Impermeable membranes effectively keep out wind-driven rain and water backed up behind ice dams, but also trap moisture that’s trying to escape from unvented attic spaces. Apply semi-permeable underlayments on inadequately vented roofs or provide ventilation before drying-in the roof.
Asphalt-impregnated felt — aka tar paper — enjoyed a 100-year run as the preeminent roofing and siding underlayment. First made by soaking rag fibers in tar, the basis later became wood fibers mixed with asbestos and fiberglass for greater strength and economy. In response to oil scarcity in the 1970s, the oil content changed — and so did product names. By shifting the pound symbol, 15# felt became #15 felt, which may actually weigh 7.5 to 12.5 pounds per square; #30 felt can weigh between 16 and 27 pounds per square.
Today, some smart people still prefer asphalt felts to synthetic alternatives. Chad Fabry, moderator of JLC’s Building Science Forum and owner-operator of Rochester, N.Y.–based StructureSmart, says, “I use #30 felt each time, every time. As far as I’m concerned, there isn’t yet a purposefully designed product that sports a variable perm rate and water shedding properties that surpasses the serendipitous invention of felt.”
Felt’s perm rating varies. Dry, #15 felt is rated at 6 perms, #30 felt at 5 perms. When wet, however, felt’s permeability increases to between 20 and 60 perms, according to Martin Holladay of TheEnergyNerd.com. A permeable underlayment may be a good thing, then, for applications such as a roof over an unvented attic or cathedral ceiling, for traditional installations of cedar shake and shingle roofs — or for a leaky roof.
Whether a permeable underlayment offers a benefit for the most common home roof assemblies is questionable, however, according to a white paper sponsored by Owens-Corning, co-authored by Joseph Lstiburek of Building Science Corporation. Its title, “Vapor Permeability Provides No Performance Benefit for Roofing Underlayments in Ventilated Attics,” tells the other side of the story. Test results provided in the paper indicate that layers of asphalt shingles built up in a typical installation form an impermeable barrier that won’t allow moisture to escape even if the underlayment can breathe. — M.C.