Understanding The “Passivhaus” Building Standard

All new properties built in the UK must be built in accordance with building regulations. Building regulations cover just about every expect of the build and include structural stability, electrical and gas safety, minimum hygiene standards such as fresh water etc etc.

They also set minimum standards for energy efficiency. Although these standards are improving, the government, when it makes legislation, has to strike a balance between better energy efficiency and affordability for the industry. As a result there is still a gulf between the minimum standards set by building regulations and the best that can actually be achieved.

What is the Passivhaus Building Standard?

The Passivhaus standard originates from Germany and is a superior standard in energy efficiency, generally recognised as the highest standard achievable. It was actually developed in 1988 and, though it is relatively new in the UK, there are an estimated 17,000 homes worldwide that meet the Passivhaus standard.

The aim is to create buildings that are comfortable whilst also being efficient and so reducing both their environmental impact and running costs. The principles can be applied equally to residential and commercial buildings.

What are the Key Design Principles of Passivhaus Homes?

There are three very simple principles to apply to create a successful Passivhaus building:

• An airtight building envelope
• High levels of insulation
• A mechanical ventilation system

Let’s examine these principles in more detail.

Creating an Airtight Building Envelope

By making the building airtight, heat loss is reduced as is penetration of cold air from the outside. To make it airtight, the design needs to be examined for gaps such as at the junction between the wall and floor, wall and roof or between two walls. Also, breaks in the insulation need to be identified. Any gaps that are found must be closed.

Draught proofing is also important, particularly around windows and exterior doors. Letterboxes should be draught proofed as well as keyholes. Thought should be given to what happens when exterior doors are opened. If possible a small cloakroom area, with a door, should be created between the living area and the exterior door, so that cold air from outside does not reach the main living area when the exterior door is open.

Installing High Levels of Insulation

High levels of good quality insulation will minimise heat loss and can be used to close gaps in the building envelope. A well insulated house will not only be more energy efficient, it will be more comfortable since it will be easier to keep it at a constant temperature.

Installing a Mechanical Ventilation System

A mechanical ventilation system with heat recovery, i.e. which extracts the heat from air that is expelled, should be installed and will further reduce the need for heating while keep the building at a constant, comfortable temperature.

What are the Standards that a Building Must Meet to be Considered a Passivhaus?

To qualify as a Passivhaus a building must achieve an overall U value of 0.15. The U value is the rate at which heat escapes through the various building elements such as walls and windows. It must also be completely airtight, which is defined as losing less than 60% of the total building volume of air per hour through unsealed joints.

It must be fitted with “Low E” triple glazed windows in airtight frames which must be positioned so as to maximise solar heat gain in winter and minimise it summer. In the northern hemisphere this done by facing the property south and installing shading carefully, so that the windows are in shade in summer but not in winter. Though tricky, this is possible because the sun will be at a higher angle in summer than in winter.

A full mechanical ventilation system system with an efficiency level of least 80% must be installed. This means that a minimum of 80% of the heat is extracted from air that is expelled and is used to heat the incoming air.

The total energy used for space heating/cooling for properties at latitudes of 40 – 60 degrees north (which includes the UK) must be no more than 15 kWh per square metre per year. By comparison, a typical family home in the UK will use around 200 kWh per square metre per year. The total energy usage, including space heating/cooling, hot water and appliances must be no more than 120 kWh per square metre per year.

Energy Efficient Appliances

A Passivhaus should have only the most efficient, A++ rated appliances, which will reduce energy usage.

Heating Systems for Passivhaus Properties

A Passivhaus building will not usually have a conventional heating system. Instead, a 3 kWh heater in the ventilation system should be sufficient.

Advantages and Disadvantages of Passivhaus Properties

Naturally a Passivhaus property will be more expensive to construct, though the difference is cost is perhaps not as great as most people would assume. In fact it could be as little as 10% more than a standard construction. The occupiers of the building need to be educated in the proper habits. In particular, something as seemingly harmless as opening a window can seriously unbalance the whole system.

A Passivhaus should use just 10% of the energy use of a standard property, that’s a 90% saving which could equal around £1,000 a year, not to mention the positive effect on the environment. As energy prices continue to rise and as governments start to respond to pressure to reduce carbon emissions by taxing them the financial benefits of the Passivhaus will become yet more apparent.

Lastly, it should be much easier to control the climate of Passivhaus, making it more comfortable.

Can Existing Houses be Converted to Passivhaus?

It is possible to retro-fit a property to Passivhaus standard however the cost may be prohibitive. Although draught proofing and insulation should be fairly simple to install, making the property airtight will be more difficult (this may for example require replacement of the wall ties) and installing the ventilation system will require more work than installing it in a new property.