What is a Passive House

The Origins of the Passive House

In 1988 Professors Bo Adamson of Lund University in Sweden and Wolfgang Feist from the Institute for Housing and the Environment in Germany developed the idea of the Passivhaus or Passive House. In 1990 they designed and built 4 Passive Houses in Darmstadt in Germany.

In September 1996 the Passivhaus Institute was set up in Darmstadt to help develop standards and improve techniques for building Passive Houses. Since then, over 25,000 certified Passive Houses have been built.

Principles of a Passive House

Concept

The main concept behind the Passive House is to design a house that does not need to be ‘actively’ heated (or cooled) by a HVAC system such as central air, hot water or gas. As with sustainable design and green interior design, the motivation is to make environmentally friendly houses that have a low carbon footprint and that reduce the use of electricity to a minimum and thus reduce carbon emissions from power stations.

The difference between a Passive House and a Zero Energy Building is that the Zero Energy Building does not seek to replace heating/cooling systems with alternative technologies. Rather the electricity for the house is generated from alternative energy sources such as photovoltaic panels. The aim of the Zero Energy Building is to make a building that produces more electricity than it consumes. The aim of a Passive House is to design a house that doesn’t need heating and cooling by heat pumps, furnaces, air-con units etc.

Passive Houses have worked very well in Central Europe and Northern Europe. Most of the certified Passive Houses have been built in Germany, Austria and Scandinavia. Typically, a passive House loses less than 0.5 °C (1 °F) per day (in winter), stabilizing at around 15 °C (59 °F) in the central European climate. This is an incredible achievement and obviously makes Passive House technologies key to formulating strategies for making buildings independent of fossil fuel energy.

How Does a Passive House Work?

One of the key components to designing a Passive house is the use of superinsulation. Superinsulation is what it sounds like – insulation of a much higher specification than normally found in buildings. One of the key components to Passive House construction is to build walls with much bigger insulation gaps than normal houses. Much more insulation is used. Although Passive Houses are normally built from scratch it is possible to retro fit a house to conform to Passive House standards.

Insulation is also used in the roof and the gaps between the walls and the roof and the walls and the flooring are firmly sealed to prevent loss of heat. All holes for wiring, ventilation etc. are also carefully sealed to remove any holes where heat can be lost. Also the doors and windows are made with air-tight seals. Typically for a Passive House the walls are Rip40 and the roof is Rip60. These figures are measures of thermal resistance.

Another key component of superinsulation is the window design. Windows are made smaller and often use tree cover and awnings to reduce heat transfer. The windows themselves are triple glazed with a low e finish to further minimize heat loss/gain.

Instead of using standard HVAC equipment such as central air, air-con units, gas, hot water etc. a Passive House instead uses a dual purpose 800 to 1,500 watt heating and/or cooling element integrated with the supply air duct of the ventilation system. Normally the heater will not be used as the house is so well sealed and insulated that the waste heat given off from electrical appliances such as fridges, washing machines, light bulbs etc. combined with the body heat given off from the human inhabitants of the house is enough to keep the house warm.

It is, however, important to use electricity to keep the house properly ventilated. Because the house is sealed it is not easy for air to circulate around the house. Without air circulation the indoor air quality deteriorates. To solve this problem as efficiently as possible, using as little electricity as possible a heat recovery ventilation system is used. The heat recovery ventilation system has a heat recovery rate of over 80%. The system uses high-efficiency electronically commutated motors (ECM) to maintain air quality. When the weather permits natural ventilation is used such as a small opening or a more effective stack effect (smaller ingress and larger egress windows).

To power the heat recovery ventilation system photovoltaic panels can be put on the roof. Because the system uses much less electricity than normal HVAC systems and because it is often not used in the summer and spring the electricity generated can be deployed for running other electrical appliances. Often a Passive House will also use solar gain technologies to heat water.

Cost / Pros and Cons

A Passive House costs 14% more to build than a conventional house. The extra expense is soon paid for by the huge savings to be made on heating bills.

Passive Houses have performed excellently in the colder climes of Central and Northern Europe. Only a few Passive Houses have been certified in hot and tropical climates. Obviously in hot and humid conditions the challenges to the green designer are different. There is a danger of the insulation in the walls and roof becoming covered in mold. There is a house in Lafayette, Louisiana, USA, which is a certified Passive House The house uses Energy Recovery Ventilation instead of Heat Recovery Ventilation. This removes excess humidity and transfers excess heat to the hot water tank. Still much of the literature and technologies associated with Passive House design and architecture are concerned with replacing conventional heating not conventional cooling systems.

Finally, Passive House design tries to limit surface area and window size. Not everyone will be happy about living in a smaller house with small windows. Still overall, the Passive House is an impressive achievement and a testament to the efficiency of superinsulation. It is also a testament to how inefficient conventional homes are.

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