Attic addition insulation

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fxportapot

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I'm in the process of designing/planning to finish our walk up attic, i need a little guidance on planning for insulation. Here's my plan and correct anything that doesn't work:
Currently there are 2x8 rafters, 24" oc, starting where the knee walls will meet the rafters up to where the ceiling will be i'm going to staple rafter vents and use R30 Roxul, cover that with a vapor barrier, and then drywall.
The 2x4 Knee walls will be roxul r15 with a vapor barrier and then drywall
The 2x8 ceiling will be roxul r30, vapor barrer, and drywall

I've gone back and forth on just using fiberglass batts, but in order to fit r30 I would have to add 2x4's to the current rafters, this won't work the planned ceiling height for the shower, and i would have to cut down doors because of height.
Thoughts? does this work?
 
Welcome to the site. Roxul r30 is 7.5 thick you would still like too add a 2x2 to the rafters to get air flow to the top of the roof. Can you make that work?
When ceiling is 24" on center, 5/8" drywall is the call.
 
Looks like I'm goin to need r38 (doh!) back to the drawing board, I'll update later with the new plan, thanks
 
Alright, so of I scab out the 2x8 rafters with 2x4's it would give me roughly 10&3/4". if i layer roxul r23 (5.5") and roxul r15 (3.5") itd leave me an inch and some change for the air gap, sound good?
 
I considered spray foam but it's a vented roof, and the thought of a roof leak or repair in the future keeps me from doing that.
 
Light weight 5/8" will be the way to go. The planned ceiling will come bellow the collar ties, the existing structure wont be touched, total flat area of new ceiling span from rafter to rafter is 7', I think 2x4's will do the trick (thoughts?), I was then planing to lay r15 in the ceiling joists and then layer r30 on top of that.
 
I used roxul to do my house and attic. I love the stuff, wicked easy to install and its a sound barrier. cant go wrong, except that its tiwce the price of the itchy pink stuff
 
drewdin said:
I used roxul to do my house and attic. I love the stuff, wicked easy to install and its a sound barrier. cant go wrong, except that its tiwce the price of the itchy pink stuff
Click to expand...
Actually you should check the numbers on the soundproofing as compared to the ichy stuff.
 
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