The presence of water or air bubbles on waterproof covers is a fact that can occur in some particular circumstances, such as in the case of thermal insulation and the absence of an adequate vapour barrier. In the latter case the vapour may rise up and meet the membrane (notorious for being scarcely breathable to water vapour) and bubbles can form, at low temperatures, which then condense into water. Another problem can occur when the concrete is not left to set long enough and being waterproofed immediately it holds on to the water still present inside it.