Hi All,

Need some advice please.

Trying to figure out what insulation to use on a new build.

ICF walls R40, R20 Slab, looking for R60 roof.

My roof is a skillion/shed style 20″ thick I-joists with metal on top. There will be no holes in the drywall other than electrical boxes (no can lights). The roof is dead square, no valleys, or peaks.

With what is available locally, BIBS and high density FG batts, here is what I have been thinking. The 6″ outside insulation from the ICF will continue up to within 2″ of the roof sheathing to allow for ventilation (is ventilation necessary?). Sealed to the joists, this give an effective air barrier from the ends. Then I was thinking of using high density r40 plus r20 batt = r60 and then ‘capping the batts with 1″ of EPS on top. Basically, the idea is to constrain the batts on the end and bottom and then constrain on top by friction fitting the EPS/XPS under the lips of the I joist and the top of the batts.

With this, the FG has essentially been encapsulated and would not be affected by air current, correct?

If correct, any issues with creating what might be a double vapor barrier? And if this approach is okay, would you use Kraft faced batts or open? And why?

We will be using the airtight drywall approach plus vapor barrier as well.’

I know that high density cellulose would be nice, but haven’t been able to find anybody locally who does it…only low density retrofits.

Normally I would be against batts, but this roof is as near perfect as could be for their use. No ducting, no corners, no intrusions…basically a perfectly square box between every joist.

Any other thoughts on how to achieve the goal are welcome.

Thanks in advance for your help.
Darren