polystyrene is better insulator than glass wool: therefore 12 cm of polystyrene is comparable to 20 cm of glass wool: some can dream better, but it's already not bad
12 = 20 does not correspond to what is in the tables and written by the manufacturers:
http://fr.wikipedia.org/wiki/Isolant_thermique
Expanded polystyrene is equivalent to glass wool !!
9 to 12 cm for around 0,33 W / m2 °.
The insulation is not the support, but the air immobilized and trapped in it, so the thermal conductivity is comparable !!
In practice there is matter for perpetual discussion according to moods!!
Indeed the polystyrene disintegrates and is reduced to powder, at a variable speed with its quality and the climatic conditions (UV and cold) and the glass wool sags with time especially in bulk and vertically.
Also we have people furious to discover their polystyrene reduced to the state of powder and others that the glass wool has become collapsed in large beads and even worse if wet by leakage or condensation !!
The less support it has for this immobilized air, the better the insulation (in the case of aerogels), but the mechanical solidity becomes negligible over time (in the case of aerogels) !!
If cold air can circulate between the insulation and the wall to be insulated, then the insulation is almost zero !! This case is easy with not careful installation, the polystyrene which peels off, the glass wool which spreads, with gusts of wind, or rats or mice which dig holes for housing.
The quality of the insulation is essential, but then its density increases and its thermal conductivity increases !!
The quality of the installation is crucial also given the easy errors.
Finally spending a lot to insulate outside in 12cm seems absurd, because what makes the price is essentially the work of laying the supports and plaster and very little the thickness of insulation !!
We can divide by 4 the heat losses instead of dividing by 2 on paper, which can if errors, be very disappointing in reality, 10 to 30% real and € 30000 amortized in 100 years !!
Before you must have detected and removed all your hidden leaks, otherwise this expenditure of € 30000 will be useless, always with a colander house, except to enrich the con artist, who makes fun completely if his work is useless !!
Since this is very new for companies, I am not convinced of the quality and the training of the staff to avoid errors, such as thermal bridges around non-displaced windows or drafts between the insulation and the poorly controlled wall.
Ask the company to see the houses thus isolated, more than 10 years ago, to find out the degree of satisfaction of their occupants 10 years later !!
I am sure that the current 12cm which looks good will become 30 to 40cm in a few years !!
The insulation (air) does not cost anything, but it is what is used to immobilize it which is expensive and the final plaster.
Straw costs almost nothing without this support!
If you do it yourself, you have it for 3000 € (see and look for examples on econology)
Are you sure that the plaster will hold on a wooden support over time?
As in the past, the thickness of thermal insulation was gradually increased: in the 1950s, concrete block or brick was enough !!
Then we went to the air gap of 1 to 2 cm at the end of the 1960s (the ultimate then !!!), then to 2cm of polystyrene in this air gap at the beginning of the 1970s (very expensive! ) then 5cm, early 1980, then 8cm, then 10cm (1990s to 2000), then now very expensive 12cm exterior with questionable installation, then 30 or 40cm exterior well installed late 2010 to early 2020 !!
The good of an era becomes bad later, endless !!