Set oven temperature at 250 degrees.
Cook one hour per every 5 pounds.
Check internal temperature periodically.
When prime rib reaches close to 120 degrees internal temperature take out of the oven and leave the bag closed. Prime rib will reach 140 degrees internal temperature (medium rare) sitting in the bag out of the oven. (Always check temperature in the middle of the roast.) Everyone's oven varies as to how it cooks. Times and temperatures given are guidelines to follow and are approximate. Always use a cooking thermometer to tell when your meat is done.