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Closed-cell spray foam on roof deck… shingle underlayment choice?

unclejemima | Posted in General Questions on

Good day,

I’m planning on doing spray foam insulation on the underside of my roof deck.

I’m about to start shingling next week, and was planning on using synthetic roof paper instead of tar paper.

Then I read an article saying that on unvented roof assemblies (what this will be), most synthetic roof underlayments should NOT be used.

I did some digging, and the product I had planned to use called “Rhinoroof” has a perm rating of 0.5, aka it would create a vapor sandwich between my shingles and under deck roof insulation.

http://www.interwrap.com/RhinoRoof/RhinoRoof-U15.html

My sprayfoam guy did not seem to conceared about it, saying that if my osb moisture content was below 15% when the foam was applied, then it would no be an issue.

Its winter, i’ve got snow on my OSB now and 1/2″ of ice. I doubt, unless I leave the roof until summer, that I will get the moisture anywhere near 15%.

Whats your guys take on this? It never crossed my mind…but glad I stumbled across it. I’m thinking #30 tar paper might be the way to go…?

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mark,
    Read the installation instructions here:
    http://www.interwrap.com/downloads/roofing-docs/RhinoRoof-Installation-Instructions-ENG.pdf

    "RhinoRoof U15 is an air, water and vapor barrier and therefore must be installed above a properly ventilated space(s)."

    So don't use Rhino Roof.

    Wait for the weather to be warm enough so that your OSB is ice-free. Install asphalt felt roofing underlayment.

    And don't install any spray foam insulation until your OSB is really, really dry -- and warm.

  2. Richard Beyer | | #2

    With ice already attached to your OSB you have other things to be concerned with when the exothermic reaction occurs during spray foam installations. Your spray foam guy should consult with SPFA (U.S. Advisors... http://www.sprayfoam.org/ ) or CUFCA (Canada Advisors... http://www.cufca.ca/contact.php ) before he moves forward knowing your current conditions. Maybe you should contact them yourself to avoid trouble with your SPF installation as well. If anything should go wrong "you" will be blamed so educate yourself before moving forward and get "EVERYTHING" in writing before your SPFI company moves forward with your project. You should also verify their insurance has pollution coverage should something go wrong. (AKA.. Contractors Pollution Liability Coverage - CPL)

    In order for SPF to perform as marketed surface temperatures are just as important as the slip sheet your preparing to use under your shingles.

  3. unclejemima | | #3

    The thing is, this is the best spray foam guy in town. Hes been around for some time and really seems to know his stuff. he says he has a cold weather foam thats good for application down to 0C.

    We have put heaters in the house to melt the ice on the roof, and then will shingle only when the roof is dry.

    From there the sprayfoam guy said they will heat the house with dry heat and they verify with moisture meter and surface temp before spraying.

    I'm not sure what options I really have for this cold time of year. I really want to insulate to my roof deck...but not sure what other options I have. I need at least R40 for my climate.

    Any suggestions would be appreciated.

  4. GBA Editor
    Martin Holladay | | #4

    Mark,
    You wrote, "not sure what other options I have."

    I don't know if you have finalized your specifications, or if you really are curious about other options. But I think that a layer of thick rigid foam on top of your roof sheathing would be preferable in all respects to spray foam under your roof sheathing. If you really want to consider other options, consider nailbase or two layers of rigid foam with staggered seams, followed by a second layer of roof sheathing.

  5. wjrobinson | | #5
  6. unclejemima | | #6

    Thanks Martin.

    No, I've not finalized my specifications. And yes, I'm curious about other options :-)

    Oh boy. My builder is going to love these ideas (not!) But I'm going to give some thought to different options and report back. Looks like I might do the styrofoam on the roof deck after all. We'll see.

    I'll post back soon...

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