Abstract: Apparatus for controlling light and heat transference to and from a greenhouse comprising a plurality of light-weight, rigid, insulative, slats operable to be selectively opened or closed in venetian blind style to selectively lighten or darken any or all portions of a greenhouse. The slats include both bulk and reflective insulation for low thermal conductivity and low emissivity of thermal radiation to retain heat within the greenhouse particularly during cooler periods of the year. The slats are arranged in pitched roof manner and are operable to drain condensed moisture, when closed.