These New Autovents Are A superb Addition To Any Greenhouse & Have A Double Spring, Designed To Hold The Roof Vent Shut In Higher Winds. Better Still It Is Now Possible To Disengage The Autovent When High Wind Is Expected & Is Supplied With A Unique Bracket That Will Lock The Roof Vent Securely Closed.these Autovents Are Very Sensitive To Changes In Temperature. When A Cloud Goes Overhead They Will Start To Close The Roof Vents Before You Are Even Aware Of A Temperature Drop. The Autovents Will Control The Climate Far Better Than You Could Achieve Yourself. please Note These Autovents Only Fit The Harrod Greenhouse.