Unvented Attic - GreenBuildingAdvisor

Unvented Attic DarrylP | Posted in Green Building Techniques on April 17, 2026 11:30am I am building a new home that will have an unvented or conditioned attic, ERV and dehumidifier.  I am in Houston, Texas, so it’s mostly hot and humid.  Framing is a traditional stick frame, so no trusses and rafter tails will extend beyond the wall and be boxed in with Hardie.  I am planning to connect the wall to the roof joint using 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 on how tight the framers get the sheathing.  The roof assembly is where I am struggling.  We will be installing a standing metal seam roof over 1×4 furring strips for an air gap.  My debate(s) are if it is worth the additional expense to use Zip roof sheathing? Or use commodity OSB and simply tape the seams, cover in synthetic underlayment (ice and water at eaves and valleys).  The second question/debate is whether I should or need to do a diffusion port on the ridges.  Rafters will be 16″ OC, so in either case will use 7/16″ sheathing, but ZIP is twice the cost, and the roof is 135 squares (13500 sq ft), so a healthy cost increase.  We will be using Rockwool in the rafter cavities (no spray foam – personal preference).  I am not sure if I need the diffusion port along the ridges if I am running a dehumidifier in the attic and will have a few HVAC vents in the attic.  I am planning to run 2 separate dehumidifiers due to the ho...