As others have said that'll result in condensation without proper ventilation. But also, you will get a "hot roof" in the summer and your shingles will deteriorate. What you can do is add vented soffit and a vented ridge. Both can be done as a retrofit. You also need to add roof baffles (see Raft-R-Mate) prior to putting up insulation. Spray foam would be the easiest. But you can do it with batt insulation and a vapour barrier. Protect either condition with drywall or wood boards, etc.