a plurality of self closing louvers 26 of known type which are pivotally mounted to move in unison to normally closed position the louvers 26 yield in response to air flow generated by fan means 18 when activated thereby pivoting in unison to an open position the louvers will close automatically when fan means 18 is deactivated an air filter indicated at 28 is provided on the outside of opening 17 in order to prevent entry of dust bugs or extraneous material into the enclosed space 10 preferably a downwardly sloping rain hood 29 is provided which shields the air filter 28 and inlet opening 17 from the elements a screen indicated at 30 may be provided at the lower entrance end of rain hood 29 since circulation of air within the enclosed space is generally desirable and since only slight superatmospheric pressure is preferred an exhaust opening is preferably provided indicated at 31 this exhaust opening is remote from inlet opening 17 in order to permit circulation of outdoor air through space 10 before it is exhausted exhaust damper means indicated generally at 32 is provided in exhaust opening 31 which operates in a manner similar to that described above for the inlet opening damper means thus self closing louvers 33 which are in normally closed position will pivot in unison to an open position when fan 18 is operating due to slight superatmospheric pressure within space 10 fig2 and 3 show installations embodying the invention in which the enclosed space which is to be cooled is remote from a source of outdoor air in fig2 and 3 parts corresponding to like parts in fig1 are given like reference numerals referring to fig2 an outside wall is indicated at 11 fan means 18 and.