So what advice did your mold experts give you? Certainly not peroxide, baking soda & boric acid?? Have you heard of a product called Boracare? It is a mold inhibitor that sprays on & will keep mold in an inactive state. Understand that you can't kill mold, just keep it inactive. If Relative Humidity (RH) gets above 50% mold growth will be activated again. You can bleach it, kiltz it...but it is just covering the mold up for alimited time. I would first look for the moisture intrusion that caused the mold growth. Get a roofer with a good reputation & good flashing education to take a look at the flashing of the skylights. Is this where the moisture intrusion began? Also look at the wall covering in your bathroom. If you have vinyl wall paper this is a source of mold as it traps moisture behind it. Oil based paintd also make a vapor barrer on top of your walls. The paper backing on the back of sheetrock is an issue. There is a brand called Densearmor or Denseguard that has no paper backing.. For mold to grow it has to have moisture and a food source eliminate the moisture first and then determine if the food source needs replacing. Have an electrician hard wire the fan to come on when the light is turned on. Bath fans should run at least 20 minutes after shower/bath is finished to remove the additoinal humidity that has been introduced into the home. Best of luck to you.

I have experienced this issue (fan randomly coming on in the night) and believe I have found both the cause and the solution and would like to share it with you. Firstly this is to do with an inline fan system where the fan draws air out of a shower light assembly and then pushes it out through an external vent. The problem is moisture getting into fan timer electronics which causes the circuit to switch the fan on, sometimes with irregular pulsating speed. The solution will be to remove the timer circuit board and dry it out. However, you will need to make sure it doesn't happen all over again. There are two things I did in an attempt to make sure the problem didn't reoccur: 1. The wires from the fan (the fan is in the moist air flowing along the pipe) connect to the timer circuit through a channel in the fan housing. Air can also travel along the wire channel bringing moist air with it where it condenses. Use a sealant to block the channel before replacing the timer circuit board. 2. The air will be forced down the wire channel if the outside vent becomes blocked - it will try every route to escape. Keep the outside vent clear. In my case it was blocked with lint and fluff. Hope this helps

The preferred configuration for bath van ducting is to exit the bath ceiling and then go horizontally (fully supported and sealed) over to a side wall with a slight slope downward toward the outside. As the humid air cools and condenses, this allows moisture to drip down the inside of the pipe toward the exterior vent and not back toward the fan or ceiling. Running the vent straight up to the roof will work, but in cool weather the condensation in a long vent has to run somewhere, and it may be back into the fan housing or onto the ceiling. One reason it may be so expensive to fix the venting is that to do it correctly includes putting one or more holes in your roof. If you have a two story house this is not a simple inside HVAC or insulation job, since someone must be both inside and on top of the roof cutting holes and installing the vent and flashing. $850 may still be too high, but its hard to say without knowing the full scope of the work. Bruce

A typical bathroom fan only exhausts so it can't provide ventilation air (i.e. outside air) but it removes moisture laden air and pulls in dryer make-up air from the adjacent room. There are bathroom fans that do provide outside make-up ventilation air that passes through a heat exchanger. As a temporary solution, you could open the window a bit or clean and repair the ceiling so it is very smooth and paint it with a semi-gloss vapor barrier paint with a mildew resistant additive. It wouldn't hurt to wash it every few months too. Flat or textured paint would speed the growth of fungus. But I suspect you will need to replace the ceiling and that is the time to install an exhaust fan. Of course you could install a simple exhaust fan through the wall near the ceiling with an humidistat and timer.