Do organic and synthetic versions of #30 felt differ in perm rating? 100 year roof.
I’m looking at roofing underlayment: at a bewildering variety of products.
Among the asphalt products there are “organic” and “synthetic” felts. I understand the organics are mostly paper, and the synthetics are generally a glass fiber. Do they differ in permeability? I assume the cotton or paper felts dry in part by the happenstance alignment of fibers from top to bottom.
Good background reading includes:
But not the specification sheets of most underlay products. They don’t list perms, or indicate how well the product seals around nail holes.
GAF RoofPro is an example synthetic base asphalt felt that does not list it’s permeability.
Delta Foxx lists (214 perms ASTM E96-05A, 550 perms ASTM E96-05B), but is hard to buy.
My specific application is repalcement underlayment for a 77 year old mission style tile roof. The old felt paper is in good condition on the steeper sections, and very brittle on the flatter section. This is a non-freezing climate. As with all ceramic tile roof, there’s a nail placement for a wire tire above each tile, so the synthetics that require capped nails won’t work.