This highly insulated roof assembly can easily achieve roof R values of 70 or more by adjusting the high heel truss depth. The roof is vented in a cavity created by blocking down a layer of rigid material at least 2" down from the upper layer of sheathing. Using a rigid board insulation for this cavity (as shown) will increase the insulating capacity of the assembly, allowing either higher R value or a reduction in truss depth. A vent channel is set in the soffit, with another at the ridge or end wall soffit of a shed roof. An OSB sheathing plane is attached to the bottom of the truss, the seams are sealed with an air sealing tape (Siga) and there is a wire chase/ceiling nailer layer of 2x2's below.
With the upper rigid material creating the vent plane, and the lower air selling OSB, it is possible to dense pack this cavity (you may need to reinforce the foam board with extra blocking above) with cellulose.
The air barrier connection to the wall can happen on the inside or outside of the top plate.
Inside: A solid (OSB) wall barrier with taped seams or a properly installed wall membrane(MemBrain, or other ) can be sealed directly to the adjacent OSB ceiling plane with air barrier tape
Outside: Bring the exterior air and water barrier (liquid applied, seam taped OSB layer or other)up and over the top plate with a top plate "cap" that extends over the TP by at least 3/4". Order the truss seat to match the TP cap width so that the ceiling plane OSB meets the cap inside of the conditioned space enough to adequately tape this seam.
From the top:
- Metal standing seam roofing
- Water barrier (felt of membrane)
- 5/8" CDX sheathing
- 1x2 spacer
- 2" rigid foam (EPS, Polyiso) -or- thin ply material
- 1x2 strips to secure foam layer
- Cavity fill dense pack cellulose
- High heel truss with custom seat cut
- OSB air barrier
- Air sealing Siga tape
- 2x2 furring for wire chase, finish ceiling nailer
- Soffit and soffit vent
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