I have a A Frame Timber Frame constructed house in Alaska that was built in 1978.  Looking to repair an issue (hole) in the roof and add insulation to prevent ice dams.  The current construction of the roof is 2×6 T&G (thats the interior ceiling) supported on 4×8 rafters exposed on the interior.  Above the T&G there is an average of 2.5 -3 inches of spray foam insulation (guessing R value around 15-18).  Then approx 1 inch air gap and the plywood sheathing, followed by asphalt singles.  Current code calls for R49 insulation and a vented roof.

 My engineer designed a plan to remove all roof materials down to the T&G, install a vapor barrier, then 9 inches of layered XPS rigid foam board insulation.  On top of the insulation the engineer wants to put a plywood sheathing layer with 2.25″ holes every square foot to vent the insulation.  On top of this sheathing the plan calls for 2×4’s to create the vent space followed by another sheathing layer, and finally a metal roof.  Seems to me there is an easier way to do this such as put 2×4 directly on top of the rigid insulation and screw them down t0 the T&G to provide a foundation for the final sheathing layer vs having the additional sheathing layer with a 2.25″ hole every square foot?

 For the insulation looking at using XPS Foamular 250 (Four layers of 2″ and 1 layer of 1″ to get to 9″ total.  Any issues with the permeability with this idea?  Its what the engineer came up with

 Most contractors in Alaska are using EPS for projects like this, likely because it’s locally produced (I’d consider EPS, but I don’t get as much R value).  Polyiso is available, but I feel its permeability isn’t good for this application plus the fact it will absorb moisture.  Per the engineer, when all R values are considered for the wood layers, vapor barrier, and XPS insulation his plan provides a total of R48.3.  My only hold back is that I’ve yet to find a roofer using XPS insulation for a project like this in Alaska, want to know I’m going down a good path.  Is 9″ of XPS a valid option or is there a better option?

  I am in a cold, wet climate.  Avg temp in winter is 20 deg (Varies -15 to 30ish, but mostly around 20), snow, freezing rain.  Summer around 65 and lots of rain.

  appreciate any useful input.  Thanks!