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Q.We have recently completed a residing job in which we installed cedar clapboards over 1-inch foil-faced foam. The clapboards have since cracked on all sides of the building, both at the ends and along the length of the clapboards. How can this be prevented in the future?

A.Cedar gains and loses moisture readily, resulting in board swelling and shrinkage. The high initial moisture content of some of the currently available cedar may contribute to this problem.

Because of this tendency, proper nailing, with just one nail per clapboard at each stud, is essential, whether or not the siding is installed over foam insulation. The nail must not penetrate the board below it. Instead, the clapboard above must hold the top of the clapboard below it with friction. If there is more than one nail across the width of the clapboard, the drying shrinkage will cause splitting. Back-priming the clapboards and sealing the ends will help reduce cupping and splitting.

Since the foil behind the siding is impermeable, daily moisture movement through the wood, due to temperature changes, causes moisture to collect on the back of the siding. This increases the cyclical wetting and drying and can cause additional warping and splitting. Many builders install a layer of #15 roofing felt between the siding and the foil. This serves as a blotter, temporarily storing moisture driven through the clapboards by the sun during the day.

It also works well to vent the back of the siding by installing furring strips over the insulation and nailing the clapboards to them. The top and bottom of the cavity created must be screened to keep insects out.