Thanks a lot for all the useful tips.
What I had in mind is not just insulation or thermal solar collectors.
The idea is to couple insulation with a heat pump (i.e. space heating).
During the day we could heat a loft using the hot air in the heating chamber between the glass and the roof...
Insulated panels to absorb and retain IR heat
I am and electrical guy but I have been involved in an insulation project for a few weeks and I have got to come up with an idea about that.
The point is that I would like to build a black flat panel made by a material with a high thermal capacity...