A soffit insulation of the attic it is interesting for its ease of implementation and the limited inconvenience of the intervention. Nothing to do with the insulation of the roof from the outside, which requires the preparation of scaffolding and the temporary removal of the mantle (with consequent unavailability of the rooms below).
However, the results will also be different: an intrados insulation of the attic, for good that it is done, it will not have the same thermal performance compared to the external insulation of the pitched roof, even if compared to the total lack of insulation the difference will be considerable.
A soffit insulation of the attic we said it is a fairly simple operation because it is done from the inside and just apply a layer of insulation on the lower face of the roof. Of course we must take into account that if the attic it is inhabited, it is also necessary to provide for a finish, generally with wooden panels or beading which increase costs a little.
The same finishing panels as the soffit insulation of the attic they can be made of insulating material, for example cork or wood fiber, and subsequently plastered from the inside. Again in the case of attics inhabited, an increasingly frequent situation, it will be necessary to worry about using an insulating material that does not release dust and that is sufficiently hygroscopic and breathable to ensure sufficient living comfort. It will also be necessary to budget for the renunciation of a few centimeters in the available height of the premises, which is sometimes a problem.
In the case of a attic uninhabited, things become much simpler and isolation intrados of the pitch can be replaced by an extrados insulation of the last slab, that is the floor of the attic itself. This, in the case of attics not inhabited, it is the solution that presents the best price-performance ratio.
The extrados insulation of the last slab in a attic not inhabited can be done by simply laying insulating material on the ground - for example rock wool, glass wool or sheep's wool - or with loose insulation. In the latter case, cork in granules is excellent and can be left as it is in the desired thickness or solidified with vegetable resins, so as to obtain a walkable substrate.
Extrados external insulation of the pitch, internal intrados insulation of the attic and internal extrados insulation of the last slab are the three systems to limit heat loss from the roof (which can reach 30-40% of total losses in small buildings) and to protect the roof from insolation and excessive summer heat.