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Another unvented roof question….

Brad Lamson | Posted in General Questions on

I’ve read through the wealth of information on this site (thanks) and others concerning unvented roof assemblies.

New house construction, with a 3/12 cathedral roof (shed), no penetrations, with a truss depth of 22 inches. Zone 6

I was considering the following method, from the ceiling to the roofing material.

-Drywall
-Furring strips
-Siga Majrex, entire interior of the structure.
-22″ of dense pack cellulose insulation, r-value of close to 80
-5/8 ply
-Siga Majavest
-2X furring strips
-1/2 ply
-Siga Majavest
-Standing seam metal roofing

I’ve seen this method used, but not on a very wide spread basis, trying to avoid using foam board on top of the sheeting or spray foam on the interior and since the truss cavities have no mechanicals or wiring I could dense pack them and surpass the insulation code of r-50. Would this be enough to alleviate condensation on the interior side?

I have seen several assemblies with out the 2X furring, but thought adding them would give me a bit more assurance with ventilation between the roof deck and roofing. Necessary or not?

House will have an air exchange system.

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brad,
    Your assembly is riskier than the usual approach. (The usual approach is to install ventilation baffles between the rafters, so that you create ventilation channels under the bottom layer of roof sheathing.)

    You have created two vapor retarders separating the top of your insulation layer from your ventilation channels. The first vapor retarder is the plywood. The second vapor retarder is the upper layer of Siga Majrex. These two layers limit vapor flow upward -- and really, what you want is a vapor-open assembly that encourages upward drying, toward your ventilation channels.

    Far better would be to use fiberboard sheathing between the cellulose and the ventilation channels (assuming your engineer approves of that approach), and to skip the Siga Majrex above the fiberboard. Remember, you want upward vapor flow at this layer.

    Or, better still, follow the usual advice for ventilation baffles, and install the baffles between your rafters.

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