With 2x6 framing, you can only fit R-19 into the space if fibreglas is used and that will compromise ventilation.
Blue and pink foam is R3.6 per inch and some foil faced foam is R7 per inch. Wood is about R1 per inch and roofing or drywall is negligable.
I would place R19 fibreglas and add an inch of foil faced foam. That gets it to R 27 with the roof sheathing. Close enough- but if inspectors are strict there, you could add 1-1/2" of the foam. Tape it on the seams and you have a vapour barrier. apply it across the faces of the joists and it helps stop thermal bridging where the joist at six inches is only R6 right next to insulation at R19 or so.
- 15 Old House Features We Shouldn't Abandon
- 17 Tiny Bathrooms We Love
- 16 Designs for a Low-Cost DIY Coffee Table
- Insanely Easy 60-Minute Home Improvements
- 12 Sheds You Could Live (or Work) In
- Assembly Required: 15 DIY Kit Homes
- 30 Things Every Adult Should Know How to Do
- 10 Surprisingly Simple Woodworking Projects
- 7 Surprising Other Uses for Mayonnaise
- 9 Ways to Make Your TV Look at Home
- 9 Totally Amazing Mobile Home Makeovers
- 11 Lessons to Learn from AirBnB's Tiniest Homes
- 10 DIY Ways to Redo Your Wall—Without Paint
- 8 Smart Shoe Racks You Can Make Today
- 7 Easy Budget-Friendly Backyard Makeovers
- Worth It: 8 Renovations That Pay You Back
- 7 House Sounds Never to Ignore
- Watch These 10 Home Trends Take Off in 2015
- 11 Things Never to Keep in Your Bedroom
- 12 Places You Never Clean—But Should!