I DON'T KNOW WHAT CLIMATE YOU LIVE IN. I LIVE IN SOUTH DAKOTA. IN OUR STATE IT IS RECCOMMENDED THAT AN R-VALUE OF AT LEAST 40 IS NEEDED FOR THE CEILING. YOU'RE TALKING ABOUT INSTALLING A PRODUCT THAT IS INSULATED ON 1 SIDE AND FINSHED ON THE OTHER. MOST "FOAM PRODUCTS ONLY HAVE AN R-BVALUE OF ABOUT 5 PER INCH. YOU ARE DREAMING IF YOU THINK YOU CAN MAKE A DIFFERANCE BY INSULATING THE CATHEDRAL CEILING IN THE WAY YOU ARE DESCRIBING.

Given your situation, you may want to consider the spray in foam insulation to insure every possible opening is sealed and maximum possible insulation. Know it is more expensive but will do the best job possible.