Allison Bailes has covered this topic before. It's a common question, but one that seems to cause confusion. To understand the issue, read this article. Persevere to the end. You don't have to learn how to use a psychrometric chart (or "psychometric" charts, as some would say). You do need to know how condensation forms, and know that when it's cold outside and you start introducing a lot of moisture into the air, moisture vapor is going to find a cold surface somewhere and condense into liquid water, bringing with it the increased the risk of mold and water damage.

Allison summarizes the issue this way:
If you have a house with a good building enclosure (airtight and well insulated), your indoor humidity should be fine without having to resort to a humidifier. If your enclosure isn't so good, do what you can to improve it before doing something that could have a negative impact on your indoor air quality.

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