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Roof truss insulation

Hi, I am building a detached 24x24, on slab, single story, 8' walls , two gable, 4/12 pitch shingled roof, brick veneered, workshop. This will be a unheated workshop. I live in Charlotte NC. This is zone 3.
Anyway.. R15 glass batt insulation is going in the walls under drywall. Since I will NOT have an attic, and NOT be using drywall to cover the bottom cords of the trusses, the 4/12 roof trusses will be visible and are spaced at 24" apart.
Here is my plan for the upper insulation.
Visualize from the shingles down,
felt paper,
osb roof deck,
a radiant barrier between the 24" separated trusses,
the 1" duro-vent from breathable soffit to ridge vent,
R19,
6 Mil plastic vapor barrier,
tongue and grove boards or 1/2" wood panel that will be nailed to the underside of the top cords.
Does that make sense ? Is the 6 Mil overkill if the R19 is faced? I am trying to keep out as much heat in the summer as I can. Remember this will only be a 4/12 pitch with black shingles, that is a lot of surface area that the sun will beat on all day, and no trees to shade the roof.

Asked by Greg Stringer
Posted Thu, 12/12/2013 - 13:38

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Greg,
If you want a radiant barrier directly under your roof sheathing, the easiest approach is to specify radiant-barrier sheathing (OSB with a radiant barrier laminated to one side).

If you ever change your mind and decide to heat and cool this building, you might be interested to know that R-19 insulation isn't enough insulation to meet minimum code requirements.

In your climate zone, you definitely don't want to install 6-mil polyethylene. If the building ever gets air conditioning in the future, the poly could cause a moisture disaster.

If you want to install tongue-and-groove boards as your interior finish material, I strongly urge you to install gypsum wallboard first as an air barrier.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 12/12/2013 - 13:52

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