Ventilation channels below roof deck vs above
One accepted assembly for the insulated roof when using fiberous insulation is to build ventilation channels, eve to peak, under the roof deck and between the rafters. It’s considered best practice to use caulk to airseal the ventilation channels from the interior side. (See 2017 FHB house baffle article, link at end of post). The insulation batts or loose fill go underneath the baffles and then drywall attached the rafters.
Now, consider a structure built with the roof deck sealed to wall sheathing, and all seams are taped, etc to make the sheathing and roof deck one continuous airsealed layer. Now 2 crossing layers of furring are added to the roof deck to create eave to peak ventilation and modest overhangs, and a metal roof is fastened to that. Fibrous insulation is installed between the rafters and drywall on the rafters.
Are these two assemblies any functionally different for moisture management?
They don’t seem any different to me, but the first assembly is normal while I’ve never seen the second suggested. It seems like the second method would offer several advantages; it’s quicker, less sheathing used, the structure is dried in quicker (if using ZIP or similar) , air sealing is easier, and you can eliminate the soffit vents and use Coravent or similar between the furring strips.
For reference, 2017 FHB house site built channels with loose fill cellulose:https://www.finehomebuilding.com/2017/08/02/ventilation-for-the-prohome-roof
GBA Detail Library
A collection of one thousand construction details organized by climate and house part