PURPOSE: To prevent the generation of dew condensation by installing a first sheet with a small permeability and a second sheet which is permeable and humidity-dischargeable to an indoor side surface of an insulation material and forming the sheets in such a fashion that they may project from one side of the insulation material.

CONSTITUTION: A first sheet 2, such as vinyl chloride with a small permeability on an indoor side of an insulation material 1 where a second sheet 3, which is permeable and humidity-dischargeable in properties is pasted on this first sheet 2, is provided. The first sheet 2 and the second sheet 3 are arranged to stick out from spots at two sides out of the four sides of the insulation material 1. A release paper is pasted on the spots. It is, therefore, possible to prevent the generation of dew condensation inside since the sheet is capable of absorbing and discharging humidity as well.