A fluid impervious wall skin provides an interstitial space. The wall is formed from a composite material, the composite material comprising a plurality of layers including:, a first layer of flexible material having adhesive on at least one surface thereof for attachment to a structural wall; a layer comprising a spacer; and a second layer of flexible material; the layers of the composite material are attached a structural wall and one to the other by adhesive between adjacent layers, and the spacer provides the interstitial space N between the said first and second layers of flexible material within the said composite material. The second flexible layer comprises heavier than the second weight, wherein the each overlapping sheet presents a free edge, and the tape is positioned over the free edge is attached to the overlapping and the overlapped sheet of woven fibreglass fabric of the first weight, and a solvent free epoxy coating applied to the surface of the second flexible layer that is distal from the structural wall, which coating cures to form a hard fluid impervious layer.