When a cold room door is opened, the cold, heavy air spills out at the bottom and warm, humid air comes in at the top to replace it. A strip curtain puts a flexible wall in the way of this flow: as a person or vehicle passes, strips open only over the width of the passage and close behind by their own weight. A strip curtain in good condition reduces the air exchange at an open door by 70–90%. Besides the energy saving, it means less frosting on the evaporator, less frequent defrost and less ice and wet floor around the door.
Sizes are chosen by the traffic: strips 200 mm wide and 2 mm thick for pedestrian doors, 300 × 3 mm for pallet-truck and small vehicle passages, 400 × 4 mm for large doors used by forklifts. The strips overlap one another; the overlap is about 50% on internal doors and goes up to 100% in frozen rooms and at doors exposed to draughts. The hanging system is a stainless or galvanised rail; the strips can be removed and replaced one by one, and on a sliding door the rail is fitted on the inner side of the door.
There are two types of material according to temperature. Standard PVC stays flexible down to about −15 °C; colder than that it stiffens, cracks and can injure anyone passing. For frozen storage rooms and blast freezers the low-temperature type known as "polar" is used; it keeps its flexibility down to around −40 °C. In food businesses strips certified as suitable for food are required. There are also insect-repellent yellow strips and coloured types for welding areas, but in a cold room the clear type is the rule for the sake of visibility and safety; the lower ends are cut to a length that does not drag on the floor.
Its weak points should be known: under forklift traffic the strips get scratched, turn dull and block the view; drivers tie the strips to one side and the curtain loses its function. Scratched and torn strips should be replaced and the curtain washed regularly, because a dirty strip rubbing against unwrapped product becomes a source of contamination. Under heavy forklift traffic an air curtain or a high-speed roll-up door is the better choice. A strip curtain does not take the place of the door; the door is still closed after every passage. By changing the door usage intensity in the cooling load calculator you can see the value of this measure.