Abstract: An apparatus for insulating against conductive, convective, and radiant heat transmission comprises a plurality of mutually parallel sheets. The sheets may be attached to a retracting device from which they can be drawn to extend in mutually parallel relation and cover a building opening such as a window or onto which they can be retracted to uncover the opening. A number of spacers which may be in the form of collapsible or nestable devices are mounted within the apparatus to separate each pair of adjacent sheets and, thus, define a dead air space therebetween. At least one of the sheets has a surface, facing on the dead air space, that exhibits a low surface emittance. This surface emittance is sufficiently low to yield a total effective emissivity of the surface and dead air space of no greater than 0.60. Important, the spacer devices are designed so as not to abrade or otherwise harm the reflective surface.