Daryl writes from Denton, Maryland: “I am adding an addition with cathedral ceilings. In hindsight I should have used 2 x 10 rafters, but I have 2 x 8.
It is very common for builders use a 2 x 8 rafter instead of 2 x 10 when the less-expensive lumber can pass code structurally. The cost of increasing from 2 x 8 to 2 x 10 can be significant. Once the 2 x 8 rafter is in place, builders will nail on a 2 x 2. This will then provide sufficient rafter cavity space to install R-30 insulation. If you have 2 x 8 rafters there should be 7-1/4″ of space. If the rafter is furred out 2″ then the rafter cavity will be 9 1/4″. An R-30C insulation batt measures 8-1/4″. This will give you the full R-30 and maintain 1″ ventilation space between the insulation and roof deck.
