Frost on roof decking above shower


Few years back I looked in the attic and noticed that I'm getting frost on the roof decking above the shower in my bathroom. Only happens when its extremely cold out. Zero degrees, somewhere around there. Obviously what's happening is that moisture from the shower is rising, hitting the cold decking and collecting. The frost than melts and darkens the decking and trusses in this localized area above the shower.
Originally, we were talking large patches of frost probably around the size of a standard shower stall. Heavy white. If you'd scratch it you would get snow. Figured that this couldn't be good so a couple years back hired a guy to take care of the problem. He installed these heavy-duty bathroom fans to suck out the moisture (and all the heat too). In addition, he put in a new light can in the shower stall that supposedly had a better seal. His theory had been that the moisture had been seeping through the fixture.
Now I take a shower with the door to the bedroom open and we're moving the air so efficiently that the mirror fogs up at the top only and even then it's not bad. This helped my frost problem in the attic a little bit.
The next year he installed ventilation in the attic to move the air up there. Unfortunately, don't know the correct lingo here. A small octagonal window at each end of the attic, and a heat register looking thing on the eaves. Again, helped a little bit.
Last winter he suggested that we paint the shower ceiling with this special vapor barrier paint.
Yesterday, Wisconsin was hit with frigid weather. Poked my head in the attic to see how we were doing. Sure enough, a faint swatch a foot by maybe two feet on the decking and a heavier one inch band following the upper edge of a truss all the way down to the eaves. This one is so white that you can not see the wood underneath. (Know these descriptions probably aren't helping much, but I do have digital pictures.)
I guess my question is is it normal to have some heavy patches of frost on the roof decking above your shower in the dead of winter. Part of me says that no, I fear that over time the wood will rot and get punky and I definitely want to avoid a mold problem. But now another part of me is starting to wonder if this is normal to have frost above your shower stall in the dead of winter. I mean it's not like there's frost up there all winter long. Maybe a week. Maybe two. Don't think it's more than that. And this morning when I woke up there was frost on the windows and I think that's normal to a degree. And I'm getting so tired of throwing money at this thing. I can think of several other things I would like to purchase.
If this isn't normal and must be fixed, now what? In addition to the work I mentioned above, I have been assured that I have a good vapor barrier between the bathroom and attic. The barrier is buried by something like a foot of those peanuts.
Thank you for your time, Brian
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snipped-for-privacy@hotmail.com wrote:

Well taking a shower put s a lot of moisture into the air. During the summer in most areas you want to get it out of the house to keep the house more comfortable. During the winter you may want to keep it in the home or get it out depending on how well insulated your home is. You don't want to keep it in the bath room in any case, you want to get it outside or distributed around the house.
Most folk plan on removing the moisture from the bathroom with an exhaust fan summer and winter. Exhaust fans come on two types, cheap contractor grade with lots of noise, short life and little effectiveness, and good quality that are much quieter, more efficient and longer lasting, also more expensive.
The fan has to move the moist air out of the home. That means all the way out. Some contractors like to vent it into the attic. That is a very bad ideal. It can cause considerable damage. Out the roof or sidewall is best, but it can be vented out the eve if there are not air intakes close to the vent.
You also sound like the advice for a vapor reducing paint would be a good idea. With or without a vent you don't want the vapor going into your attic.
--
Joseph Meehan

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Should have mentioned that the exhaust fans that were installed (expensive top-of-the line I'm told) are jetting the moisture not into the attic but rather through a ribbed hose that goes directly outside.
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snipped-for-privacy@hotmail.com wrote:

"ribbed" hose, aka dryer vent hose can be a problem. If the hose run is long and/or if the hose is not pulled tight, the coiled steel (ribs) creates wind resistance, which makes it harder for the fan to expel the air. Replace with PVC pipe.
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Seems a shame to keep throwing all that nice warm moist air out into your nasty Wisconsin weather. Just speculation, but could it instead be recycled into the heating system via a small cold air return to the furnace plenum? A competent HVAC engineer could help with pros and cons on such an approach. This is being done with dryer vents these days . HTH
Joe
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