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Gambrel roof details

john_fassler | Posted in General Questions on

I’ve got a gambrel roof/wall detail and need some ideas on a safe building assembly. This is in CZ 5A marine. 
I’ve attached section drawings of the top and bottom of the assembly. 
The vertical wall will be structural in  a high-wind, hurricane area. The engineer has specified 1/2″ plywood sheathing. 
The drawing currently shows 2″ ZIP panel below the shingles. I don’t think that will work. What about dense packing the entire assembly with cellulose and not using any exterior insulation? Would that result in any drying problems?
I’m open to any ideas for this assembly with the constraint that the vertical wall is a shear wall.

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  1. Expert Member
    Akos | | #1

    If you are in an area with a lot of wind, I would vote for an unvented assembly. Less chance this way of wind driven rain making it into the house during one of those hurricanes.

    In cold climate there are only two ways of making an unvented roof. Rigid insulation on the outside or closed cell spray foam on the inside. Unvented denspack only roofs can have moisture buildup and end up with rotten roof deck down the road.

    For exterior rigid option look for nail base panels. Unlike ZipR these are rated for roof install. You can look at Hunter or GAF thermacal. They are not cheap and you'll need a decent framer to install them on a complicated roof.

    If you want to save some money, you can go with a hybrid insulation setup where you only put enough rigid above the deck or closed cell SPF bellow for condensation control and the rest of the insulation can be either batts or open cell spray foam. You can read more about the details and ratio here:

    I would use the same insulation setup for both upper attic and the knee wall area. In the knee wall area, you want the insulation in between the rafters and make sure to carry the SPF all the way down to the top plate of the floor bellow to ensure air barrier continuity. By insulating the rafters in the knee wall area, you can now use the space as closet space, storage or for running ducts.

  2. walta100 | | #2

    Your drawing looks like a classic foam sandwich to me.

    The Zip panel is trapped between 2 layers of foam with no way to dry to the interior or exterior.

    Consider moving all the insulation to the exterior.


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