Attic insulation depth above wall is less than wall insulation
I live in Madison, WI (cold climate). I have an old ranch home with 2×6 rafters with a deep soffit overhang (2ft). The rafters are notched at the top plates and I have about 2 inches of free space between the top plate and the plywood roof sheathing (assuming I keep an inch free for ventilation). The walls are 2×4 with plywood and Homasote sheathing.
I have read here and elsewhere (i.e., buildingscience.com) that I should not insulate the wall more than the depth of the attic insulation. I’ve got to pull off a of siding this summer to replace with fiber cement and was thinking of adding 2″ of XPS foam, boosting the wall value to R13 batt + R10 XPS + Homasote value (maybe 1). I’d end up with a wall at R23 and an attic right above it at R13ish at best (assuming closed cell insulation). Rest of attic is combination of loose fiberglass and fiberglass batt, R30 at best.
My question is, why would this be a bad thing? I’d air seal the heck out of things (airtight drywall) and if there is 2x more heat transfer through the attic than the wall, what is so bad? Wouldn’t the heat transfer through the attic be unchanged regardless of the heat transfer through the wall? Or do I need a course in thermodynamics? Ice dams are a plague in our climate, so I don’t want to screw things up, but would like to boost the wall’s insulation value. But that contradicts most guidance I’ve seen on the subject.
Am I stuck with R-13 walls forever without reframing my attic rafters?
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