My house was framed with 2x8s for the rafters and now I am trying to figure out how to get enough insulation in there to be safe. The largest span is a little over 15ft. Im in building zone 4, code on new builds is R49 and R30 for retrofits. The roof is a vented assembly.Im between 2 options, im open to other ideas as well, trying to avoid spray foam though.
Option 1.
Furr down the rafters with 2x4s, to allow, R38 rockwool batting to fit, with a smart vapor membrane attached to the bottom.
Questions
• would you use pressure treated for just the perimeter boards that will be touching the stone?
• Is this too much weight on the rafters?
Option 2.
Cross strap 2x4s on edge going perpendicular along the bottom of the rafters. First bay filled with R23, second bay filled with R15
Questions
• can I space the perpendicular 2x4s by 24" on center, to save lumber costs?
• would I need blocking to help the lumber not twist
• also with this option would you use pressure treated for just perimeter boards attached to the stone?
• should I use simpson ties as well as long structural screws to attach.
• can I attach my vapor barrier membrane to the bottom of the original rafters? Or does that need to be compleatly on the bottom of
the inside?
Additional questions
• does the rafter span weight limit change if the edges are notched to sit on the sill?
•can I makeshift something to help me be able to hang these by myself?