Polyiso foamboard (foil-faced on both sides) used in unconditioned space
I live in Portland,OR climate zone 4 & 5 marine.
I consider it mainly a heating climate and lots of rain.
I have a cape cod style house with 13/12 pitch roof on the main roof.
I have three gable dormers on the front, two new ones and one existing all with 20/12 roof pitch.
I just had ridge vents installed on the main roof and all three gable dormers.
I have managed to keep my new HVAC ducts in the floor joists and the are only exposed in the knee wall area before to going down into the (conditioned) floor/ceiling joists on the way to each room.
As a consequence, code requires R9/R10 insulation for ducts in unconditioned space. So, my builder and I put in some 2″ polyiso foil faced between existing 2×4 rafters (3.5″ deep).
Soffit vents to be installed next to connect with the ridge vents, created about 1.5″ vent baffle.
I have read that ideally I should put the polyiso on the exterior but it may be outside of my budget. So, I am trying to stick with insulating on the interior of the walls (2×6) . Our currently plan just calls for R21 unfaced fiberglass in the knee wall and exterior walls.
I have a short section of catherdral ceiling which I am trying to maintain my air path, but also meet an even higher Rvalue requirement (R30). I was thinking of going with either 4″ or 4.5″ polyiso foil is on both sides from Homedepot Rmax Thermasheath-3.
My foam board serves two purposes:
1. baffle path for ridge vent
2. insulate the rigid duct from unconditioned space.
So, my question is should I be using XPS both areas as my site built baffles of the polysio ?
The area is partially in the knee wall area, but then becomes a sloped catheral ceiling it the upper portion before it hits the flat ceiling part of the room.
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