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Cracking Clapboards by William A. Lotz We have known for several years that there is a compatibility problem between wood clapboards and extruded-polystyrene sheathing. When the two products are installed tightly together, the clapboards can fail by splitting, shrinking, cupping, curling, and popping the nails. The failure of the clapboards is more severe on the sides of the building that get sun-especially morning sun. The problems are worse when the siding has a dark stain or dark paint, or is left Splitting (above) and nail popping (right) are common on horizontal wood sidings installed directly over foam sheathing. One solution is to put the foam on the inside. unfinished. The problem is also worse when the nails are driven too hard. The foam-insulation manufacturers claim