Hopefully this is the right forum for this. I just recently installed a bathroom fan/light combo in a bathroom that had no venting before. I ran it this morning and peeked outside to see if steam was coming out and the louvers were shut. I went outside and got a pole that would reach and tapped the louvers till the unfroze and then it proceeded to exhaust properly. It got down to around 20F last night for the first time since the install. There was a light snowfall last night (dusting) but no rain.

What should I do to prevent this?

Current setup is bathroom vent ducted with 4" insulated flex duct goes straight up about 3 feet in attic then connects to 4" PVC which runs horizontally (with slight pitch in case of condensate) out to gable end. It does take a 45 degree angle down the last 3 feet in order to exit via the louvered vent. I had to run it up 3 feet then back down 3 because otherwise it would have impeded access to that area of the attic.