High Permeability-Rated Underlayments
by Mark Robins | 1 November 2022 12:00 am
Examining the water vapor permeability of metal roof panel underlayments
Clip corrosion spreads throughout the backside of the steel roof panels due to uncontrolled moisture being released under the steel roof panels.
Perm rating is a standard measure of the water vapor permeability of a material. The higher the number, the more readily water vapor (condensation) can diffuse through the material. A perm rating of less than 0.1 is considered a Class I impermeable vapor retarder, which is also considered a vapor barrier). A perm rating between 0.1 and 1 is regarded as a Class II semi-permeable vapor retarder. A perm rating between 1 and 10 is a Class III permeable vapor retarder. A perm rating greater than 10 is highly permeable and not considered to be a vapor retarder at all. A permeable underlayment prevents moisture from getting into the building but allows for the passage of internal moisture (condensation) from the building out, through the underlayment, and under the roof panels.
Panel section cut out and removed to show that moisture under the panel has corroded the galvanized panel clip. The clip corrosion then spread uncontrollably and caused corrosion into the steel roof panels.
Roofing underlayments for metal roof applications typically have a very low perm rating, and are considered Class 1 Vapor Impermeable with a perm rating of 0.1 or less. As an example, AEP Span’s Underlayment HT has a perm rating of 0.02, some have a perm rating of 0.05, and 30# felt has a perm rating of around 0.5.
Not Advised
The use of a high perm-rated underlayment is not advised under metal roof systems unless cautionary steps have been purposely built into the roof panel assembly and design. Some case study trade articles on the subject can be found online regarding the use of high-perm underlayment.
High perm-rated underlayment provides no benefit unless the roof system is properly vented to release trapped moisture. A metal roof system installed over the perm-rated underlayment can act as a vapor retarder itself, effectively eliminating the benefit of using a perm-rated underlayment.
Typical details for metal roof designs are intended to keep moisture out of the system altogether. If moisture is designed to vent internally through the substrate assembly, the trapped moisture will need a way to get it out from behind the roof panels by way of a vented ridge or roof-mounted venting system. Only having a vented underlayment will not suffice, and can accelerate backside corrosion of the panels, flashings, G-90 galvanized coated clips, and zinc-plated fasteners.
Jeff Haddock is technical services manager at AEP Span, Fontana, Calif. To learn more, go to www.aepspan.com[1].
Backside panel corrosion appears as a paint blister with some evidence of visible corrosion. Beware, as what can appear to be minor surface rust, could be caused by uncontrollable moisture release below the roof panels that will spread rapidly.
- www.aepspan.com: http://www.aepspan.com/
Source URL: https://www.metalconstructionnews.com/articles/high-permeability-rated-underlayments/