Abstract: A thermal insulation system for a window comprising a roller blind carried by a roller blind mechanism which is arranged to be fixed to a window frame above a window to be insulated. The blind is adapted to be detachably connectable to the window frame by means of touch fastener (Velcro) strips carried by the longitudinal side edges and the free transverse edge of the blind and mating touch fastener strips carried by the frame. A mounting mechanism for the roller blind enables it to be selectively displaceable for sealing the upper end of the blind.