Passive House Definition
The Passive House is not an energy standard but an integrated concept assuring the highest
level of comfort. The exact definition is as follows:
“A Passive House is a building, for which thermal comfort (ISO 7730) can be achieved solely by post-heating
or post-cooling of the fresh air mass, which is required to achieve sufficient indoor air quality
conditions – without the need for additional recirculation of air.”
This is a purely functional definition which doesn't contain any numerical values and is valid for all climates.
This definition shows that the Passive House is a fundamental concept and not a random standard. Passive Houses
have not been “invented” by anyone – in fact, the Passive House principle was discovered. It may be debatable
whether the term “Passive House” accurately describes this concept – well, there is no other term better suited for
it. Thermal comfort is achieved to a maximum extent through passive measures (insulation, heat recovery, passive
use of solar energy and internal heat sources).
The following considerations help to shed light on this concept:
All airtight buildings (any low-energy building needs to be airtight) require the use of an efficient
ventilation system. In Passive Houses this system can also be used for heating purposes, without the
need for additional ducts, major technical interfaces, auxiliary fans etc.
This concept allows for the construction of houses equipped with a highly efficient heat recovery
system at a very affordable price. This is usually rather difficult to do since the ventilation system
costs just as much as a heating system – a double investment which hardly pays off. The way to go
therefore involves cutting back on one of the two systems: either on the ventilation system, e.g. by
installing an exhaust system only; in this case the building will become a low-energy house with
conventional heating; or on the heating system by using the ventilation system for heating as well – in
this case the building will become a Passive House.
This heating concept automatically implies extremely low energy consumption. After all, using the fresh
ventilation air for heating without an additional heating system can only work in buildings with minimal net
losses. This requires an excellent insulation of the building envelope – especially in cold climates to
keep the desired warmth inside the building, but also in hot climates to keep undesirable heat out. An energy
balance will help determine the level of insulation that will be required in a given building and climate.
Basic principle of a Passive House: The ventilation system supplies at least the amount of fresh air
required for good indoor air quality.
Can’t this air be used for heating as well? – Yes it can – provided that the building requires very little heat
to begin with.
Heating load - the Passive House requirement
The following calculation illustrates the heating load Passive House requirement:
To ensure good indoor air quality, one needs about 30 m² of fresh air per hour. This supply air can only be
heated up to 50°C to avoid the scorching of dust and the specific heat of air is 0.33 Wh/(m²K) at normal
pressure and a temperature of approx. 21°C. From this follows:
|30 m²/hr/pers * 0.33 Wh/(m²K) * (50 - 20) K = 300 W/pers
Hence: Fresh air heating can supply 300 Watt per person. Assuming 30 m² of living space per person the
maximum heating load at a given point of time may not exceed 10 Watt per square metre of living space –
independent of the climate: As these figures refer to that day of the year where the maximum amount of
heat needs to be supplied to the building (heating load), Passive Houses require different
levels of insulation depending on the individual climate: more insulation in extreme climates, less insulation
in milder ones.
The specific values for heating loads (measured in W (Watt))are not identical to the ones for energy
(measured in kilowatt hours (kWh)), the numbers for which are often easier to come by. The Passive House
heating demand criterion of 15 kWh/(m²yr) typically relates to a heating load of 10W/m² in Central European
climates, however, it is only supposed to serve as a rough benchmark which may vary with different climatic
conditions: in Stockholm a house with a heating load of 10W/m² may use more like 20kWh/(m²yr); in Rome it might
be as low as 10kWh/(m²yr).The Passive House criteria allow buildings to go by either criterion - the 15
kWh/(m²yr) heat demand OR the 10W/m² heating load.