Rigid insulation under mineral wood in rafter bays? (Zone 6 Vermont)
My wife and I are adding an addition with a vaulted vented ceiling with no dormers, skylights, or electrical. It is 18’X10′ added to our existing bedroom. We ran a 2X12 ridge beam with 2X10 rafters 24″ oc , Advantech roof sheathing, Ice and Water shield then asphalt shingles. I am now scratching my head trying to get to R-49 in the ceiling. I thought of furring out the rafters to fit more mineral wool, or doing a layer or two of rigid insulation under the rafters, with strapping, and finishing with sheet rock. There are a couple articles and Q&A’s I found that mention this strategy, mainly:
However, most situation seem more complicated (dormers + skylights). I was wondering which strategy makes the most sense (furring out / ridged), and if anyone has more info on adding ridged insulation to the underside of rafters.
Cheers and thanks for any suggestion/information.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part