Room and board are not qualified expenses for the interest exclusion of cashing Savings and I-bonds. You must allocate tuition to the Bond interest exclusion. Even books don't count.
If you have more bonds, the "trick" is to cash the bonds and contribute the proceeds to your 529 plan (within 60 days). Then you can take a distribution from the 529 and allocate it to room and board.
You do not allocate expenses to just the interest. You have to cover the entire bond proceeds, to exclude the interest.
