Insulation for condensation control changes from IRC 2009 to 2012
I’m building a small house in Portland Oregon, climate zone 4C marine (according to 2009 IRC). I have 2×12 rafters with blown cellulose, then a layer of taped plywood sheathing for air barrier, then 3″ of XPS followed by another layer of ply and the roofing on top of that. Since the ceilings are vaulted, it’s an unvented attic situation, so I’m worried about moisture. I’ve got R36 in the cellulose and R15 in the XPS.
I was reading in the 2009 IRC about the prescriptive rules for condensation control in unvented attics and in table 806.4 it says in order to avoid my first layer of plywood getting wet, I need R20 of exterior insulation as part of the required R38 minimum ceiling insulation.
So while I’m way over the minimum IRC2009 insulation requirements, my ratio of exterior foam to interior cellulose is way off- 52% of the stack should be exterior according to IRC2009.
Then I looked at the 2012 IRC, they have upped the minimum ceiling insulation to R49 but kept the exterior portion in an unvented attic to R20, meaning the ratio is now 41%.
Does this mean that the ratio was wrong in 2009, or that ratio isn’t the important thing, but rather then total amount of insulation above the first layer of sheathing?
I would think that the important thing is to keep the difference between outside air temp and the temp of the sheathing below a certain amount. (no idea what that amount is). I would also think that the more insulation you have below, the less of a problem it would be, since the heat from the house wouldn’t be warming the sheathing as much in the first place. So I’m questioning the idea that the ratio of above/below insulation is the important metric.
I’m a diy builder on my first project, so could someone more experienced and educated weigh in on this seeming inconsistency?
BTW, my city building plans inspectors (notoriously nitpicky) had no problem with my plans for R36 below and R15 above- either they missed it, or it’s not an issue for them. They use IRC2009 currently.
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