Factory-adhered polyiso/OSB over rafters
A number of products are on the market that consist of polyiso foam insulation adhered to OSB. The manufacturing process looks to be one of the foam being sprayed directly on the OSB with no glues involved. Can this product be laid down on top of the rafters and screwed in place? Or is it required to have an OSB or plywood deck first?
i.e.: Drywall, rafters with cavity insulation, Hailgard, roofing paper, standing-seam metal panels
In a house with lots of cathedral ceiling building, an unvented roof seems quite reasonable. Laying down this combination insulation/nailbase product would not only provide a fully adhered closed-cell foam on the first condensing surface, it would provide a thermal block over each rafter. The rafter space could then be filled with foam board, glass batts, or other insulation. But all the insulation would be under the roof deck.
Any moisture that made it into the OSB deck could dry to the outside if a vapor-permeable roofing paper were used. Then a standing-seam metal roof over that, where the standing-seam panels are held off the roof deck by 3/8″.
Any moisture that made it into the rafters could dry to the interior if a vapor permeable cavity insulation were used along with drywall for the ceiling.
For instance, Hailgard only states that it is to be laid like any other OSB. It would be great if that meant it could be laid directly on top of rafters. But does it meet code and will it be an easy sell to an inspector?
-barlyfield
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Replies
AJ's right, you'll have to check with your local inspector and run it by the manufacturer first. Bring them into the conversation earlier rather than later.
But I think your building science is fairly sound, re: the vapor open (but airtight!) layers and drying potential. Couple things: I wouldn't use more foam in cavities - blown fiberglass or cellulose, being vapor open, would be a somewhat safer bet there. More importantly though, what's your climate zone and planned thicknesses of the foam sandwich and framing/cavity layers? It's the ratio of the R-values of these that determines your first condensing surface, and thus how critical the drying potential is.
We are not your building inspector nor are we the manufacturers. Direct your inquiry appropriately.
There are standard tables that building engineers use to figure shear. Your products are not standard. So to get from your idea of using said products to OKs from others to build is such that you need to make the inquiries.
Let us know the answers.
Arthur,
If you want to install an insulated panel directly on top of rafters in a new-construction situation, you should use structural insulated panels (SIPs). These consist of a layer of rigid foam sandwiched between two layers of OSB.
Nailbase panels differ from SIPs: they consist of a layer of rigid foam with OSB on one side only. Nailbase panels are used for retrofit situations; they are usually installed on top of a layer of existing roof sheathing.