "Passive House" is a holistic approach and it should be considered from the start, which is why it is best to appoint a designer who is familiar with the Passive House principles.
Dittrich Hudson Vasetti architects in Bristol are certified Passive House designers and look forward to assist their clients on Passive House projects. 
 

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Certification

Passive House buildings need to be certified by an accredited Building Certifier (such as the BRE). The certifier will check the PHPP model and all the relevant documentation prepared by the designer, as well as reviewing site photographs and materials invoices to make sure that the construction follows the design. The certification is a guarantee that the building will performs as stated. "Passive House" is a rigorous, tested approach based on sound physics principles. Post completion energy monitoring has consistently shown that the actual energy use in a PH is very close to the design predictions.
 

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