I am building a brand-new home that will have an unvented or conditioned attic, ERV and dehumidifier. I remain in Houston, Texas, so it’s mostly hot and damp. Framing is a standard stick frame, so no trusses and rafter tails will extend beyond the wall and be boxed in with Hardie. I am preparing to link the wall to the roofing system joint utilizing a saw tooth in the Zip around each rafter tail and sealing with Proseco gap and seam, Proseco R guard or Zip Liquid flash, depending upon how tight the get the sheathing. The roofing assembly is where I am struggling. We will be installing a standing metal seam roofing system over 1 × 4 furring strips for an air space. My debate(s) are if it is worth the additional cost to utilize Zip roof sheathing? Or use product OSB and merely tape the seams, cover in artificial underlayment (ice and water at eaves and valleys). The 2nd question/debate is whether I need to or require to do a diffusion port on the ridges. Rafters will be 16 ″ OC, so in either case will use 7/16 ″ sheathing, however ZIP is two times the expense, and the roofing is 135 squares (13500 sq feet), so a healthy cost increase. We will be using Rockwool in the rafter cavities (no spray foam– individual choice). I am unsure if I require the diffusion port along the ridges if I am running a dehumidifier in the attic and will have a couple of HVAC vents in the attic. I am preparing to run 2 different dehumidifiers due to your house and attic size, and likewise redundancy. I would have liked to do outside polyiso and a 2nd layer of sheathing, however too expensive for my budget plan. Open to ideas, ideas, and useful criticism (harassment). Value any suggestions.
