In the atic you will also want to use insulated duct. you do not want the exhaust air to get cold until it is outside otherwise you will have condensation in the new pipe. Check the manufacturer's directions if you are using an in line fan you may need to connect it with flex because of noise / vibration. There is nothing wrong with flex, I have used it with your problems. Flex has a bad rap because it and shoddy workmanship go so well together.
