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A.Mark Parlee, a general contractor in Des Moines, Iowa, responds: In my experience, water that gets trapped against fiber-cement siding can be absorbed by the material, causing joints to swell and shrink during freeze/thaw cycles.
On our jobs, we don't leave gaps for caulk at butt joints, because I think that this detail contributes to the problem. Instead, we snug the joints up tight and protect the seams with a piece of metal flashing installed behind each joint.