Perfect, what are the drawbacks ?
How much is it ?
Builder would instantly answer you, without any survey in support : "It's expensive".
The outside surface is a bit larger than the inside surface because of the foundations, apart from that, placing polystyrene inside or outside is the same work. I would even say that it is faster outside, because there is less cut-out and adjustment.
With outside insulation, the roughcast is included, about 30€ per m2, but with inside insulation, you do not have the plaster, about 15€ per m2, as there is about twice more surface inside, these make up each other at the cost level, but on the other hand, at the aesthetic level, after one or two year (appartitions of the joint lines and undulations of the plasterboards).
By following the norms, the insulation under the flagstone and the insulation of the roof timbers should be of the same price.
If there is a slight overcost for outside insulation, do not forget that you consume three times less heating energy, whatever is your heating system, and you win in comfort in your house.
Though a heating pump compressor is just a fridge compressor five times more powerful, and is not more complex than a oil-fired boiler (which handle explosives and must respect drastic security norms, it turns out that, because the market is not important, a 8 kW heat pump, that is to say 2 electric kW, cost three times the cost of a very good oil-fired boiler of 20 kW, that is to say about 6000€.
In total, compressor, sensors (burried pipes or air radiator), diffuser (pipes in the ground), a thermodynamic heating system cost between 15000 and 20000€, that is to say 30% to 50% more than a not renewable fossil energy heating system. But do not forget that you consume three to four times less billed energy.
Insulation and heating represents about 20% of the cost of a house, if the outside insulation and the heating are worked out 50% more expensive, it is largely estimated, this makes 10% of the cost of a house, that is to say 20000€ for a house of 200000€.
If a traditional house consume 4000 l of oil per yeat, that is to say with current price 2800€, you would save 2500€ per year, hence a return on investment of 8 years, with the current cost of the oils, with the comfort and the security into the bargain. But these hypothesis are willingly very pessimistic.
Durability
See opposite pictures of my house built in 1995, and now in 2006 under at the end of the work.
Some claim that you can not lean a bike against the wall, other claim that children can not play the ball against the wall : obviously, Mr Zidane must not come and train making penaltys against the walls. Of course, you should'nt attack the wall with pickaxe or a hammer. Anyhow, this can be easily repaired with some foam, some glass fabric and some coating. It is easier to repair than a ship hull.
In return, no harmful toxic emations from the polystyrene and the glue in the house, no problem for hanging pieces of furniture in the kitchen or curtains, and no appearances of glue strips.
Other talk about the mice : every precaution are taken for the glass fabric, which the mice do not like, and which wrap the polystyren, ensures a perfect airtightness. Some metallic protection canvas are planned at the top and at the bottom. There is no more risk than in inside-insulated house, maybe even less considering the dispositions taken.
Beware of the ants, like in any house, ant roads can cross through any passage of pipe, even between the bricks. I had some once, along my bathtub, at the thrid floor of a brand new construction, in the middle of a big city, and I had great difficulties to get rid of them because they were resistant to most of the insecticides. It is wise to spread an insecticide powder every year when it is hot at the bottom of the wall around the house and mainly on the southern side. There is no survey which say that ants eat polystyrene. But all the houses have polystyrene. I think that, because the ant feed themselves with insects corpses, they must not appreciate polystyrene.
Have you ever heard about the red micro-fungus which develop themselves under the traditional roughcast and that can be seen under the form of long red trails on the walls of the gable, well they do not survive the synthetic coats used with outside insulation. It is nevertheless recommanded to make every decades a pulverization of water repellant.