Exposed rafters and Insulating the Roof Deck

I’ll be using 4×10 rough sawn timbers for the rafters. The roof deck will be 2×6 tongue and groove cedar planks. Exposed on the interior of the cabin those elements should look rather dramatic. Moving from drama to the practical side, the question of where to put the insulation surfaces. In a house with an attic, loose fill insulation is blown into the attic and settles on the floor.

In a house without an attic, insulation batts are typically installed between the rafters. Then the rafters and insulation are covered up with sheet rock and plaster. 

To leave the rafters and underside of the roof deck open the only place left for the insulation to go is on top of the roof deck. The plan is to attach 6″ of rigid foam to the topside of the roof deck. Polyisocyanurate rigid foam has an R-value of 5-6 per inch. That combined with meager R-value of the other roof components give put R-value of the total roof somewhere in the 33-40 range. Recommend roof R-value in our region is 30-60. 

I do have some other ways I’m planning to mitigate heat absorption including:

  • Locating the structure where it will be shaded by trees in the afternoons.
  • Using light colored metal roof sheathing
  • Running continuous soffit vent and venting the underside of the metal roof sheathing.
  • Installing solar panels (Solar panels will shade the roof somewhat)
Here’s a slideshow the roof installation process: