Abstract: This invention relates to a ventilation system for buildings and comprises for use at a venting duct outlet (opening into the outdoor atmosphere or into a room) a movable duct closing means and a temperature sensing means which is connected to operate said duct closing means and is mounted to be exposed to effects of prevailing air temperature and wind conditions in the atmosphere outside the venting duct (outdoor temperature and wind force) as well as to indoor temperature and, as the case may be, indoor air flows to be actuated thereby and accordingly to operate the venting duct closing means, whereby the sensing means will operate so as to gradually close or open the venting duct closing means in dependence on the combined effects of outdoor wind force and outdoor and indoor temperature and, as the case may be, the rate of air flow of indoor temperature in the vicinity of the temperature sensing means.