Hi Andy, I am also renovating a property in Finistere and can recommend the ideal solution.The product is multi-layer foil/foam sheets (approx 20/25mm thick) that come in rolls and is stapled to the u/s of the existing rafters.Before plasterboarding over provide timber cross battens @450c/c to fix plasterboard to.This product originates I think from France and is very popular in the UK at the moment.I am currently using it on my own house loft conversion it is currently accepted by Building Control as a suitable product in lieu of alternative solutions. Benefits obviously are...easy to fit,provides air space to aviod condensation risk and is user friendly unlike fibreglass etc.The product is well known in France and is readily available at builders merchants. Hope this is useful. Regards Chris