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full article as a PDF document.  A.Mike Guertin, a builder and
remodeler in East Greenwich, R.I., responds: Some questions have
complicated answers, but this isn't one of them. Yes, the IRC
2003 says you have to put a "secondary weather-resistant
permeable membrane" [R703.5.1] over any walls that will be
covered with cedar shingles, and it makes no distinction between
shingles covering conditioned or unconditioned spaces. The Cedar
Shake & Shingle Bureau's standard instructions for shingle
installation also recommend an underlayment (they specify #30
tar paper). But if you were installing vinyl, aluminum, steel,
or wood lap siding, you wouldn't have to install housewrap or
tar paper; for some reason, the IRC doesn't require it beneath
those claddings.