Q. Is it necessary to use felt paper or Tyvek behind cedar shakes on gable walls that are not heated or cooled?
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.
