The humidity in the house might be too high.

It was very very cold and now the temps are milder--almost 30' today. The Aprilaire humidifier is attached the furnace and the dial on it lets you set the humidity according to the outside temperature. I keep a close eye on it :) The house is fine, it's the attic where we had trouble.
I keep the temp pretty low (around 64 or less) and turn it down at night. Good idea about the fans, I usually have them on but forgot!

Humidifiers are not anywhere near as accurate as the dial leads you to believe. They really don't take into account how many showers are taken or food is cooked, or even how often people go in and out of the house. When it gets that cold, it should be turned off completely. If you see frost building, open the windows for a few minutes and remove the humidity. Putting new windows in often leads to more humidity, because the house is sealed up better. Well sealed homes need a heat exchange for fresh air and make-up air for what the bath fan tries to suck out. You could actually be sucking more humid air INTO the house if you have a crawl space under the house where the make-up air is now coming from when using any exhaust fan. Keeping the ceiling fans on only helps by a few degrees. Keeping the attic cooler than the rest of the house will actually speed up the transfer of moisture from the house into the attic. Curtains are the biggest cause of frost, because they make the inside window pane even colder. Unfortunately, the energy conservation practices always seem to contradict house health practices.

hmmm :) I understand what you posted, I'm not sure some it applies to this house. It isn't sealed that tightly, because we have drafts coming in around the doors (despite having replaced the outside ones with good storm doors) The front has a porch that never gets iced up but there is a draft coming in from the three doors (yes, it's like an airlock :) --the middle door has weather stripping, go figure. We rolled up a blanked and use it for a draft stopper. The side door has ice forming inside and the back door usually has some too, but now it's not as bad since we put back the old door inside the kitchen. again, go figure :)
We finally got rid of the bats and mice!!! there were mice in the attic!!! We had the crawl space under the back porch fixed--insulated, etc. as well as the foundation of the whole house.
Bats were still getting into the attic so we had a carpenter put in vents that eliminated having those window screens which got iced up anyway and probably provided enough space for them to crawl in. There is no off position on the Aprilaire--it's quite old--I had the furnace system checked and they said the Aprilaire is fine and works well.
There is insulation in the attic floor and around the bath vent, but the vent might be leaking?
I've read that the humidifier should be turned off, but I can't figure out how to do it, maybe it's just turned down? The attic is subject to all of the bad weather because the house is taller than it's neighbors. The old windows were so decayed that one of them almost fell out, thus the new windows :)

If you quit making mice, then you would not need to get rid of them :) You could turn the water off that supplies the humidifier, but if it is a cheap saddle valve, I would not mess with it. The condensation always happens somewhere when it is much colder outside than inside. A leaky window does not show it as much sometimes because the moving air clears it. If you go out and start your car in that cold of temperature and then turn on the warm de-froster, your window will fog up and get icy until the inside of the window is closer to the temperature of the hot air your blowing on it. If you turn the de-froster on cold, it doesn't frost up. You may still have enough leaks to not be sucking humid air out of the crawl space, but when it is that cold, there is still enough moisture in the air to cause problems. Every little thing you change will effect where things ice up and at what outside temperature it will happen. Your bath vent might be leaking, but that is going to be much less moisture than what migrates up through the ceiling 24/7. I am not sure if I missed something, but with the windows, it sounds like this is usable space, or is it just some planks over the ceiling joists with insulation between the joists? Is the attic ceiling insulated? If not, and I am picturing this correctly, I might suggest pulling up any floor boards and adding more insulation. Then, put the floor up above that. This would keep the house warmer and the attic cooler. The cooler attic air will hold less moisture because it is closer to the outside temperature.

I haven't made any mice for a long time :( haven't been knitting because of arthritis issues..
when we moved here there was no insulation. It took 4 days to fill the house, including the attic floor. we were advised at that time that with proper venting it would be okay. also had knob and tube wiring removed :) Now there are soffit vents and the windows are insulated. I don't know if the ceiling is or not ( I think not, although the roof is fairly new and the chimney is removed)
The attic is the footprint of the original house, so it is quite large. I have a lot of stuff store up there including my yarn stash :)
we checked the ceiling vent today and it seems fine. I think it was just so cold that condensation was inevitable. I am going to ask my contractor for sure though. I appreciate your suggestions and help.

I think it is all the hot air coming out of your mouth lol :)))))))

Margo might have something there. Drive a car in winter conditions in where the car heater doesn't work. I noticed that my breathing made frost on the inside of car windshield. En route, obviously i couldn't repair the heater core. Solution: Roll down the window. Brrr.... talk about the wind chill factor

you guys are so very helpful!!! :) I need help today! I am exhausted :) I can't even think of anything clever to say!

Hi mouse, was this as a direct result of the polar vortex or is this happening regularly?Sorry you are exhausted. Cup of hot chocolate helps warm me and cheer me up :-)

Could have been the polar vortex (Fancy name for "Thank you Siberia") Haven't noticed it previously but I will be checking more often now. Summer cannot come soon enough!!! :)

I blame it on Canada :)