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Community and Q&A

Insulation in roof

user-6825743 | Posted in Energy Efficiency and Durability on

I am looking to insulate the roof plane of a small library addition in Coastal Maine.  This area is over the entry and librarian station in a section that connects our super-insulated addition (using dense pack cellulose in a double stud wall construction and dense pack cellulose in roof trusses) to the existing building.  It has only 2×10 rafters.  Can I specify dense pack cellulose between the rafters, Zip system sheathing on the roof, and then 1 or 2 layers of a new product:  1″ rigid VIP, Vacuum Insulated Panel by Kingspan, on the underside of the rafters and get the r-value needed?  I need at least r-50+, preferably r-60.  Then where does the vapor barrier occur?  I understand that Zip panels have an integrated vapor barrier.  Will I be introducing another vapor barrier on the under side with with the VIP panels?

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  1. Expert Member
    Dana Dorsett | | #1

    Most roofing materials are already a vapor barrier, which is why roof decks are normally vented on the underside of the roof deck. ZIP isn't particularly better or worse in that regard.

    Putting foil faced goods on the underside of dense packed cellulose tight to the roof deck would create a moisture trap between the roofing materials and the VIP (and a code violation.) If the underside of the roof deck has at least an inch of air space and is vented to the outdoors on both ends of each rafter bay it would have a code-approved drying path.

    To go unvented in coastal Maine is all US climate zone 6, and at least half the total R has to be on the exterior of either the roof deck or the fiber insulation to have adequate dew point control. With closed cell polyurethane on the underside of the roof deck it's almost a moisture trap, but being well bonded it doesn't leak air, with no convective moisture transfer of moisture to the roof deck from the interior. With sufficient VIP or rigid foam or fiber insulation above the roof deck it's fine to use dense-packed cellulose on the interior side with only standard interior latex paint on gypsum board as the interior side vapor retarder.

  2. GBA Editor
    Martin Holladay | | #2

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

    Whether or not you can use a vented assembly approach depends on the slope of the roof. If this is a flat roof or a low-slope roof, you can't really make it vented. For more on low-slope roofs, see "Insulating Low-Slope Residential Roofs."

    If the roof has a slope of 3-in-12, or anything steeper, you can follow Dana's advice for a vented roof assembly -- assuming, of course, that there is an easy way to install soffit vents and a ridge vent.

    For an unvented assembly, your choices are:
    1. Install an adequately thick layer of continuous rigid foam on the exterior side of the roof sheathing, with or without some fluffy insulation between the rafters. For more information on this approach, see "How to Install Rigid Foam On Top of Roof Sheathing."

    2. Install an adequately thick layer of closed-cell spray foam on the underside of the roof sheathing, with or without some additional fluffy insulation on the interior side of the cured spray foam. For more information on this approach, see these two articles:

    "How to Build an Insulated Cathedral Ceiling"

    "Flash-and-Batt Insulation"

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