GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Phased vented to unvented attic conversion plan

sazerac | Posted in Green Building Techniques on

I live in Zone 3 (San Diego), and I have a combined roof assembly of a cathedral ceiling connected to a 2×4 truss system. The cathedral ceiling is 2×10 and has insulation sitting against the drywall with a several inch air gap leading to the truss roof assembly as well as soffit vents (see attached terrible sketch). The truss section has no insulation whatsoever. My roof needs to be replaced soon but I won’t have enough money to complete it for about 2 years.

Existing conditions:
* Climate Zone 3B (San Diego)
* Soffit vents in cathedral ceiling at bottom and air gap at top into truss roof assembly
* Soffit vents and gable vent in truss assembly
* No ridge vent
* Roofing is plank + plywood sheathing
* Truss section is gable on one side hip on the other
* Mini-split AC
* All exhausts vent to exterior through roof penetrations

Phase 1:
* Seal soffit vents and gable vent in truss roof assembly 
* Fur out 2×4 truss assembly to fit R-30 rockwool (or some other insulation retention option)
* Seal air gap connecting cathedral assembly to truss assembly
* Install R-30 rockwool against interior of roof sheating
* Install R-15 in gable end

Phase 2 (within two years):
* Install 3″ (2x 1-1/2″) of continuous insulation on exterior of roof on top of peal-and-stick
* Cover CI with more sheathing and furring strips to create an air gap between roofing material
* Install standing seam roofing
* Fill air gap between roof sheathing and drywall in cathedral ceiling with blow in insulation (not sure if this is necessary)
* Seal remaining soffit vent in cathedral ceiling

Sealing technique for soffits and gable is rigid insulation with spray foam can, or 2 part closed cell if required (cut and cobble).

My main concern is the amount of moisture / rot that could develop prior to installing the continuous insulation. I had originally planned to just install the batts assuming a vented assembly then just move them to the under side of the roof deck after the CI was installed, but it’s extra work.

Dana’s comment here: makes me think it might be okay to do it in phases (climate zones are different though and a ridge vent might have been assumed)

This post: has me concerned, since they repair the rot but it reappeared within a year and they’re likely in the same climate zone as me.

* Am I safe to do it in phases?
* Is my vent sealing technique sound?
* Any tips/tricks holding R-30 batts against 2×4 truss top cord?


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |