Doug,
I made the mistake of using that flex duct stuff ONCE. I installed it in October of 1984 back when I was still in Winnipeg and I was back a month later because the HO complained (and rightly so) that the fan "gurgled" when it was turned on. We had just gone through a week of minus 30C weather and it had obviously collected condensation which froze up when the fan was off because I had taken the easy way out and run it out the side of the house and had a few feet of semi-horizontal run (about 3 feet of diagonal)
I wound up doing something completely different and used 4" white PVC going straight up through the roof with a really good vent cover at the top of the roof that closed up when the fan was off. I also wrapped the PVC with about 6 inches of fibreglass batt insulation to try and stop the condensation issue but it still dripped a bit of water when the fan came on the next time.
I wound up wrapping the pipe with a 25 watt strip of heat tape that was hooked to the bathroom light and putting the fibreglass insulation back on and it's worked fine ever since. That was about 25 years ago too.
I could have used aluminum or galvanized pipes here but I've seen way too many of those corrode away to a piece of nicely vented pipe with pinholes all over it because of the corrosion from the moisture.
