Thick insulation in buildings offers great potential not only for decreasing cooling load but also for changing its calculation method, and for changing system design, operation or control of the system and the thermal environment of the room. The research results of room air temperature changes in well-insulated buildings, show the effects of the daily swing of outdoor temperature and solar radiation are quite small. A simple calculation method for cooling load based on steady state theories is available for such buildings. Cooling loads can also be minimized by using the thermal mass and small swings of room air temperature. As the cooling load decreases, floor cooling becomes an effective system with no risk of vapor condensation. It is also effective to remove a portion of the radiant heat gain on the floor from lighting and from sun coming through the windows before it turn to the cooling load of a room. A water system is very suitable for continuous cooling and thus the roles of the air circulation system are simplified to supply fresh air and to control humidity. A simple air conditioning system without recirculation of air can also be realized by decreasing the cooling load.