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Vapor Permeability Provides No Performance Benefit for Roofing Underlayments in Ventilated Attics

Testing Shows Breathable Underlayment Provides Little or No Moisture Transfer Value in Asphalt Roofing Systems (June 2011)

Dr. Joseph Lstiburek, Ph.D.,
P. Eng., ASHRAE Fellow,
Building Science Corporation

Dr. Achilles Karagiozis,
Director of Building Science,
Owens Corning

Paul Gassman, P. Eng.,
Product Engineer,
Owens Corning

Introduction

The roofing market has seen an onslaught of new synthetic underlayment products in the past 10 years. These products bring many advantages to the installer: increased speed of installation; lighter weight; and significantly stronger physical characteristics, resulting in increased wind uplift performance than typical asphalt felt underlayment. These products also benefit homeowners by protecting their homes as a durable moisture barrier from the elements over an extended period of dry-in during construction or re-roofing.

Some synthetic products claim to improve performance of the roofing system by including “breathability” as an added feature. But does this feature truly add any benefit to a typical asphalt shingle roofing system? In a typical installation, underlayment is sandwiched between the plywood or oriented strand board (OSB) roof deck and a covering layer of asphalt shingles. Does a breathable underlayment allow attic moisture to escape? Does it allow a roof deck to breathe?

Defining Breathability

The 2009 International Residential Code (IRC ) defines a vapor permeable membrane as “a material or covering having a permeance rating of 5 perms or greater, when tested in accordance with the desiccant method with Procedure A of ASTM E 96. A vapor permeable material permits the passage of moisture vapor.”2

However, traditional convention within the building industry defines:

• a material with a perm of less than 0.1 as vapor impermeable
• a material with a perm of between 0.1 and 1.0 as vapor semi impermeable
• a material with a perm of between 1.0 and 10.0 as vapor semi permeable
• and a material with a perm of greater than 10 as vapor permeable

Additionally, a vapor barrier is defined as less than 0.1 perm and a vapor retarder is defined as less than 1.0 perm. As such, some inconsistency exists between the IRC and traditional convention.

Shingle Installation Instructions

Owens Corning™ Classic® 3-tab shingles were used in all testing. Application instructions for these shingles include a 5" vertical exposure on the 12" high shingle and a 6" offset on the horizontal dimension for shingle lapping. This is the industry standard practice for shingle installation.

This method provides an overlapping “water shedding” construction necessary to keep rainwater out. The same principle greatly increases the travel path or flow length for air movement through the same assembly. This resistance to airflow is likely the greatest contributing factor in creating the vapor resistance which this testing demonstrates.

With a 12" height and a 5" exposure on the individual shingles, the overlapping system results in air path which always has a double layer of shingles, and a
triple layer of material at each vertical intersection for air and moisture vapor to migrate through. The 36"width on the shingles also introduces a complicated
path for any air and moisture vapor to travel through in the horizontal—or lateral—direction. Additionally, shingles are relatively heavy and flat and have a rough surface, all three of which are physical characteristics that increase the resistance to airflow in the roofing system.

Roofing Shingle Layer Testing

A 24" wide by 36" long commercially available plastic pan was selected as the “test dish” for the ASTM E 96 testing on large-scale system components. This size allowed for a typical asphalt shingle application with seven overlapping
horizontal rows of shingles, and three vertical butt joints between adjacent shingles. To simulate the shingle layer during application, an OSB frame was fabricated with large slotted windows to allow moisture vapor transfer through this layer with minimal interference from the OSB , while the OSB allowed the shingle attachment as per manufacturer’s instructions. Repetitive testing on this application showed the multi-layered asphalt shingle system—when installed per the manufacturer’s instructions—has an average moisture vapor transfer rate of 0.65 perms. Demonstrating a measured perm of less than 1.0 showed that moisture transfer through the multi-layer asphalt shingles is negligible.
This important fact begs the question: If the asphalt shingles act as a vapor retarder on the roof, then what value is added with the introduction of a breathable roof underlayment below it? Any moisture within the roof deck or the attic will not be able to move through the roofing system, regardless
of the permeability of the roofing underlayment layer.