Effi,

would like to create a cathedral type ceiling in a small area of 24"
on center roof rafters 3.25" thick
within the 3.25 inches thick want: 1" airflow space; and 2.25"
insulation

2.25" won't provide much in the way of insulation, especially in a ceiling
where it's needed most. You might get R-5 to R10 if you're real lucky.
That's a far cry from the R38 (in my area) that normally is required in
cathedral ceilings.

To maintain the airspace, just use the foam air baffles you can get from
Home Depot or most any lumber yard. The ones I used have little "bumps" in
the middle to prevent them from getting pushed against the roof sheathing
when you push the insulation in.

Alternatively, you could nail (or glue) some 1x boards (or strips of
foam insulation) along the sides of the rafters and span the gap with a
piece of plywood or hardboard.

If you MUST stick to that 2.25" insulation space, your best bet would be
solid foam insulation. It will provide the best R-value per inch, and
because of their rigidity will probably not need the air baffles above
them.

Compressing 4" insulation into the space won't work very well either. It's
the air that gets trapped in the fiberglass that provides the insulation
ability. If you compress the fiberglass, you greatly reduce the air spaces
and correspondingly the R-value.

If there's any way you can shim down the rafter space to gain more depth
(i.e. nail 2x3's on edge along the bottom of the rafters), you could put in
more insulation.

Alternatively, or in combination with the above method, you could go ahead
and fill up the available rafter bays, then frame a second roof on top of
the existing roof sheathing, providing the 1" air space there.

Finally, you may want to look into sprayed foam insulation. It'll provide
the best insulating ability for the space, and will completely seal off the
rafter bays. While this would also eliminate the 1" airspace, I hear it
works well because no air currents can flow within the space. It's a lot
like structural stress skin panels. Unfortunately, it's not a do-it-
yourself application.

Good luck!

Anthony