Passivehouse Principles

Definition

A passivehouse is a building in which you can maintain a pleasant temperature in the winter as well as in the summer without a separate heating or air conditioning system. It offers a high living comfort with a heating requirement of less than 15kWh/m²a and a primary energy consumption of less than 120kWh/m²a including hot water and electricity.

To this definition we add one important point characterising passivehouses from Zero-Carbon Solutions: a passivehouse is only a true passivehouse if it is certified by the PassivHaus Institut or a UK company authorised to do so. Neither passivehouse nor Passivhaus are protected concepts - anybody can call anything a 'passivehouse' - so the certification is key to assuring you get the genuine product!

This heating requirement corresponds to an annual heating bill of £50-100 based on current UK gas prices for a 100m² dwelling, less than one-tenth of the price most householders currently pay, and 20-25% of what you would pay in a house built to current building regulations. You attain these sensational savings because of two overriding design principles used consequently in passivehouse design:

avoid heat losses; and
optimise free heat gains.

Ultra-insulated building components, including wall and roof thickness of up to 40cm and triple-layer noble gas filled glazing, keeps the heat inside the house. Fresh air and humidity control is supplied by a mechanical ventilation system which recovers 80% or more of the heat in the used air it expels and transfers it to the fresh intake air. Hence, if the outside temperature is 1°C and the room temperature 20°C, the heat exchange system heats the intake air to around 16.5°C using only the heat in the used air. For this reason the fresh air only needs to be heated another 3.5°C to reach room temperature - instead of 19°C without heat recovery.

At the same time the fresh air is filtered to remove most of the pollen and dust before being led into the dwelling, a blessing for sufferers of allergies and asthma. A passivehouse is healthy and comfortable to live or work in.

Solar energy is free

Even in the UK, the solar intensity is sufficiently high to supply most housing energy requirements. When designing passivehouses you attempt to use passive solar energy to cover as much of the energy requirement as possible, e.g. by including large south-facing windows and placing building components with large thermal mass where the sun rays hit, thus storing solar energy for later use. That way you cover nocturnal heating requirements and may even be able to bridge over shady days. To avoid over-heating in the summer months you normally need to use shading, e.g. by placing balconies between the large glass surfaces and the sun when it is in its highest position in the summer sky. If, in the winter months, you do need additional heating a small amount of heat can be added to the ventilation air, e.g. using a small heat pump. A separate heating system is not normally required in a passivehouse.

The passivehouse concept, originating in Sweden, has been under development for the past more than ten years and more than 10.000 have been built. Measurements over many years have verified the viability, high quality and high comfort level of the system. Even in the winter 1996/97, with a prolonged period featuring an outside temperature of -14°C a 20m² room in a passivehouse required only an energy supply corresponding to two normal 75W light bulbs to maintain a temperature of 20°.

Why passivehouses work

Let us dig a little deeper into the building physics of what it is that makes a passivehouse work. Basically it is constructed like a ventilated Thermos flask. The requirement of an annual space heating of only 15kWh/m²a is not a coincidence. In a northern European climate it is the value which makes it possible to avoid having to install a normal heating system in the house when all the available sources of heat are taken into consideration, e.g. cooking, tv and computer operation, body heat and solar gains, coupled with state of the art ventilation heat recovery. Heat is lost from a building through two channels:

by transmission of heat through the fabric of the building, i.e. walls, roof, windows, etc. How much heat is lost (or gained) in this manner is determined by how a building is constructed and insulated, and can be calculated at the design stage, or even for existing buildings; and
through ventilation, controlled as well as uncontrolled through temporary or permanent leaks in the building envelope. Opening a window or door causes a temporary leak, through which an amazing amount of heat can be lost. Permanent leaks are caused by poor workmanship, settlement or ageing, causing cracks e.g. between window frames and walls.

By calculating the losses through the building fabric and controlled ventilation, and subtracting the gains through solar heating and internal heat sources you arrive at the annual residual space heating requirement. For a house to qualify as a passivehouse this, as mentioned, must be below 15kWh/m²a. It is important to understand that this is a calculated value. Reaching it depends on behavioural and operational issues. If the inhabitants of a passivehouse leave windows or doors open in the winter for example, it is unlikely that the projected operating costs will be attained.

