Conditioned Attic with Ventilated Roof Deck (Assembly Question) - GreenBuildingAdvisor

Conditioned Attic with Ventilated Roof Deck (Assembly Question) cathg | Posted in General Questions on April 6, 2026 07:59pm I have a 1930s colonial home in climate zone 4A and my hvac unit is in my vented attic.  I want to have my hvac and ductwork in a conditioned space and trying to figure out an assembly that won’t trap moisture but also won’t allow wintertime condensation. The attic has a simple gable roofline with soffits and a ridge vent.  I just replaced the roof, so I can’t do exterior insulation at this point, and closing up the soffits would be too tricky, so here was my proposed set up: 1.) 2″ ventilated channel under the sheathing2.) 3.5″ of Timer HP filling the remainder of the 2×6 for an r12 (I was hoping that the timber hp could also double as a baffle if I use the exterior board).  I would have 2″ spacers in place to ensure that I don’t accidentally push it too far towards the roof sheathing3.) 4″ of Thermax polyiso foil faced insulation across the attic rafters (2 – 2″ boards staggered seams and all seams taped) for an r28 My questions are: 1.) Is the any risk of  moisture getting trapped in the Thermax layer since it’s a foil faced vapor barrier?  I’m thinking no since the ventilated gap allows drying, but vapor barriers make me nervous.2.) Is there any risk of the timber HP getting moldy since it’s not treated with borate?  Should I used foam boards in the rafter bay instead of Timber HP?  I didn’t want to stack too many foam layers that could trap mois...