This paper documents the thermal insulation effect of a screen installed inside a vertical rectangular enclosure (e.g., double-glazed window). The screen is a venetian blind system made out of horizontal strips that can be rotated. The focus is on the “closed” position, where the strips almost touch. The effect of this permeable screen on the temperature field, the flow field, and the overall heat transfer rate is determined numerically. The study shows that there exists a ceiling (critical) conductance for the air leakage through the screen, above which the screen does not cause a significant drop in the overall heat transfer rate. A numerical example shows how this critical conductance can be used to calculate the critical spacing that can be tolerated between two consecutive strips in the screen.

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