Abstract: The panel assembly comprises an elongate panel preformed from mineral fibres having a density of not less than 180 kg/m.sup.3 and having two major faces and two longitudinal side edge faces, two elongate metal sections, each comprising a web engaging on one side thereof one of the side edge faces, a first flange at one longitudinal edge of the web and extending at right angles thereto from one side thereof, a second flange at the other longitudinal edge of the web and extending at an inclination thereto from the one side thereof and towards the first flange, and a channel in the web opening to the other side thereof for receiving a vapor seal.