A.Joe Lstiburek responds: Wood siding, including cedar siding, should be installed over an air space, regardless of the type of sheathing used. With foam sheathing, an air space is essential, both to reduce the siding’s water uptake and to provide a receptor space for the moisture in the siding. If the cedar siding is installed directly on foil-faced foam sheathing, it will be prone to cupping, splitting, and premature paint peeling.
Since sidings are not watertight, no matter what we do, moisture will get into any wood siding. Rain water penetrates the siding at joints, nail holes, penetrations, and overlaps, and runs down the back surface of the siding. Additionally, capillary action can pull water upward at siding overlaps.
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