A polythene vapour control layer is used in the new construction extension's warm roof. Warm moist air rises and condenses into a liquid when it comes into touch with the cooler surfaces above the insulation. A vapour control layer is intended to be installed on the room side of the insulation to prevent warm damp air from entering the structure. For fully adherent and mechanically fixed systems, 140mm PIR tissue-facing insulation is optimal. To get a U-value that meets the architect's requirements, 140mm insulation was used. The rate of heat movement through a structure divided by the temperature differential across that structure is known as thermal transmittance, or U–value. The lower the U–value, the better insulated the structure is. W/m2K is the unit of measurement.