Thermal insulation of exterior walls is effective to reduce heating energy requirement, but in summer it occasionally brings an increase of cooling load and keeps room temperatures at a high level without air-conditioning. In this case, the utilization of water evaporative cooling by wetting thermal insulation materials laid on outside surface is considered to be one of the passive cooling methods. The authors carried out two series of field experiments with twin test houses and simulations for these houses, to improve thermal performance of outer insulated walls by using evaporative cooling. 1) Evaporative cooling effects of the outside insulation system whose insulation materials were saturated with water were examined using the twin test houses. Surface treatments of the insulation material were considered to keep insulation materials air-dried in winter. 2) After the houses were reconstructed, we examined thermal effects of the double envelope system, which had panels for solar shading at outside surfaces of the evaporative cooling system. Panels of the roof were movable and could be opened at night to use atomospheric radiation cooling. 3) Simulations for these houses were made by using the calculation program PSSP proposed by the authors. The results showed that the outgoing heat flow through exterior walls in the evaporative cooling system increased by reason that the outside surface temperatures lowered and thermal resistance of the insulation materials decreased. Furthermore this phenomenon was promoted in the double envelop system because outside surface temperatures lowered near by the wet-bulb temperature of ambient air.