The second value a house must attain in order to be classified as a passivehouse, is a primary energy factor not exceeding 120kWh/m²a. Primary energy is energy derived from non-renewable sources such as coal, oil or natural gas. This factor includes the total energy requirement of the house, including heating, hot water and appliances. The original reason for including this condition possibly was that builders might be tempted to improve on the space heating figure by building in more heat sources than strictly necessary, e.g. lights. However, as the quality of passivehouses has increased to the point where space heating is no longer the largest energy sink, the primary energy factor becomes increasingly significant, and with that advanced intelligent building energy controls and integrated power generation, in step with the UK 'zero carbon' home concept.

Whereas the original passivehouse calculation criteria were based on reasonable assumptions if you live in Middle or Northern Europe, expanding the concept to a pan-European, or even global, scope, has required some changes of focus, most of which have been incorporated in the most recent incarnation of the Passivehouse Project Planning software, PHPP 2007, which among other factors include UK climate data. As a consequence the passivehouse definition is moving towards a limit for the actual heating load of 10W/m² with more emphasis on the primary energy factor. Hopefully this moves the Europe-wide acceptance of PHPP as the primary calculation tool for passivehouses closer, although in the UK, the Building Research Establishment (BRE) is still playing catch-up with their SAP calculation. Reading the Scottish building regulations closely, PHPP may be acceptable in Scotland although SAP is the reference system. We would love to find a Scottish passivehouse customer to test this out!

The mathematical description of transmission heat losses and gains is really quite simple. A house is after all an insulated box of some geometrical shape that can be described. The inside of the box is separated from the outside by an inhomogeneous assembly of walls, floors, roofs, doors and windows. We can calculate the areas of all these components as well as their respective U-values (see Introduction in the menu to the left to learn about U-values). When we know the orientation of the box and some climate data we can also calculate the basic influence of solar energy. So, assuming a comfortable interior temperature and knowing how the sun, local conditions and the climate influence conditions we can make credible calculations regarding transmission losses. As a result of these we arrive at the conclusion that we should strive for U-values below 0.15W/m²K in all building components.

In order for a normal masonry wall to reach that insulation level it will have to be more than 1m thick. This is unrealistic (and perhaps a slightly unfair comparison since we moved away from solid masonry walls 50 years ago). However, it does serve to illustrate that we need different construction methods in order to reach ultra-insulation. (It also illustrates why the current UK policies of ultra-dense construction and conservation of old buildings without any architectural or historical value is incompatible with modern ecological considerations).

When we consider heat losses through unplanned and uncontrolled ventilation one factor stands out more than anything else: a passivehouse must be airtight. From a construction point of view airtightness is extremely difficult to attain. From planning to execution this goal requires the utmost attention by architect, engineers, site foreman and craftsmen. As a rule of thumb for architects it is necessary to be able to trace a path with a pencil on the plans around the building components separating heated and unheated areas irrespective of which intersection you display. If you have to lift the pencil from the paper something is not airtight.

Similarly, when designing building component details, engineers must make sure they do not break the airtight layer in the building. This requires attention to all connections between building elements, e.g. between window and door frames, and walls, and also in the floor and roof areas.

All the architects and engineers in the world can not compensate for bad craftsmanship and poor site management. To successfully reach passive house level you need excellent site management (which contributes considerably to the cost of passivehouses built on site rather than being pre-fabricated), you need site tidiness and you need workers who are proud of their craftsmanship and of course appreciated through bonuses when a passivehouse certification is at the end issued.

Airtightness being such an important factor, at the end of a build, and/or when the building envelope is finished, it is verified through a blower door test. For a passivehouse the maximum acceptable leakage corresponds to an hourly air change of 0.6, i.e. a maximum of 60% of the volume of the air in the building is allowed to leak out/in in one hour at a pressure difference between the outside and the inside of 50 Pascal. We write this n₅₀<= 0.6 h^-1.

(In some English literature you sometimes see the term ac/h or 'air changes per hour'. No metric unit called 'air changes' exists and this is incorrect. The correct usage is h^-1, 'hour to the power of minus one', most often pronounced 'per hour', hence 'the rate of air change is below, or equal to, zero-point-six per hour').