Roof Insulation High Wind Area
c_legato
| Posted in Energy Efficiency and Durability on
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).
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. If I already have totally sacrificed permeability to the interior, I could add 1″ of foil faced 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. My thought is to couple the R26 ci exterior with oc SPF (R 12.25) and 1′ polyiso (R6.5 ci). A more expensive alternative would substitute ccSPF at R24.5 for the open cell.
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, would you please comment on this choice and any other ways to optimize this situation. One further point, 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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Replies
Two thoughts:
1) it is my non-expert understanding that the vent locations (like perforated soffits) are not the problem when it comes to wind uplift but rather having any soffits at all are the problem. In other words, perforated or not, they will face the same wind uplift challenges. If that’s true I would do an unvented attic but have a rainscreen/vent gap between the metal roof and the exterior insulation. You didn’t say what type of roof but for wind concerns a standing seam metal with no exposed fasteners would be a great choice.
2) if you go thicker on your polyiso exterior insulation layers you avoid all of this headache on what to do with interior insulation. Why are you limiting yourself to R26?
You also didn’t talk about the roof WRB but I would suggest a peel-and-stick WRB on top of the sheathing underneath the polyiso. You can also tape the seams of the polyiso if you want for a secondary barrier for little extra cost.
Thank you for your comments.
1. The roof will be OC Duration Storm, Class 3 asphalt shingles (resilient to hail and wind-blown debris). Unfortunately metal would be out of the budget range for this project.
2. What is the advantage of peel and stick directly over the first plywood sheathing if there is zip system red sheathing at the top of the assembly?
3. I am intrigued by the idea of increasing the continuous insulation on the exterior. I have only used 4” of foam on roofs previously and worry about the additional windage on additional foam and how to secure thicker foam to the roof. Any experience with building thicker assemblies would be greatly appreciated. Again I am concerned about the 139 mph wind zone.
4. The problem definitely is openings not just the soffit itself. All of the references that I have read say that the soffit can be protected against uplift if there are no vents and that vents are the problem. One such source is Joe Lstiburek.
Thanks