Air Barrier for Unvented Metal Roof
I’m building a new home in southern Idaho. From the top down, the roof will be as follows: Standing seam Cor-Ten A606 metal roofing over 7/16″ OSB, over 10″ Type IX EPS (2 5″ layers w/ staggered joints for R50), over exposed 3/4″ tongue & groove decking, over exposed 4×8 rafters. The pitch is 2:12 and mostly a shed configuration, and mostly south facing.
It’s not often these days that we get much snow that sticks around for long…though there has been more this yr than most in the last decade+ w/ 3-6″ that has been around for ~10 days. Because I’m not in a mountainous region that receives a lot of snow…and because I’ll have R50 w/ no thermal bridging, I rationalized it was not critical that I ventilate my roof…at least based on my understanding of the principles explained by Joseph Lstiburek. If money was no object, I would ventilate….but, again, unless I’m misunderstanding, it’s not a critical factor in my case….at least as far as ice damming goes. As far as condensation though….I’m not clear on that.
Following Lstiburek’s guidelines in figure 7 in the following link, https://www.buildingscience.com/documents/reports/rr-0404-roof-design/view ….. I was going to place an air barrier (sheet poly) between the tongue & groove decking and EPS. He shows “roofing paper” over the OSB though….and, as I understand, the heat from the metal roofing will bake it. The solution, as I’m interpreting, is to use a high temp synthetic roof underlayment instead. In my case, I was planning to use self-adhered Sharkskin Ultra SA as it’s locally available.
In my case, am I ok to build as I planned…like shown in figure 7, with no ventilation and with the metal roof and Sharkskin instead of shingles and “roofing paper”?
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You may need to place an OSB or plywood sheathing over the T& G for the roof diaphragm, for seismic resistance. That would be a good place to place the air barrier. You will need a underlayment under the metal roof - for a class A rating - some thing like versashield or similar that the mfg has a rated assembly. It is a fiberglass underlayment.
The 7/16 OSB seems thin for the screws for a metal roof, make sure they calc the fasteners for the roofing based on the OSB thickness, it may need more fasteners than normal.
Thanks for your reply, Tim.
The T&G has been engineered. It will take a lot of nails but it will work for the diaphragm. And yes, I was placing the air barrier on top of that.
As far as 7/16" OSB being thick enough, the roofing manufacturer says it's fine.
My question though is if a high temp underlayment under the metal will be the only change I need to make to the aforementioned Lstiburek's detail? Or......will using metal roofing in this particular context, and in my region, necessitate venting my roof? My budget is lean so I'd prefer not to but if the lack of ventilation is guaranteed to cause issues, I'll add it.
In watching Lstiburek's video last night again, he said this style of roof "has to be" ventilated when the ground snow load is 50# or higher (I'm at 15#). But....he was referring to preventing ice dams. I'm not clear if that type of roof w/ metal requires venting for other reasons in my locale.