What underlayment for asphalt shingles?
I have read a bit on this website and other places about underlayment permeance/permeability, and I’m still not clear. Some building science experts say (source: Fine Homebuilding article from Oct./Nov. 2011) that underlayment permeance doesn’t matter when asphalt shingles are installed. I’m 900′ from the salt water in coastal NC where it is hot, humid, and you get hurricane driven rains. My house was built in 1938, has thick T&G deck boards, ridge vent, and soffit ventilation, and I have had no leaks w/ a 2002 installed roof (I’m assuming it has asphalt felt underlayment). I am replacing the roof w/ Certainteed Landmark Premium shingles (using Type 316 stainless steel nails) and was going to apply I&W shield only at the valleys and around the penetrations (concerned about I&W shield being a vapor barrier and the difficulty of removing it in a future re-roof). I was thinking of using GAF Deck Armor (16 perms), but it is not stocked at any of the roofing supply stores, is going to be expensive to order (only need 4 rolls), and I can’t find it online. I could easily purchase ASTM D 226 #30 (Type 2) asphalt felt which breathes, but my roofer has dissuaded me from doing this (contends asphalt felt will degrade faster than synthetics and will blow off in a storm). I could easily purchase a vapor impermeable roof underlayment and move on. So does roof underlayment permeability matter at all in my case (asphalt shingles, have proper ventilation, and live in a warm coastal environment in N. Carolina)? Thanks!