442 it is desirable that a shrinkage proof groove 446 be formed at an upper surface of the sensor seating rib 442 along a circumferential direction of the sensor seating rib 442 of course it is possible that a sealing member such as rubber packing be interposed between the sensor seating rib 442 and the humidity sensor 100 to achieve the desired air tightness therebetween fig6 is a sectional view illustrating the humidity sensor 100 mounted on the sensor seating rib 442 the humidity sensor 100 includes a case 110 having a plurality of vent holes 111 perforated through the bottom thereof facing the interior of the fan housing 44 and a humidity sensing unit disposed in the case 110 the humidity sensing unit includes a heat conductive temperature compensation plate 120 a humidity sensing element 130 and a compensation element 140 the humidity sensing element 130 is mounted on the temperature compensation plate 120 at one side location of the plate 120 and adapted to measure a resistance value that is variable in accordance with the humidity of the air introduced into the case 110 the compensation element 140 is mounted on the temperature compensation plate 120 at the other side location of the plate 120 and adapted to compensate for a resistance value depending on the surrounding temperature although not shown in the drawing each of the humidity sensing element 130 and compensation element 140 is internally provided with a micro heater if a voltage of approximately 5 to 10v is applied to both ends of the micro heater a surface temperature of the micro heater is changed causing a variation in the resistance value the humidity sensor 100 is operable on the basis of a property in that.