Ice on the Windows!

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Dunno about the 50% RH being "ideal." IME, it's whatever's sufficient to avoid respiratory problems, without notable condensation on windows of cooler parts of house.
In the case I'm familiar with, those windows are low-e, with original el-cheapo storms, and the humidistat is set to around 25% RH.
If moisture is condensing on glass, consider what's happening inside wall cavities and above the ceiling too.

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