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Roof Insulation Tradeoffs

c_legato | Posted in Energy Efficiency and Durability on

High Wind Area Roof Insulation Tradeoffs

I am asking for advice on a specific situation in case it is not completely covered by all of the many excellent past threads on attic roof insulation options which have informed my analysis (thank you!). The new construction is on a river completely open to the ocean three miles distant (Westport). The Massachusetts building code design wind speed for this location is 139 mph but insurance companies do not recognize that and would require over 140 mph at a minimum. 
Because of the risk of hurricanes or other high wind events, I am designing the roof with no ventilation. Any openings in soffits, for example are a definite significant risk in high wind events. With this in mind, I am designing the roof to have the following in order from the trusses outward: ¾” plywood + two 2- inch layers of un-faced polyiso staggered and taped + Zip System Red (5/8”OSB). That provides R26ci
on the exterior which under R806.5 is more than adequate for zone five to maintain the temperature of the innermost sheathing above 45° (R20). For the inside, the 2×4 trusses limit the assembly R value. (R 60 is the prescriptive standard but I could probably work with less under the Mass DOER Stretch Code performance standards that apply because the structure will have other energy-saving features that should compensate). If I were to fill the 3 ½ inch cavity of the truss chord with open cell foam it would provide R 12.25 and at that thickness it would have little permeability. Filling the cavity with closed cell foam would give me R 24.5 on the inside. If I already have totally sacrificed permeability to the interior, I could add polyiso foam board to the underside of the truss chord without any further permeability penalty since it is already totally impermeable. Adding 1 inch of foil faced polyiso would also eliminate the expense of intumescent paint on the foam for fire protection. I am thinking ocSPF on the inside with 1″ polyiso nailed to the underside of the truss top chord and taped. (along with the ci on the top side). 
I recognize that impermeable insulation on the underside will prevent any drying potential of the sheathing and that does create some risk to the sheathing. It probably creates less risk than the risk of the roof blowing off, however. Although based on prior posts on the site, I think that this is the basic trade-off I face I just wanted to ask whether there are any other ways to optimize this situation. One further point, for the same reasons I am not designing  venting on the top side because venting there would
be similarly risky in high wind.
Thanks very much.

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