Roof ventilation channels above the foam?
I am in Vermont. I am insulating my roof on the exterior only with polyiso. The roof will be strapped and metal hung on the strapping. I was going to just use rough cut 1x’s horizontally across the insulation. Having watched a number of videos of people installing rigid roof insulation all of them are putting the strapping vertically first to create an air channel to the ridge and then strapping horizontally. I can understand the thought process in that, I just question whether it is necessary on a strapped roof to begin with. This is 5 channel metal roofing so there is a way for air to flow upwards through the ridges in the metal panels. Pitch is about 8/12 . Can anyone offer any thoughts or actual experience? Thanks
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squarepeg,
To me the strapping is just a necessary backing to fasten the metal to - it serves no other useful purpose. The question is, once you have created the air space, will it accumulate enough moisture that it needs more venting than will occur through the profiles of the panels, or does it even require a drainage path? I suspect not, and it may even be the double layer of strapping that allows enough moist air to accumulate, which necessitates the venting the first place.
If I was building a roof like that I would be tempted to fill the spaces between the rough-cut horizontal strapping with 1" foam, so the metal roofing lay on a flat substrate with no air gap at all.