I have to add fiber glass insulation on my cathedral roof, for a small cottage 16' x 24', new construction, in Canada. The roof rafters are 2" x 8". For that size, I should use R20 (6") in order to keep 2" for ventilation on the top of insulation. I though I could to be able to use add another 2" x 2" lumber in order to extend the rafters wide to 2 x 10 (instead of 2 x 8) and to use R28 (8") insulation instead of R20 (6"). Is that a big difference for R-value? The cottage is used to spend weekends and some vacation days and not (yet!) for permanent living. What would you recommend to me? And another question: In order to keep the 2" for ventilation I though to use raft-R-mate. Is it wrong to put this raft-R-mate the entire long of rafters or only at the beginning should be enough?
Thank you, Chris