I just had a new system installed before Christmas.
I must have had the humidity level too high as the condensation on my windows was freezing. I literally had a 1/2 inch ice chunk on the bottom of my window by front door. On the flip side it visibly exposed to me every major air leak in my house.
A word of caution - be careful of the combination of whole house humidifiers, high efficiency furnace, premium windows, and cold weather.
I had all of the above last year and thought my roof was leaking one day when I went into the attic storage area. I called the roof installer when I noticed moisture dripping from damp roof joists. The roofer first asked me if I had the combination listed above as they get a lot of calls about excessive attic moisture with those conditions.
It turned out that he was correct. I turned the humidity down to the lowest setting and the problem disappeared. I was lucky to find it in time. The roofer knows of cases where the moisture was not dealt with resulting in mold developing to the point that all of the insulation and many of the roof decking had to be replaced.
It turns out that if you have a tight house that the normal steam from showers and cooking is almost enough to maintain proper humidity. If you pump more moisture into the air than cool air can hold, it ends up condensing on cold temperatures in your attic.
I studied this extensively and went round and round with contractors discussing this prior to the construction of our house. Here's what I found:
With no vapor barrier in the ceiling, all the moisture in the house goes up into the attic. Cellulose and the rafters will hold that moisture and lwill eventually lead to mold if the humidity level is kept at too high of a level. Blown fiberglass will hold some moisture, but lets most of it pass thru and doesn't mold, although it might rot your rafters. As you said, it doesn't become a problem until the humidity gets too high.
The answer for this is to install a vapor barrier in the ceiling. However, in our northern climate, this results in way too much humidity in the house, resulting in mold and mildew in the drywall of the ceiling rather than the insulation and rafters. This is because none of the humidity can escape.
What I concluded was that if one wants to truly control humidity, the best way is to install the vapor barrier along with an air exchanger to remove the excess humidity. You can adjust the exchanger to control the humidity level and can keep it quite consistant. If you have extensive wood trim or old wooden family heirlooms, you can maintain their quality better by not allowing them to shrink and dry out.
However, by this point of our construction, our drywaller had already convinced my wife that a vapor barrier would result in nothing but mold so the vapor barrier and air exchanger went out the window. But I did save two grand by not going that route, and every time she complains about dry skin in the winter I'm sure to remind her why.
Walking on water is easy. Just do it in December when it's frozen.
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