If you intend to use your room in the roof for workspace, then you will benefit from insulating the walls, roof and floor. If it is to be residential then you will need to satisfy the Building Regulation requirements.
High-performance Celotex insulation board complies with the thermal requirements of building regulations It is lightweight, durable and easy to install.
Thermal Conductivity: 0.022 W/mK
Finish: Foil Faced
Usage: Floors, walls and roofs