Summary: You should duct the bathroom fans outside. The moisture will result in growth of mold in your attic. Ducting of the bathroom exhaust fans ought to be through the wall or the roof.
Question: The master bathroom ventilation fan directly vents into the attic space in our home. There is a power attic fan and a number of rooftop stationery vents. The covers of all the vents were dented due to a major hailstorm a year ago. The insurance inspector assured us that in spite of the dents, the vents and roofing were leak-free. But I found that there were white fuzzy mold has emerged on the rafter tops located at the crest of the roof. What could be its cause? What is the problem?
Answer: I am not so sure about the hailstorm being the cause of the mold. Also I do not rule it out. But the leaks caused by the hail leading to the mold are a remote possibility. Mold spores and humid air in large quantities are pumped into the attic space by the master bathroom fan and the other fans. The growth of mold is supported and nourished by this moisture. The moisture is an essential need for their growth. The mold which is growing gets a boost every time you run the fan.
Almost all model building codes stipulates that the ducting of the ventilation fans is to the outdoors of the home. A number of builders believe that all that is required is to extend a plastic hose that is flexible to the stationary roof vent from the fan. This is quite regrettable. This may be fine in certain situations but I don’t think that this should be the norm. Large volumes of humid air can still be discharged into the attic space even if the pipe terminates close to a roof vent inside the attic. Ducting of the exhaust from fans should be carried out outdoors. This is absolutely essential.
This issue is serious enough to merit your attention. Since you are living in a colder type of weather, considerable and swift wood rot is indeed a strong likelihood. The humid air can quickly condense and become liquid water underneath the roof sheathing and the roof framing lumber that is cold in the winter months due to the humid air discharged from the fan. You also say that the tips of the roof rafters have molds. It is but natural with the warm air rising. If there are wood rot in a number of rafters it should not be surprising.
The attic needs to be kept free from the entry of the moist air. You need to pay attention to this first. Termination caps are provided for soffits, roofs and walls by the manufacturers of fan. My choice is to ventilate the bathroom fans all the way through the roofs. The special roof hood that I use accepts the galvanized metal ventilation pipe which is 4 inch in diameter. The progress of air faces least resistance from the solid pipe. This solid metal piping is recommended by a large number of fan manufacturers.
The damper provided for in the roof termination cap prevents snow and cold drafts from wafting back down into the pipe and it functions along with the small damper which is normally there in the fan itself.
The complete length of the pipe is to be insulated properly. This is critically important otherwise water is likely to drip in winter months from your fan. Whenever the fan is not turned on, the metal pipe if not insulated properly, will become cold. The warm and moist air condenses into liquid water as it travels through the cold pipe. You could easily mistake the leak as coming from the roof when it in reality is condensed water running down in the direction of the fan cover from the inner side of the pipe.
Using spray-foam insulation is possibly the finest way to insulating the metal pipe. The whole pipe can be coated with the foams that expand. It also prevents condensation. A pipe that is wrapped around with bat insulation and taped may not work. The insulation is likely to come off the pipe or the metal pipe is likely to get partially exposed when the tapes fail in extremes of temperature. Insulation of the entire exterior of the pipe with a coat of expanded foam of thickness of one inch will be adequate.
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