The issues of the methodological approach 
for designing low energy buildings
and their economics is addressed in this lecture.
Building envelope design, if we aim to achieve
the maximum comfort
with the minimum primary energy consumption, 
requires the integration
of the parameters of comfort and energy into each of the critical steps of the design process.
Factors such as climate, master plan, building shape, façade design,
thermo-physical characteristics of the materials, and mechanical services, must all play a role in decisions.
To achieve this result, it is required a high level 
of integration
among the skills called into play in the design process.
The design process, nowadays, is generally
based on a linear path, in three steps:
architectural design, followed by the design 
of the mechanical systems
(when the building is equipped with an air conditioning system), and then construction.
This approach is incompatible with the design
of low energy, high comfort buildings.
It is necessary to change the design methodology,
and make use of an integrated design model
that includes the introduction of a new professional expertise:
the energy expert, which must have a deep knowledge 
of building physics.
The energy expert must be able to interact
with the architect and with the mechanical engineer;
he must not only be capable of managing simple rules of thumb, but must also be able to use sophisticated simulation tools
for evaluating the energy performance of the building, and not only;
also thermal comfort, daylighting, natural ventilation 
and all the passive means of reducing energy demand.
The design process becomes circular, iterative.
Ideally, the first design sketch is developed jointly 
by the architect, the energy expert 
and the mechanical engineer.
Then the energy expert makes a closer check
of the initial solutions by means of energy,
thermal and visual comfort simulations.
If the result is not satisfactory, 
the initial sketch is modified.
If and when the result is satisfactory, 
then the most appropriate mechanical system is chosen
and energy consumption evaluated.
Again, if the evaluation shows that energy
consumption is too high,
either the HVAC system is modified or some 
design choices are revised.
The next step is to check the possibility of providing partially or entirely the necessary energy with renewable sources.
Next step is the economic feasibility analysis.
If satisfactory, the process stops here and the project 
is consolidated, otherwise, the loop starts again,
until a satisfactory result is achieved. 
This is the integrated design approach.
Most important are the first two stages 
of the design process.
The reason is that the most critical decisions 
about the energy performance of the building are taken in these early phases.
The earlier the energy expert is integrated
into the design process,
the greater the impact on building’s performance, 
and the lower its cost.
It is not sufficient, however, that 
the new design process integrates a new expertise,
and that from linear it becomes circular;
it is also necessary to define a planning strategy,
the one characterising the energy
efficient and comfortable building.
In the design process, the maximum effort
must go into minimising the amount of energy
needed to provide thermal and visual comfort,
by means of appropriate architectural design.
It is the most important stage.
Only after this goal is reached, the design team must maximise the energy efficiency of mechanical systems.
If the two previous jobs are performed well,
the amount of primary energy needed is quite small
and can be supplied by renewable sources,
in economically and financially sustainable way.
This is because the higher the energy efficiency,
the lower the size,
and hence the cost, of the renewable energy 
production system.
About cost, there is a common belief,
according to which low energy, sustainable buildings are necessarily far more expensive than conventional ones.
This graph, dealing with a sample of German
buildings, shows that it is a wrong belief.
Buildings with the same cost show very different
energy consumption,
and low energy buildings can also be low-cost, 
if appropriately designed.
In conclusion, integrated design not only is a necessary condition for energy efficient buildings design,
but it is also the main way to control their cost.
