One major consideration in our house is the level of insulation to the main roof. One superior product that is being installed in french houses is Airflex Insulation - an aluminium backed foam' geotextile insulation. However, it is not cheap and I am yet to be convinced, 100%, about is qualities and its price justification
. Leclerc have had an offer recently on a similar product, that is a lot cheaper, but because we have not used it I cannot compare.
Airflex is not cheap. 100 square metres cost 1,000 euros
Airflex is about 1cm thick and enables us to sandwich it between the plasterboard and the batons without much modification. In our case we presented the Airflex from underneath and then presented the plasterboard onto galvanised batons which are suspended on galvanised brackets.
The alternative, common insulation, is 10cm thick and we would not have had the space so in our case we had very little choice.
One thing also to bear in mind is if attached from the top (which we could not do) it can be used as an under-felt. Because we had to pierce the aluminium to push the Airflex over the batons this might cause some water seepage if the tiles were to fail.
