Most experience with
Passive Houses is available from Central Europe. Can Passive Houses
also be built in warmer climates, where hot and possibly humid conditions
prevail in summer? A theoretical study dating from 2009 has considered
this question in great detail for the warmer climates of south-west
Europe. The major finding is that indeed buildings which can provide
high thermal comfort in both winter and summer by solely heating,
cooling and/or dehumidifying the supply air required for good indoor
air quality can be realised here. Some key results from the study
are given on this page.
First, it is important
to note that, like in central Europe, space heating is the dominant
energy demand in most climates of the Mediterranean.
On a European level,
the annual energy demand for residential space cooling is still
relatively small, but major problems are expected due to the strongly
growing market for air conditioning devices. High peak loads, which
are bound to occur simultaneously in all households of a larger
region, tend to cause blackouts already today. Therefore, addressing
both heating and cooling demands is essential.
Given the milder climate
of the Mediterranean, the heating task can easily be solved using
components which can be of a somewhat lower efficiency than in central
Europe. Some interesting issues concerning the winter arise:
In Southern Europe with its higher level of solar radiation, single
glazing is already sufficient to achieve net solar gains on a south
façade during the heating period in many locations, provided
there is no significant shading. Nevertheless, for the sake of thermal
comfort double low-e glazing appears to be the appropriate choice.
Triple glazing is not recommended south of the Alps: its energy
balance is usually worse because of the lower solar gains.
Proper orientation can
make things easier to a much greater extent than in more northern
climates. The influence of orientation on the heating demand is
typically twice as high in the Mediterranean as in central Europe.
For the heating load, this factor may even rise to 3 or 4. Additional
south-facing glazing areas can reduce the heating demand and heating
load significantly (provided that the building site has good solar
access, of course).
For Passive Houses in
central Europe, it has long been known that the influence of thermal
mass on heating demand is negligible. In the sunnier Mediterranean
climates this influence becomes notable: The effect on heating demand
may reach as much as 5 kWh/(m²a). The influence is most significant
in lightweight buildings. In heavier structures additional thermal
mass has no considerable effects any more; other factors such as
hygrothermal interaction may even outweigh the thermal storage effects.
Insulation between the heated envelope and the ground can be omitted
in climates with annual average temperatures between approximately
15 and 20 °C, such as lower regions of southern Spain and Italy.
It would not save any significant amounts of energy, and the surface
temperatures on top of the basement ceiling are sufficiently high
even without insulation. In both hotter or colder climates, the
building should also be insulated to the ground.
Cooling by means of the
small amounts of supply air that are required for good indoor air
quality, i.e. 30 m³ per hour per person, is indeed possible
throughout the Mediterranean climate zone. Some aspects deserve
Contrary to some publications,
good thermal protection also helps to provide high thermal comfort
in summer (particularly if applied in the roof) and to reduce temperature
fluctuations. A compact building shape is beneficial, too. The double
low-e glazing recommended for winter comfort simultaneously protects
against heat transfer from the window blinds in summer.
A good, movable exterior
shading is indispensable. Fixed shadings or solar control glass
become equivalent to movable shading only in extreme cases, but
result in very high additional heat demand in winter. Without movable
shading the cooling load can increase by 5 W/m² even in the
study's comparatively robust reference situation, having moderate
window areas and all windows facing north or south. With east or
west oriented windows the influence is even bigger.
An important distinction
must be made between places which experience relatively low summer
humidities, such as most of Spain and Portugal, and locations with
higher humidities like the Italian coastal locations and the Po
valley. The semi-arid climates of the Iberian peninsula do not require
dehumidification, whereas to the south and east of Nice dehumidification
can become the major task in summer. Palermo, as an extreme example,
has typical summer temperatures between 25 and 30 °C throughout
the day and dew point temperatures which hardly drop below 20 °C
for longer periods.
The potential for night
ventilation may become zero in such humid situations, in spite of
sufficiently low ambient temperatures, because any additional ventilation
air would bring excess humidity into the building. The latent heat
ratio of any combined cooling and dehumidification device also needs
be chosen according to the local climatic conditions. Separating
cooling and dehumidification is not essential in European climates.
Some general remarks should be added:
The installation of a
supply and exhaust air ventilation system (to be combined with an
airtight envelope) must be recommended in most locations, for different
reasons. Either ventilation heat recovery is preferable to excessive
insulation thicknesses for the winter case, or active cooling/dehumidification
is required in summer, such that heating and cooling via the supply
air appears advantageous to installing a multi-split unit, particularly
since it also improves indoor air quality and thermal comfort. Any
heat recovery should have an automatically controlled bypass to
make use of low ambient temperatures during summer nights.
The Mediterranean Passive
Houses with, as compared to central Europe, lower insulation levels
and higher solar fractions, are more sensitive to changing boundary
conditions such as different indoor or ambient temperatures or changes
in user behaviour. Greater safety margins than in Central Europe
would be a good idea.
Although the Mediterranean
climate prevails only in a relatively restricted area of the earth's
surface, the incongruent variations in winter and summer temperatures,
daily temperature variations, solar radiation and relative humidity
require different solutions for different locations. A specific
calculation of the energy balance and the space conditioning loads
will be required for every Passive House to be built. A PHPP
calculation will usually be sufficient.
An interesting option
is the use of so-called cool colours, i.e. colours with low solar
absorption, for the exterior surfaces to reduce the solar load during
summer. Even for well-insulated buildings exterior colours can change
the cooling demand by up to 5 kWh/(m²a) and the peak cooling
load by as much as 3 W/m². Unfortunately, reduced solar gains
in winter may compensate for the advantages in summer.
Central European Passive
Houses frequently use compact heat pump systems. These devices integrate
all mechanical systems required in a dwelling: a heat pump is added
to a ventilation unit with heat recovery, using the sensible and
latent heat of the exhaust air as a heat source for producing both
supply air heating and domestic hot water in an integrated storage
tank. One great advantage of these systems is that they are factory-made,
including all controls, and the different components can be expected
to cooperate smoothly. In addition there is a potential for a mass-production
similar to white goods, with the respective cost reduction.
Such systems could in
principle easily be supplemented by a cooling function, without
impeding the above advantages. Apart from several technical details
which need to be solved, the exhaust air cannot be used as a heat
sink because its temperature rise would be excessive under full
cooling load. Other solutions, such as ground condensers, mixing
with ambient air, or outside units from conventional cooling systems,
can resolve this issue.
heat pump systems which are appropriate for Mediterranean conditions
are not available on the market yet. Current demo projects will
probably have to fall back on multi-split units or other conventional
technologies. Alternatively, space heating and cooling via large
radiant areas, such as floor heating or active structural mass,
appears to be an option (but probably a more costly one) because
of lower temperature differences and the corresponding higher COPs
of a heat pump. Panel cooling in humid locations will require an
additional dehumidification system to prevent condensation, rendering
the system more susceptible to problems.
and sample pages (pdf 464 kB)
Details and further reading
can be found in the following publication, to be ordered from www.passiv.de:
Passive Houses in South West Europe. A quantitative investigation
of some passive and active space conditioning techniques for highly
energy efficient dwellings in the South West European region. 2nd,
corrected edition. Darmstadt, Passivhaus Institut, 2009.
Schnieders 2009, PHI
to the homepage of the Passive House Institute: