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What underlayment for asphalt shingles?

NCarolina | Posted in General Questions on

I have read a bit on this website and other places about underlayment permeance/permeability, and I’m still not clear.  Some building science experts say (source: Fine Homebuilding article from Oct./Nov. 2011) that underlayment permeance doesn’t matter when asphalt shingles are installed.  I’m 900′ from the salt water in coastal NC where it is hot, humid, and you get hurricane driven rains.  My house was built in 1938, has thick T&G deck boards, ridge vent, and soffit ventilation, and I have had no leaks w/ a 2002 installed roof (I’m assuming it has asphalt felt underlayment).  I am replacing the roof w/ Certainteed Landmark Premium shingles (using Type 316 stainless steel nails) and was going to apply I&W shield only at the valleys and around the penetrations (concerned about I&W shield being a vapor barrier and the difficulty of removing it in a future re-roof).  I was thinking of using GAF Deck Armor (16 perms), but it is not stocked at any of the roofing supply stores, is going to be expensive to order (only need 4 rolls), and I can’t find it online.  I could easily purchase ASTM D 226 #30 (Type 2) asphalt felt which breathes, but my roofer has dissuaded me from doing this (contends asphalt felt will degrade faster than synthetics and will blow off in a storm).  I could easily purchase a vapor impermeable roof underlayment and move on.  So does roof underlayment permeability matter at all in my case (asphalt shingles, have proper ventilation, and live in a warm coastal environment in N. Carolina)?  Thanks!

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Replies

  1. NCarolina | | #1

    Bump, is anyone able to help me by weighing in on this subject?

  2. Jon R | | #2

    This (page 14-15) suggests that it makes little difference (in a vented design) and that a fully adhered underlayment is probably a better upgrade. I'd expect an un-vented design to benefit more from the upward drying.

    http://rdh.com/wp-content/uploads/2015/03/The-Problem-with-Ventilated-Attics-RCI-March-2015-Nov-5-2014-For-Printing.pdf

  3. Debra | | #3

    From studies that I've read, it's not particularly beneficial to install vapor permeable underlayment under asphalt shingles, as properly installed asphalt shingles are themselves impermeable. I'd rather install fully adhered underlayment, such as ice and water shield. If the shingles get damaged or blown off, it provides good backup protection. It also seals around every nail driven through it.

  4. Tim R | | #4

    The
    Insurance Institute for Business & Home Safety has info on what should be done for roofs exposed to hurricanes at. They like fully adhered underlayment to prevent damage when the roof shingles get blown off.
    https://disastersafety.org/hail/roofing-the-right-way/

  5. User avatar GBA Editor
    Brian Pontolilo | | #5

    NCarolina,

    If your roof assembly is vented, or as long as there is not impermeable insulation like closed-cell spray foam applied directly to the underside of the roof deck, you should be fine with an impermeable roofing membrane. If the sheathing gets wet, it will dry inward. If you don't want to use roofing felt and still want a permeable membrane you could take a look at Solitex Mento 5000. It's a robust synthetic roofing underlayment with a perm rating of 38. You can order it at 475 Building Supply. Also, here's an article that you may find helpful when it comes to understanding when vapor permeance is important at the roof plane: https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers

  6. User avatar GBA Editor
    Peter Yost | | #6

    Asphalt roofing shingles installed have a vapor permeance of about 0.2 - 0.3 perms, so a solid Class II vapor retarder, but NOT a Class I VR (< 0.1 perms). Is there some outward drying potential with asphalt shingles as roof cladding? Yes, and more on south slopes, less on north because energy matters. Modified bitumen membranes are Class I vapor retarders. If there is drying potential to a roof with this underlayment, it would be to the interior. Since bulk water management always trumps drying potential, it's not hard to understand preference for a material that adheres to the substrate (roof sheathing), is essentially "waterproof" and seals around fastener penetrations. Peter

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