When the door is open, cold air flows out at the bottom and warm air flows in at the top; this air exchange means cooling load as well as humidity and icing. An air curtain blows a vertical wall of air across the door opening from a fan unit mounted above the door; it reduces the air flow between the two spaces by 60–80%. It is the most practical solution for warehouses that cannot close the door under heavy forklift traffic.
The PVC strip curtain is the simpler and cheaper alternative: overlapping flexible clear strips that part during passage and close again immediately. It is very effective for pedestrian and hand-truck traffic; with forklifts the strips wear over time, and because they can stiffen and crack in frozen rooms a low-temperature type (rated to −25 °C) is chosen. High-speed roll-up doors are a third option that replaces both the air curtain and the strip curtain.
Which measure? In 0 °C rooms opened more than 50 times a day, a strip curtain at the least; in distribution warehouses with forklift traffic, an air curtain or a high-speed door; in frozen rooms both together (an anteroom or "vestibule" in front of the door is the most effective method). In the cooling load calculator you can set door usage to "heavy" and see how large the infiltration load is; these measures cut this item directly.