The most common method is attaching a horizontal 2×4 about 4 or 5 feet above your baseboards, called a hip board. The plastic is attached from hip board to hip board, and hangs below the hip boards to the ground. The hanging plastic is wound around a metal pole that has a long handle that you can roll up and down. Venting in warm weather is only through the sides below the hip board, and the open doors on each end wall. Warm air gets trapped, and air above the hip board is quite warm. Also, air is vented by wind, so on still days it can still get hotter than desired.
One variation on the roll-up sides is installing a polycarbonite (corrugated greenhouse plastic) panel below the hip board that is screwed on in the fall and taken off in the spring. The upside to this is that the rigid polycarbonite is stronger than greenhouse plastic film, and therefore you can mound up mulch or compost along the sides in the winter. Also, polycarbonite will last longer than the greenhouse film. However, this option is expensive, as polycarbonite sheets are not just expensive to purchase, but expensive to ship. It also doesn’t allow for the daily adjustments in venting which can be important during unexpectedly hot days during the early spring.
The second main method to covering a hoop house is with pieces of plastic that are 20 feet wide and as long as the length of your hoops. These pieces are put parallel to the hoops and attached to the baseboards, and overlap by about five or six feet. They open at the top by putting a piece of wood in between two pieces of plastic, creating a vent. The upside is that this causes convection; cool air comes in from the doors and warm air exits the open vents on the top, even on still days. Also, because the plastic is attached to the baseboards, there’s less of a chance of animals getting into your hoop house. However, this method makes your hoop house impossibly hot during the hotter months. Also, putting wood placeholders in your plastic vents takes more time then simply rolling up a side and wears out the plastic.
A better, but more expensive method is using shade cloth. Many green house supply companies sell custom cut shade cloth in different shade percentages, allowing you to order the precise size with grommets in the correct placement to secure it to your hoop house. I suggest using 50% shade cloth, unless using it for a wash station or mushroom production, in which 90% is best. Place a grommet at each hoop and every four feet along the endwalls, and it will withstand the typical summer thunderstorm. The big benefit of shade cloth over Kool Ray is that you don’t need your plastic on at all, allowing you to actually reduce the temperature when compared to outside temperatures. This allows you to use your metal frame hold shade cloth, extending your season for cooler crops into the summer months.
No matter what, you’ll need end walls that open entirely for maximum ventilation. Build your end walls with a door-within-a-door design, so that you can have it closed in the winter, but open wide in the summer.