I'd suggest you research this further, but
AFAIK creating an hollow area is asking for trouble.
Water vapor will work past the sheetrock/T&G/foam panels/whatever, and accumulate against the underside of the cold roof surface. Then it collects and rots.
My suggestion is to either vent under the roof sheathing from soffit to ridge(helps keep the shingles cooler during the summer), or fill it in completely with spray foam insulation. The open-cell type foam does "breath" a little bit(slowly) I've been told.
You can buy DIY foam from Tigerfoam and other vendors but it's tricky to use.
A few years ago, I found out the P.O. didn't install proper soffit venting with active circulation when she built the second story addition only 20 years before. The bottom of the sheeting wasn't in good shape because of this, and squirrels tore up most of the batting, so it was all replaced.
