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Vapor barrier at unvented roof…..

user-7125415 | Posted in Energy Efficiency and Durability on

First….what a great resource. Thank you.
I’m in zone 3 B/C a little north of Los Angeles. Hot and dry. I have a partial cathedral ceiling in a straight gable run roof line. The construction assembly is to have:

Structural rafters (exposed)
2×6 T&G sheathing
Two courses of poly iso rigid (R-19) the top most is nail base
Peel and Stick (Grace)
Standing seam metal roofing

In the article “How to install rigid insulation on top of sheathing” it is recommended to apply a vapor barrier at/above the structural sheathing. T&G in my case. When discussing this with my local building official, in fact the plan review engineer, he is telling me not to put a vapor barrier here, but to let the rigid insulation seal it.

I suppose my question is….Is it detrimental to install a VB at the structural sheathing level ?

Thank you for any opinions.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell use your name? (I'm Martin.)

    Q. "In the article 'How to install rigid insulation on top of sheathing,' it is recommended to apply a vapor barrier at/above the structural sheathing."

    A. That's not true. The article recommended an air barrier at this location, not a vapor barrier.

    There is no requirement for an interior vapor barrier in your climate zone. That said, if your polyisocyanurate is faced with foil, you already have a vapor barrier, whether you need one or not.

    Concerning the need for an air barrier (which you didn't ask about): It's possible to use the polyiso as your air barrier, as long as the seams are carefully taped, and the two layers are installed with staggered seams. That said, rigid foam can shrink, and may even shrink away from the tape after a few years. So the best practice would be to install an air barrier between the tongue-and-groove sheathing and the bottom of the rigid foam, as described in my article. The article notes, "If your roof has board sheathing, install an airtight membrane (for example, Solitex Mento, a product available from 475 High Performance Building Supply), a layer of synthetic roofing underlayment, or a peel-and-stick membrane."

  2. Expert Member
    Dana Dorsett | | #2

    With the t & g exposed to the interior space it's essentially indoors, and it's FINE to have a vapor barrier on the exterior side, not that it's vapor retardency is particularly relevant in your proposed stackup & location.

    But the stackup needs to be changed, putting the Peel 'n' Stick on the much leakier t & g, not above the nailbase panel (which is NOT the structural sheathing.) Use the standing seam manufacturer's recommended underlayment for the roofing.

    With t & g decking a self-healing membrane such as Ice & Water Shield is the right way to go for better long-term air tightness. As Martin points out, foam expands/contracts with temperature and humidity, and even with the seams taped won't be nearly as tight as a self healing membrane or broad-sheet underlayment air barrier between the foam and the t & g.

    Even without foil facers, a 2 layers of fiber faced R19 polyiso you'd be pretty close to a true vapor barrier, at the very tight end of Class-II vapor retardency. But you still don't want indoor air to be able to pass through the t & g and into the micro-spaces between sheets of polyiso to "the great outdoors" and beyond.

  3. user-7125415 | | #3

    Hi Martin......
    Thank you for the immediate reply. Again....what a great resource. How often does one get to engage the author of an informative tech article ?
    I'm afraid I am my own victim here. I suffered from un-careful reading. You are of course correct. The distinction between air and vapor escaped me....they are only two of our principle elements.
    For my own further edification, are we concerned with air loss because it may carry vapor with it, or simply for thermal loss.
    Thank again,

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Are we concerned with air loss because it may carry vapor with it, or simply for thermal loss?"

    A. For both reasons. Attention to airtightness is a fundamental principle of green building, or any building attempting to comply with building science principles. For more information, see these two articles:

    "Questions and Answers About Air Barriers"

    "An Introduction to Pressure Diagnostics"

  5. user-7125415 | | #5

    Thank Dana, for expanding further. It's very helpful.
    And Martin for the response and links.

    One of the great things about a Q&A discussion format is one may resolve obstacles to understanding at a fundamental level....encouraging further development...for me at least.

    Best - Coleman

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