Five industry experts — Peter Barrett, roofing product manager, Cosella-Dorken; Michael Coulton, director of new product development,
Benjamin Obdyke Inc.; Charlie Lake, director of marketing, residential roofing accessories, GAF-Elk; Gary H. Schinning, division
manager, roofing products, InterWrap Inc.; and Mark C. Strait, president, Kirsch Building Products LLC — discuss the state
of underlayment:
Ventilation is a complex topic, but what's your short take on breathable versus non-breathable underlayment?
Synthetic underlayment offers many advantages over its felt counterpart, including durability, enhanced ultraviolet resistance,
and mold resistance. (Photograph courtesy of Johns Manville)
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Barrett: As the amount of insulation increases, space available for ventilation decreases. In fact, this has long been a problem area
for non-breathable membranes in configurations such as cathedral ceilings. In this context, a breathable underlayment will
act as the air barrier in the system. It will allow moisture to diffuse and mitigate air movement. High-tech polymer construction
allows the membrane to remain liquid watertight, even when exposed to a driving rain. Placing an impermeable permanent roof,
such as asphalt shingles, onto a permeable underlayment will not lead to any benefit. When used with other roofing materials,
such as metal, tile, or slate, significant gains in moisture control can be seen.
Coulton: If a roof is properly ventilated with a balanced ridge-and-soffit system, it shouldn't matter whether the underlayment is
breathable. Many builders in extreme wind-driven rain climates have successfully installed ice-and-water barrier-type underlayments
on the entire roof without problems, and they are not "breathable." In a sealed attic situation, non-breathable may actually
be preferable. After a rainstorm, solar drive may drive moisture in the form of vapor toward the roof deck. A breathable underlayment
will allow that vapor drive to reach the roof sheathing. If the sheathing has foam sprayed to the underside that is vapor
impermeable, the moisture may be trapped in the sheathing. If the sheathing is OSB (oriented strand board), this becomes especially
problematic as it will change dimensionally more than plywood. This could lead to visible buckling of the roof sheathing.
Coulton
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Lake: GAF-Elk believes that the ability of an underlayment to breathe is essential in preventing structural decay, mold, and mildew
in a home. Unlike non-breathable underlayments, breathable underlayments work in tandem with traditional attic ventilation
systems to exhaust a maximum amount of hot and moist air from the attic and overlying roofing.
Schinning: Construction design has a lot to do with the need for breathable or non-breathable underlayment. In Europe and other countries,
the roof in many cases is an open beam/timber/truss with no plywood or OSB deck. In this design, breathable is the most widely
accepted. In the United States most roof designs include plywood or OSB decks. The plywood industry states that plywood does
not breathe when dry. Asphalt shingles, whether lightweight or heavyweight multi-layer laminate shingles, all have the same
perm range as air and water-barrier synthetic underlayments. If a structure has the minimum or more ventilation, air in and
air out in an open attic in normal conditions is not a problem. If the house is built with a closed-sealed attic in which
the space is being used as part of an efficient design-installed heating and cooling system, this mechanical system will accomplish
the same.
Strait: There is no real ventilation value to a breathable roof underlayment. The term breathable applies to moisture permeance. No
breathable synthetic manufacturer, that I am aware of, states perm values in an installed condition. The installation of a
breathable underlayment installed over 15/32-inch plywood has a greatly reduced value. Add to this the wide range of perm
ratings along with varying deck applications.
