On most homes built within the last 50 years you should find vapour barriers behind walls and ceilings. Vapour barriers keep warm air for escaping a room. They can be made of plastic, Polyvinyl sheets, paper, foam and yes even that old wall paper at Grandma’s house can be considered a vapour barrier. The vapour barrier for homes in our climate should be installed on the warm side of the wall, followed by an insulation. But when plastics and such are installed incorrectly such as in the attic at this skylight well, warm air can pass through the wall and the insulation and be left to condensate against the cool plastic non-effective surface in the attic. The water can then feed molds, cause staining and damage structure. That warm air you paid to heat escapes your home without the proper installation of a vapour barrier.