Can I put insulation under roof between joists
As heat (vapor) escapes up through the ceiling and insulation in the joist space, it turns from vapor to liquid when it reaches its “Dew Point”.  (You can google Dew Point. )

That moisture (liquid) will cause dryrot and mold if it is not dried out. The easiest way (and customary way) to dry this moisture out is by ventilating the space between the insulation and roof sheathing. 

In order for ventilation to work, it has been determined that you need a 1” air space between the insulation and the roof sheathing. (See ICC Code R806.3.) Also, cross ventilation is recommend (soffit vents on both sides of your house or gable end vents). 

The amount of venting is 1/150th of the area of the attic, unless you install not less than 40% and not more than 50% in the top one-half of the attic (gable vents or ridge vents) See ICC R806.2, then it can be reduced to 1/300th the area vented. 

After you install the insulation in between the roof joists, the existing blown in insulation in the ceiling will not help thermal resistance. (However, it will provide sound control between floors.)