In the first few years of our existing house, I struggled in various ways to vent the bathroom shower upstairs. Everything I tried led to condensation and ice problems. Even with double R-19 cotton floss over galvanized ventilation ducts, I still encounter problems of condensation and freezing. Are there any proven solutions to this problem of cold weather condensation when we are preparing to build a new house? It's a good question, because it's not something I often see doing well. Before we get some specific suggestions about pipeline installation in cold weather, let's start with some basic pipeline suggestions.
It is recommended that our customers blow out their bathroom fans from the wall as much as possible. If you exhaust through the roof, the condensate will drip back inside. If you ventilate through the vault, which is usually located in the attic vent, moisture will be sucked back into the attic or roof vent. Use rigid metal pipes (aluminium or galvanized steel) with longitudinal joints facing upward, and the joints are sealed with aluminium foil tape or pipe cement or rigid plastic pipes. Be sure to tilt the pipe a little so that it can be discharged from the outside. Rigid pipes are superior to flexible ones. Larger ones are preferred, especially if they run longer. Best practice is to wrap the entire galvanized ventilation duct in R-3 to R-8 insulation, but if nothing else, you should wrap at least 6 feet of duct in the end, which is the most likely place for condensation. Keep the vent low and run the insulation layer on the pipeline.
For the freezing problem of the external vent, the 70 degree F indoor air with 25% relative humidity (quite low) will condense at 33 degree F, so condensation will occur in cold weather. Push 110 cubic meters with a fan. At a foot per minute, the air temperature is 80 degrees F and the relative humidity is 80%. As you do with a beautiful steam shower, in a 10-minute shower, when it's 25 degrees outside, you produce almost three-quarters of the condensed water.