Not only the construction fair, but elsewhere you can meet with distributors of reflective plastic sheets, who convince customers that this system can save a lot of energy. They even offer reflective sheet as a replacement of insulation for opaque structures (eg. loft insulation). We decided that we calculate the influence of reflective sheet on the heat loss from the building. This article only deals with heat loss in winter in a typical house. It does not address the question of reducing summer overheating, when reflective sheet may be useful in protection against radiant heat.