What is the Passivhaus (or Passive House) standard? Passivhaus is a low energy building standard and methodology that can be used for new buildings. It has been applied to homes, offices, hospitals and even skyscrapers.
Here are a few key principles that underpin the standard:
It's very low energy
Passivhaus buildings use around 80% less energy compared to a traditional home. Take a look at this article to learn more (Would a Passivhaus building reduce my energy bills?).
Passivhaus buildings typically have significantly more insulation compared to a traditional new build. By way of example, Phase 1 of Mulberry Park in Bath, completed in 2016, has 100mm of wall insulation. A detached Passivhaus building will have around 350-400mm of insulation.
Air Quality & Ventilation
A key rule of low energy building is to 'insulate and ventilate'.
Traditional buildings commonly have areas where air can pass through the building fabric. For example, chimneys, around windows and through porous materials (such as stone or brick). This causes a number of issues:
Warm air escapes the building in winter and enters the building in summer
Traditional buildings are often over-ventilated. If too much air passes through the building, moisture is removed from the internal atmosphere and humidity drops to uncomfortable levels. This leads to conditions such as dry skin and can aggravate respiratory conditions like asthma. This is especially the case in winter as cold air holds less moisture than warm air.
At it's core, the Passivhaus standard is about taking a rigorous and measured approach to the design of the building. The building fabric is well insulated and airtight. The amount of air entering the building is controlled and balanced through a mechanical ventilation system. This ensures that the humidity of the air is comfortable; not too high to cause mould nor too low to cause dry skin and other irritations for occupants.
An option to recover heat is also a common part of the ventilation system. This is known as Mechanically Ventilated Heat Recovery (MVHR).
An MVHR unit consists of:
Two fans: one supplying air to the building, another extracting air from the building
Two filters: one on each fan; filtering out pollutants like dust, pollen and air pollution
A heat exchanger: originally made from paper, this is a honeycomb shaped structure that allows the supply and extract air paths to overlap (but not mix).
The system is underpinned by the second law of thermodynamics (ie: that heat flows naturally from regions of higher temperature to regions of lower temperature, but that it will not flow naturally the other way). As a result, hot stale air that is extracted from the building is overlapped (but not mixed) with cold fresh air entering the building. Heat is transferred from one air path to the other, leading to the supply of warm fresh air into the building.
The Passivhaus standard requires a minimum heat recovery of 75%. Some units can achieve 90% heat recovery; reducing energy bills.
No Heating System
MVHR systems can be paired up with a 'post heater' (basically a low energy hair dryer) that can top-up recovered heat to the desired internal room temperature (typically 18-20°C). As a result, the ventilation system is also the heating system. No conventional heating system is required.
The filters within the MVHR system are very high quality; filtering out external air pollution, pollen and dust. This reduces allergies and increases air quality within the home. Considering we spend an average of 22hrs a day indoors, this is an important feature.
Heavily insulated buildings are at risk of overheating. The Passivhaus standard requires that buildings do not exceed 25°C for 10% of the year. Good designers will aim for 2% of the year; equating to 1 week of overheating. This insures the building is resilient in a changing climate.
The orientation of the building and position of glazing is critical to managing overheating. In addition, it is important to provide external shading (fins, blinds, vegetation or shutters).
The windows open
A requirement of Passivhaus is that all habitable rooms have at least one opening window.
Passive house | Passive occupant
A common mis-conception is that a Passivhaus building has lots of technology and is high maintenance. The only additional technology within a Passivhaus building is the MVHR. Ideally, we would not have any other space heating system within the building. We would need something to generate hot water (for showers, etc). This is usually a heat pump.
Unless the client asks for specific 'smart' technologies, we wouldn't expect a Passivhaus to have significantly more technology compared to a conventional new build home.
There is a very active community of Passivhaus enthusiasts throughout the world; many of whom are not professional consultants but people who are interested in and dedicated to environmentally responsible low energy building. A few communities include the Passivhaus Accelerator (that has a good podcast), the UK Passivhaus Trust, the German Passivhaus Institute and International Passivhaus Association.
Certification & Process
Passivhaus is an open-source and free methodology; meaning that you can construct a Passivhaus building without needing to pay for any certification. However, there are a number of benefits to getting certification and it is generally very good value for money. See this link to learn about Certification and Process.
Materials & Carbon
Passivhaus buildings can be constructed from anything. There are no requirements on embodied or operational carbon. In fact, concrete and other 'heavy' materials are popular because they have good thermal mass.