Low Humidity=Cracking?

In the last two weeks I have had numerous cracks appear on the plaster, from one end of the house to the other. Generally, these cracks have formed around doorways and arches, and along the main beam in the that runs in the middle of the house from end to end. I can't figure out why this is happening all of a sudden.
My only guess is that a recent covering of the soil in the crawlspace dramatically lowered the humidity down there. This lower humidity could have caused the 18" posts that support the main beam to shrink slightly. Or, the lower humidity could have somehow impacted the beam itself. Does anyone have an experiences with this?
I jacked up the beam by one of the posts, and it alligned a doorway correctly (the door was hitting the jam), so it does appear to have something to do with the main beam or the posts.
Todd
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How old is the house?

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If jacking up a post tended to reverse the problem, it isn't humidity. The posts would not shrink or swell in along their length. It sounds as if you are experiencing settling at a dramatic rate. I don't know how "covering the soil" could produce settling. Yes, the beam could shrink, but the amount of movement this would produce is virtually undetectable.
A new house constructed with very wet lumber would dry out and cause various cracks and nail pops etc, but if this is an older place, and it is suddenly moving, then something serious is wrong with the foundation. If you lived in Amherst (suburb of Buffalo), you would have a lot of company. About a thousand houses have foundations which are breaking up due to building on unsuitable soil using unsuitable methods; some of these places suddenly started to move after 20 years of apparent stability. A falling water table appears to be the culprit in Amherst, though they really haven't figured it out yet, except that building a house in a swamp has never seemed like a real good idea.

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The house is roughly 50 years old. I had a contractor out the other day and he says my foundation is fine, and he suggested using jacks. I bought a bunch of jacks to stabilize the main beam, and hopefully this will have some impact on the cracks. New cracks show up every day now in my ceiling. I can hear the plaster crack at night. I call my home "The Little House of Horrors" now.
Todd
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