A "passive house" building is often considered to be the ultimate low-energy or "passive solar" building, as it is designed to the practical limits in terms of insulation and in terms of using solar for heating. The Passive House concept, is an extension of the principles explained in the overview of passive solar buildings above, i.e. based on energy-efficient construction combining low heat losses with maximum use of free solar gains.
A Passive House requires as little as 10 percent of the energy used by typical European buildings, which means an energy savings of up to 90 percent. Passive Houses have high levels of insulation and an extremely airtight design with controlled ventilation including heat recovery system. Another important feature of passive houses is that of a design free from "thermal bridges", which means that the insulation all over the building is applied without any "weak spots" , which eliminates cold corners as well as excessive heat losses and moisture build up.
Any competent architect can design a Passive House. By combining individual measures any new building anywhere in the world can be designed to reach the Passive House standard. In order to correctly design a passive building, it is necessary to use the "Passive House Planning Package" or PHPP. This is a piece of software which calculates the optimum levels of insulation, glazing etc. based on a given building design. The major advantage of using the PHPP to assist with building design is that elements of the building such as glazing size/type, or insulation thicknesses/types can be tweaked to find the optimum levels.
The PHPP also takes into account the free solar heat gains, and heat gains from occupants, to calculate the final energy requirement of the building. It is important to note that the PHPP does not consider the Irish Building Regulations, and your architect or designer should also ensure that the design also meets these requirements. The Passive House Standard is also increasingly being used for non-residential buildings. A list of certified Irish passive house planners is available on the Passivhausplaner website.
The Royal Institute of the Architects of Ireland also have an architect service which is very useful, as you can describe your project brief and engage a suitable practice to assist.
Main Characteristics of a passive house
The main characteristics of a passive house that might seem to be "new" concepts, are in fact, very easy to grasp :
The better the level of insulation is, the more energy and money will be saved. Other features are also positively affected by good insulation:
- Heat losses through external walls and roofs account for more than 70% of the total heat losses in existing buildings, Therefore, improving thermal insulation is the most effective way to save energy.
- Thermal comfort is increased and there are better options for effective heating
- The risk of mould formation is reduced,
- During summer months, good insulation also provides protection against heat.
Airtightness is another high-priority area in passive house construction, for the following reasons:
- Building damage caused by water vapour present in air draughts can only be prevented by airtightness of the envelope, and building damage is mostly due to the absence of airtightness in the roof area.
- Draughty living spaces and cold air pockets lead to a hard to heat building
- Ventilation systems with supply air and exhaust air spaces only function if the building envelope is sufficiently airtight.
- Airtightness results in better sound protection
- Where a building is well insulated, unwanted air infilatration accounts for a high percentage of heat loss, thus reducing the overall effectiveness of the insulation.
Interestingly, new 2011 building regulations prescribe a good level of airtightness.
Eliminating thermal bridges
Thermal bridges are junctions where insulation is not continuous , the main reasons to eliminate thermal bridges are :
- They lead to decreased interior surface temperatures and in the worst cases this can result in high humidity in parts of the construction.
- Where a building is well insulated, thermal bridges make up a high percentage of heat loss, thus reducing the effectiveness of the insulation.
As well as reducing heat loss as much as possible through insulation, airtightness and eliminating thermal bridges, it is important to maximise the solar energy available to the building in order to contribute significantly to the low heat requirement of the building.
There are a number of areas to consider, which you will have seen already in the overview of passive solar buildings.
- Compact form and good insulation
- Southern orientation and shade considerations
- Energy-efficient window glazing and frames
Note, one of the benefits of using the PHPP is that it can help find the balance between passive solar heat gains and overheating, so as to indentify the optimum form (shape), orientation, glazing type and size and shading techniques to use.
Controlled ventilation and recovering waste heat
We are accustomed to living in dwellings that have no controlled ventilation, and rely on the gaps and cracks to provide the air necessary, but this type of house loses a large amount of heat. An airtight construction reduces the infiltration of unwanted cold air and the escape of heated air, which is very important if a low-energy building is the goal. However, in an airtight building, where there are no gaps and cracks to supply air, there is a need for controlled ventilation, or controlled air supply to the building. For this reason, in very airtight buildings, there is usually a Mechanical Ventilation system installed to control air flow, and to ensure that the occupants have sufficient air flow for comfort.
In a passive house, where the heat requirement is so low, energy from waste heat (such as cooking, clothes drying, showers etc.) can be used to pre-heat the incoming air in the Mechanical Ventilation system using a heat recovery exchanger. This is also known as Mechanical Ventilation with Heat Recovery (MVHR). Note that in a house that isn't airtight, MVHR will be of no extra benefit to the occupants.
Although a passive building is designed with very low heat losses and maximises free heat gains from the sun, there is always a remaining energy requirement which must be satisfied. A building meeting the requirements of the passivhaus standard (see www.passiv.de) does not need a conventional radiator system or underfloor heating. Because of its low heat demand, the heat can be distributed through the mechanical ventilation system, which allows the building services for heating to be very simple.
The following features are characteristic of building services in a passive house :
- The remainder of the heat demand is supplied by systems such as:
- A small heat pump (compact heat pump unit)
- A small condensing boiler (compact condensing unit)
- A small heat generator based on biomass fuel e.g. wood pellets
- Heating and hot water production assisted by solar thermal - See Active Solar Heat Technologies.
In a passive house, at design, planning and construction stages, the main goal is to achieve a building that meets the heating requirements of the passivhaus standard. It is important to note that the passivhaus standard is designed with an overall low energy requirement in mind, and electricity use cannot be ignored as it can make up almost a quarter of energy used in our homes. For every unit of electricity energy we use in our homes, it takes 2.7 units of energy to produce and transport that to our house. This is known as the "primary energy". What we are using at our plug points is "delivered energy". The passivhaus standard limits the amount of primary energy that a house is resonsible for, and some of the common features are:
- Low energy refrigerators, freezers, lamps, washers, dryers, dishwashers.
- Low energy lighting
- Energy efficient ventilation