Heat insulation tiles (20) are supported in a grid (14) of support bars (15 and 16) of a ceiling assembly (10) with the ceiling tiles being in the form of insulation boxes (21) that form a dead air spaces (24) therein. Reflective material (22) is applied to at least one of the interior surfaces of the box that faces the dead air space (24) and a layer of phase change material (40) is applied to the insulation boxes.