Inadvertent vapor barrier?
I live in MN and have a 12’x16′ porch addition that I’m about to insulate the vaulted ceiling. I’m calling the porch a 3-1/2 season porch since I’m installing electric base board heat which allows me to heat the room, but only when I intend to use the room, which may not be that often in the dead of winter. The scissor trusses are just 4/12 exterior pitch with a 2/12 vaulted ceiling and 7″ heel height at the wall line, so not a lot of room to meet R value requirements here.
My thought to insulate the ceiling was to install 2″ thick XPS foam boards cut to fit between the top chords of the truss with fiberglass batts under the rigid foam. The foam would be installed up the roof slope with a 1″ air gap beneath the roof sheathing to ventilate from the soffit vents. At the truss peak the foam would run flat so the vented air is free to travel along the ridge and to the ridge vents. The edges and seams of the ridged foam would be sealed with spray foam. Using the 2″ XPS foam gives me more R value in this small insulating space and would prevent the batt insulation from compromising the air ventilation space.
My local inspector will expect a minimum 4 mil vapor barrier on the warm side of the insulation. Am I creating a harmful double vapor barrier with this concept? Also, and I’m not sure if this is important, though the porch ceiling would be tightly sealed The vented ridge does run from the new gable wall to the new attic space created over my existing house and neither of those would be sealed. I thought I had a handle on this, but the more I research I do the more confused I become.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part