Unvented vented roof with exterior rigid insulation
I am doing an energy retrofit on a Zone 5 simple box gable roof. Part of this calls for redoing the roof. I am sold on the exterior rigid foam idea to cut down on thermal bridging and also since my roof slope is very low and the eaves are shallow.
Code calls for R38 but I want to shoot for about R50. The planned assembly going upward is:
1. R20 of fluffy between upper truss cords
2. Structural sheathing with taped seams
3. Air barrier
4. First layer of 3″ polyiso rigid foam
5. Second layer of 3″ polyiso rigid foam with staggered and taped seams
6. Roof underlayment
My question is in venting the underside of a metal roof to add to the performance since ice damming can be a problem around here.
It seems you would need to use horizontal purlins to add a gap between the metal roofing and the underlayment. However horizontal purlins seems to be contradictory to air flowing up to the ridge and water possibly damming.
Could anyone make any recommendations on this scenario?