unfinished attic - new insulation
 

I have a 1940's house with an unfinished attic. I would like to add
insulation to the roof and use the space as an office.

First, my roof rafters are a non-standard spacing of 20" on center,
instead of the 16 or 24 that the rolls are sold in. Does this mean I
will have to cut each roll on length??

Second, the rafters are 2*6 (actual 5.5 wide). If I am to maintain an
inch for ventilation between the insulation and the roof, I can only
add 4.5 deep of insulation. Is that true? For rolled insulation I
would get an R-15 max. I am looking into the rigid foam board to use
since it provides higher R per inch.

ANy insight would be appreciated.

Geoff Oldham
ATLANTA, GA