I own a chicago style brick bungalow. I am going to put a bonus room in 1/2 of the attic. I am going to put down 1/2 " osb for a subfloor over 1 x 6 floor joists. i am going to build 2 knee walls down the length of the room and hang 3/8 inch dry wall on 2x 6 roof rafters
16" on center. Do I have to double the roof rafters to carry the increased load
Thanks,
Jim

Your not adding any load to the rafters, you'll need to add something to the bottom edge of the rafter in order to have insulation in the ceiling, but just the drywall isn't much load, also using 1/2 osb will not work. You need 3/4 subfloor to do it right, otherwise its going to have a lot of bounce to the floor.

You mentioned 1X6 floor joists. I would assume you meant 2x6. Even with 2x6 they may be a little flimsy for a floor. When they were installed they were probably not intended to carry much of a load. Check with your local building dept. and see what the code is for the space.

Chicago has pretty strict building codes and it appears you would be violating several with this plan. Did you pull a building permit? If so you will need to submit your plans and the 1/2" osb is out the window. I disagree with Glavine a hair on the roof load. Once you attach dryway to your roof rafters your have created a load increase around 1lbs per sq/ft. You will need insulation behind your knee walls and in your new ceiling. Your 2x6 rafters will only give you room for R13 since there must still be an air space above the insulation.

All these answers have been a great help. I don't live in chicago, but in a nearby suburb. What is the best way to give myself 2 extra inches for air space behind the insulation?

I live in the Chicago burbs too. 2x6 rafters in Chicago weather are pretty light, we now use 2x6 in the walls and 2x10 as rafters. You added to the load with the drywall so I'd sister in 2x10s. It will better carry the snow load and give you 4 more inches for insulation. Remember to not insulate into the eave andblock the incoming air from the eave. Leave at least 2" between the insulation and the underside of the roof. I know I just made you roof lower but even Mayor Daley did this to his bungalo.