How to Manage Moisture on Steep-Slope Roofing Systems

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For many years, the importance of proper attic ventilation has been supported by most major roofing manufacturers—including GAF—for helping to manage moisture and prolong the life of the roof covering and exhaust warm air. Attic ventilation has also become widely accepted amongst architects, specifiers, and roofing contractors as a critical component of high-performance roofing systems.

With the moisture and temperature-control benefits of attic ventilation widely known, the challenge has become centered around managing moisture and temperature on structures that are not suited for typical intake/exhaust attic ventilation systems. For example: steep-slope roof structures with conditioned or living spaces directly below the roof deck, cathedral ceilings, glue-lam construction, and post & beam construction continue to become more prominent in residential construction, churches, institutions, and retail.  All of these may be covered with asphalt shingle, metal, slate, or tile roofs. As you can imagine (or may have experienced), providing the proper attic ventilation for these types of structures can be quite a headache.

So in order to solve this difficult challenge, ventilated nail base insulation—as it’s typically known in the roofing industry—was developed. It combines polyiso insulation, an air space (typically 1″ deep), and a top layer of sheathing to serve as a nailing base for installing the roof covering. Ventilated nail base insulationwas designed to insulate against heat drive into the space below the roof deck. The ventilation capacity, provided by the air space, helps exhaust excess moisture before it could condense in the deck or roof system. Together, the insulating and ventilation functions of ventilated nail base insulation are similar to those of typical attic ventilation systems. Ventilated nail base insulation is installed on top of steep-slope structural wood or steel roof decks, and can be used with shingle, slate, tile, and metal roofs.

With the design and use of structures continuing to evolve, ventilated nail base insulation will continue to grow as a prominent solution for architects, specifiers, and roofers who are looking to manage both moisture and temperature on steep-slope roofing systems.

The acquisition of Cornell Corporation has now allowed GAF to add ventilated nail base insulation products to our product portfolio. We’re proud to offer a complete line of ventilated nail base insulation product options under the GAF Cornell brand, featuring ThermaCal® 1 and ThermaCal® 2 Ventilated Roof Insulation Panels—which are both available in various polyiso insulation thickness and sheathing options.

For more information on GAF Cornell Ventilated Nail Base Roof Insulation, please visit www.cornellcorporation.com or call 1-800-ROOF-411.



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  1. Genaro

    James, that spray foam insulation is only making things worse. The moisture is now getting trapped between the insulation and the decking, most likely causing the deck (if it’s wood decking) an perhaps rafters to rot. Now the damage is not visible to the homeowner. This is definitely something to be concerned about. Have a reputable contractor thoroughly inspect your roof.

  2. James

    I have a sort of thick sandpapery-rubber type of tile on my roof (I don’t know exactly what it’s called), and so does everyone in the neighborhood I live in. One of the problems that this type of tile has though is that in heavy storms and wind, tiny amounts of water seep under the tiles. That’s why everyone I know has spray foam insulation lining almost their entire attics, which helps prevent any serious damage

  3. Eli

    Thank you for posting this article and explaining why proper ventilation is so important for prolonging the life of your roof. Air is, in itself, an insulator and can be used in spaces between walls, panes of glass, and under the roof to regulate temperature. The bigger issue is improper protection against moisture and humidity because these can greatly weaken the structure and the integrity of building materials.

  4. Brooke Cason

    I’ve never thought about the architecture of an attic as a component to the longevity of a home’s roof. When I own a home I want to use metal on my roof. The metal is durable and long lasting and easier to replace than roofing tiles.


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