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Q.Can vinyl siding be installed directly over foil-faced rigid foam?

A.Corresponding editor Paul Fisette responds: Some vinyl siding manufacturers void their warranty for heat distortion when their siding is installed over foil-faced foam. One study has shown that when vinyl siding is installed over foil-faced foam, the temperature of the back of the siding can be 8°F warmer than when it’s installed over foam without foil. Installers who are concerned about heat buildup should select rigid foam without foil facing.

Nevertheless, some manufacturers have no objection to the installation of vinyl siding over foil-faced foam. Whenever vinyl siding is installed over rigid foam (with or without foil facing), studs should be spaced no more than 16 inches on-center, and the thickness of the foam should be limited to 1 inch. Because rigid foam doesn’t provide much resistance to vinyl siding sagging under its own weight, such an installation requires threaded nails that are long enough to pass through the rigid foam and penetrate at least 1 inch into the sheathing and studs.

Some building codes may require a secondary weather barrier (asphalt felt or housewrap) under the siding, so check with your local building authorities before proceeding. Where allowed by code, rigid foam will perform adequately as a weather-resistive barrier, as long as all seams and flashing are taped with foil tape.