drywall corners opening up

Over the seven years we have been in the house, all of the exterior vertical corners in my house have opened up about 1/16" or more in the middle part of the corner (starting about 18" up from the floor and extending to about 18" short of the ceiling). The framing is 2x6, of course. Because of the layout of the house, there are three such corners in the house; there are two corners in the attached garage, which is 2x4 construction, and these corners have not cracked to any significant degree. None of the interior partition corners, nor corners where partitions meet the exterior walls, have opened up. Two possibilities occur to me: the nature of an exterior corner post assembly is different from other corners, and 2x6 lumber moves 60% farther under moisture changes than does 2x4 lumber.
So, what is likely going on with these corners, and what might I have done with the drywall to prevent this? By now, probably not much can be done to fix it, since the corners open and close with the seasons to some extent (it will be less noticeable when summer humidity and heat return). I am going to clean out and fill the three corners with flexible white caulk, but I expect there will still be some cracking, at least of the paint layer.
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donald girod wrote:

I am not sure what you mean by exterior corners, but I am going to suggest that the drywall was not properly installed using standard procedures. Each outside corner should have had a metal or plastic corner bead
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Joseph Meehan

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I suspect you are seeing the results of normal shrinkage of framing lumber, it is just compounded by the 2x6 framing. Rather than re tape and bed the corner, I would wipe the corner in with painter's latex caulk at the next paint job. Lumber shrinks across the grain exponentially as compared to with the grain. ______________________________ Keep the whole world singing . . . . DanG (remove the sevens) snipped-for-privacy@7cox.net

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By "exterior corner" I mean the corners of the exterior walls of the house--these are, of course, interior (cove) corners of the drywall. Anyway, lumber shrinks from moisture loss just twice as much radially as it does tangentially, but of course almost not at all lengthwise (it does expand and contract lengthwise to a measurable degree with temperature changes). A 2x6 should shrink .1", 5.5" to 5.4" going from 19% to 12% moisture content, and I suppose that is what is causing the trouble. I intend to put acrylic caulk in the crack when I paint.
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donald girod wrote:

In that case I would tend to believe they used green wet lumber.
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This sounds more like a framing problems than a drywalling problem or a problem with shrinkage. Properly built corners inside or outside - as in whether they'd be taped (inside) or beaded (outside), or internal (walls interal to the house) or external (an outside wall) should not crack as you've defined, whether made of 2x4 or 2x6.

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